PHD2 Manual - PHD2 Guiding

PHD2 Manual - PHD2 Guiding
PHD2 v2.4.1i
1. Introduction ..............................................................................................................2
2. Main Screen .............................................................................................................3
3. Basics: Using PHD2 Guiding ..................................................................................5
a. Equipment Connection
b. Exposure Time & Star Selection
c. Calibration
d. Guiding
4. Darks and Bad-pixel Maps.....................................................................................13
5. Visualization Tools ................................................................................................17
a. Overlays
b. Guiding Performance Graph
c. Star Profile Display
d. Target Display
e. AO Graph
f. Dockable Windows
6. Advanced Settings .................................................................................................22
a. Global Parameters
b. Guiding Parameters
c. Camera Parameters
d. Mount Parameters
e. AO Parameters
f. Rotator Parameters
7. Guide Algorithms...................................................................................................31
a. Guiding Theory
b. Guide Algorithm Parameters
8. Tools and Utilities ..................................................................................................34
a. Manual Mount Control
b. Auto-select Star
c. Calibration Data
d. Server
e. Logging
f. Drift Alignment
g. Lock Positions
h. Comet Tracking
i. Guiding Assistant
j. Equipment Profiles
k. Simulator Controls
9. Keyboard Shortcuts ................................................................................................43
10. Trouble-shooting and Analysis ..............................................................................44
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PHD2 is the second generation of Craig Stark's original PHD application. PHD has become a fixture of the
amateur astronomy community with more than a quarter million downloads. From its inception, it has
successfully embraced three seemingly conflicting objectives:
1. For the beginning or casual imager, to deliver ease of use and good guiding performance "out of the
2. For the experienced imager, to deliver sophisticated guiding algorithms, extensive options for tuning,
and broad support for imaging equipment
3. For all users, to consistently exhibit a commercial level of quality while being available free of charge
In order to extend PHD to more platforms and further expand its capabilities, Craig released his program to the
open-source community, and PHD2 is the direct result of that generosity. It has been substantially restructured
to make it more extensible and supportable going forward. Moreover, the initial release of PHD2 already
includes a substantial number of new features and refinements while retaining all the core strengths of the
original. Users of the new PHD2 can be confident it will remain committed to the three objectives that made
the original application so successful.
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Main Screen
The PHD2 main window is designed for ease of use and clarity. Its intent is to support a quick and natural
sequence of interactions to start and control guiding. The basic steps for doing this are as follows:
Connect to your guide camera and mount
Start a sequence of guide exposures to see what stars are available in the field of view
Choose a guide star and calibrate the guider
Continue guiding on the target star while using various display tools to see how things are going
Stop and resume guiding as necessary
The majority of the screen is taken up by the display of the star field from your guide camera. The display is
automatically adjusted for size, brightness, and contrast so you can have a clear view of available
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stars. However, these adjustments are done only for display purposes. Internally, PHD2 operates on the raw,
un-adjusted data in order to maximize guiding accuracy. This display is also used to select a guide star by
simply clicking on it.
Basic control
Near the bottom of the screen are the main controls. PHD2 is largely controlled by these buttons and sliders,
with additional pull-down menus at the top of the window for more detailed functions. Moving from left to
right in the window, the primary buttons are as follows:
1. The camera icon - used to connect to your camera and mount devices
2. The loop icon - used to start a sequence of repeated exposures with the guide camera ("looping"), with
each resultant image (guide frame) being displayed in the main window. If guiding is subsequently
started, clicking on the 'loop' icon again will pause guiding while continuing to take guide exposures.
3. The guide/target icon - used to start calibration, if needed, and then to start guiding on the selected star.
4. The stop icon - used to stop both guiding and looping
To the right of the stop icon is a pull-down list of exposure durations (0.01s - 15s). You use this control to
quickly set the guide camera's exposure duration. If your camera does not support an exposure duration, PHD2
will do its best to emulate that duration. For example, if you use a short-exposure webcam, your maximum true
exposure duration might be only 1/30th of a second. If you select one second as the desired exposure time,
PHD2 will automatically acquire images for one second and stack them on the fly to create a composite image
for guiding.
The next control to the right is a slider for adjusting screen stretch and contrast, essentially a "gamma"
adjustment. PHD2 automatically adjusts the display accounting for the darkest and brightest pixels in the
image, and the slider is used to fine-tune the display to better see the stars in the field of view. This may be
useful, for example, if you are trying to focus the guide camera for the first time and need to see the large, outof-focus star image. Moving the gamma slider only makes the display brighter or dimmer for your viewing.
PHD2 always uses the raw pixels from the camera for guiding, and moving the gamma slider has no effect on
guiding. A display of "completely white" or "completely black" is usually an indication that no stars are
available in the field of view.
Next to the gamma slider is the so-called "brain button." This button brings up an Advanced Dialog for making
detailed adjustments to PHD2's guiding operations. A major design goal of the program is to minimize your
need to change these parameters, but "the brain" is nothing to be feared - there are adjustments available here
that can significantly improve your experience. Over a period of time, you should take a look at this dialog and
learn what it can do for you.
The rightmost control in this row is a "camera dialog" button. This may be disabled ("grayed out") depending
on the type of guide camera you have selected. If it is enabled, you can click on it to set various parameters that
are unique to that type of camera - binning and specialized gain settings are examples of such parameters, but
there could be others depending on the camera.
The pull-down menus above the main guider display are used to access a variety of functions. These are
described in the Darks, Tools and Utilities, and Visualization sections of this help document.
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Status Bar
The status bar at the bottom of the main window is used to display messages and status information that will
help you keep track of guiding operations. For example, the rightmost three panels on the status bar will show
you whether the camera and mount are connected and whether the guider has been successfully calibrated.
Basics: Using PHD2 Guiding
There are five basic steps to start guiding.
1. Press the camera button and connect to your guide camera and mount.
2. Pick an exposure duration from the drop-down list.
3. Hit the loop button and look at the available stars, adjusting focus if necessary. Move the mount or
adjust the exposure duration as needed to find a suitable guide star.
4. Click on a non-saturated star that's not very near an edge for use as the guide star.
5. Press the PHD2 Guide button.
Details of these operations will be described in the sections below.
Equipment Connection
Exposure Time and Star Selection
Equipment Connection
In order to begin guiding, PHD2 must first connect to your hardware: the guide camera, the mount, and,
optionally, an 'aux' mount, an adaptive optics (AO) device, or a rotator. When you click on the camera icon,
you'll see a dialog that looks like this:
Camera Selection
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The Camera drop-down list shows all the camera types currently supported by PHD2. With the exception of
ASCOM-compatible cameras, the camera drivers are bundled with PHD2 and will reside on your system after
the PHD2 installation. ASCOM-compatible camera drivers are available from the ASCOM web site or from
the camera manufacturer, and the drop-down list will show only those ASCOM cameras that are already
installed on your system, if any.
The list of supported cameras at the time of writing is shown below - but you can contact the open-phd-guiding
forum to inquire about your specific camera or mount.
Supported cameras, December 2014:
ASCOM v5/6 compliant cameras
Atik 16 series, color or monochrome
Atik Gen 3 color or monochrome
CCD-Labs Q-Guider
Fishcamp Starfish
iNova PLC-M
MagZero MZ-5
Meade DSI series: I-III, color and monochrome
Orion StarShoot DSCI
Orion Starshoot Autoguider
Orion Starshoot Planetary Imager and Autoguider
SBIG rotator
Starlight Xpress SXF / SXVF / Lodestar
Webcams (LXUSB, parallel, serial, OpenCV, WDM)
Fishcamp Starfish
Meade DSI series: I-III, color and monochrome
Orion Starshoot Autoguider
Starlight XPress SXV
The Imaging Source (DCAM Firewire)
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Mount Selection
The Mount drop-down list displays options for connecting to your mount. There are generally two ways to do
1. Use an ASCOM-compatible telescope driver that sends guide commands to the mount over a serial
cable (or more commonly, a USB/Serial connection)
2. Use the ST-4 compatible guide port interface on the mount with a specialized cable and an intermediate
device like a camera or a Shoestring box
The ASCOM interface relies on third-party drivers to communicate with the mount. These drivers are available
from the ASCOM web site (ASCOM Standards) or from the mount manufacturer - they are not distributed with
PHD2. So the drop-down list will be populated by only those ASCOM drivers you already have installed on
your system. The ASCOM driver must support the 'PulseGuide' interface, which has been a requirement for
ASCOM compliance for many years and is widely supported. With this type of mount control, guide
commands are sent from PHD2 to the mount over the serial interface. The high-level PHD2 guide commands
(e.g. "Move west 500 mSec") are translated by the mount firmware into the appropriate motor control signals to
execute the command. With the ASCOM interface, PHD2 can also obtain the pointing position of the mount,
especially the declination and side-of-pier, which can be used as factors in guider calibration.
The "Guide-port" interfaces use a specialized, hardware-level control port available on most mounts. To use
this type of interface, there must be another device in the link between PHD2 and the mount:
1. Any of the guide cameras which have an ST-4 compatible "on-camera" guider interface. Use the 'on
camera' mount choice for these setups.
2. Any of the Shoestring GP-xxx devices
3. A supported AO device with a guide port interface
With this style of interface, PHD2 guide commands like "Move west 500 mSec" are translated by the
intermediate device (camera, Shoestring box, AO) into electrical signals necessary to drive the mount motor for
the correct length of time.
Aux Mount Selection
If you have selected an ST-4 style of guiding in the 'mount' section, that interface cannot be used to query the
pointing position of the telescope. As a consequence, guider calibration won't be automatically adjusted for
declination, nor will it be automatically flipped when the side-of-pier changes. You can restore these features
by specifying an "aux" mount connection that will be used only to get the telescope pointing information. An
example is shown below:
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For Windows users, the "aux" mount can use any of the ASCOM-compatible mount drivers, while Linux users
can take advantage of INDI drivers. The "aux" mount choice will be used only if the primary mount interface
cannot return pointing information - it will otherwise be ignored.
Adaptive Optics and Rotator Selections
With PHD2, you now have the option of controlling the Starlight Xpress adaptive optics unit and/or any of
several ASCOM-compatible camera rotators These can be specified by clicking on the 'More Equipment..."
button in the above dialog:
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If you don't have these devices, just leave the selections at 'None.' If these devices are connected, you'll see
additional tabs in the 'Advanced Settings' dialog that provide access to various device-related properties. PHD2
does not control a rotator, but it will read the current angle setting from the rotator and adjust the guiding
calibration if needed.
All of the PHD2 devices - camera, mount, AO - include built-in simulators. You can use these simulators to
explore how PHD2 works and to decide how you want to use the program. There's no reason to waste valuable
dark-sky time learning to use PHD2! Virtually all of PHD2’s features, including full calibration and all the
graphical display options, will work properly when device simulators are used. You will even see fairly
realistic guiding performance to give you some idea of what to expect in the field. To get started using the
simulators, choose 'Simulator' for the camera type and 'On-camera' for the Mount type.
Equipment Profile
At the top of the 'Connect Equipment' dialog are some additional controls for managing equipment profiles. All
of the guider settings in PHD2, default or otherwise, are automatically stored as part of an equipment profile. If
you have only one guiding setup - you use the same camera and guide scope combination each time - you will
only need one profile; and you can just use the default profile. But you may have multiple equipment
configurations - for example, an off-axis-guiding arrangement for a long focal length scope and a separate guide
scope/camera configuration for s shorter focal length imaging scope. The PHD2 guide settings for those
configurations are likely to be different, so you would want to use separate equipment profiles. The controls at
the top of the 'Connect Equipment' dialog let you choose the profile you want to use and to create/edit/remove
profiles as you see fit. When you select a profile and connect to its associated equipment, all of the settings last
used with that profile are automatically reloaded. Once you've established the profiles you need - perhaps only
the default one - you can simply click on the 'Connect All' button and you're ready to move ahead.
The recommended way to create a new profile is to use the "Wizard" capability. The wizard takes you through
a sequence of dialogs that explain the various settings and help you decide how to set them. It will also
calculate a recommended set of guiding algorithm settings that are likely to get reasonable results right
away. When you run PHD2 for the first time on your system, this wizard will be automatically
launched. Subsequently, you can use the new-profile wizard by clicking on the 'Manage Profiles' field in the
'Connect Equipment' dialog, then choosing 'new using wizard...’ If you are changing cameras and want to keep
the dark libraries and bad-pixel maps associated with the old camera, you should create a separate profile for the
new camera. When a camera selection is changed in an existing profile, the previously built dark library and
bad-pixel map data will no longer be usable.
If you already have a suitable default equipment profile and you simply want to connect to the equipment "just
like before", you can do a< shift>-click on the main screen 'Camera' button and PHD2 will automatically reconnect to your hardware.
Exposure Time and Star Selection
The guide star can be selected (clicked on) while "looping" is active - in fact, this is the recommended
method. It can also be selected after looping has been stopped, but this opens the possibility that the star might
have moved since the last exposure. No great precision is required in clicking on the star - PHD2 will find the
star nearest to the cursor. After you do this, a green box will appear to frame the star. If you pick a star that is
too bright, a message box will tell you the star is saturated, and you should either use a different star or decrease
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the exposure duration. The choice of exposure time will depend entirely on your equipment, sky conditions,
and the available stars. The exposure time you choose has two major implications:
1. It affects the signal strength (brightness) of the selected star - a brighter star will stand out better from
the background and will generally produce better guiding results so long as it is not saturated.
2. It also determines the frequency with which guide commands are sent to the mount - guide commands
cannot be sent any more frequently than once for each exposure cycle. Some mounts benefit from
frequent small guiding adjustments while others do not - you may need to experiment to understand
what works best for your situation.
As a starting point, try using exposure durations in the range of one to five seconds. Rather than choosing the
star yourself with a mouse-click, you can let PHD2 Auto-select the guide star by using the Alt-S keyboard
shortcut after stars are visible in the main display.
There is also an Auto exposure time selection available. When exposure is set to Auto, PHD2 will attempt to
adjust the exposure to keep the selected guide star at a constant signal-to-noise ratio (SNR) value. The Auto
setting can be especially beneficial for AO users who want to minimize exposure time without losing the guide
star. The settings to control Auto-exposure are on the Global Tab of the Advanced Dialog.
Automatic Calibration
Conventional Mounts
Two things need to be measured by PHD2 as part of guider calibration:
1. The angle of the camera relative to the telescope axes
2. The length of the guide pulse needed to move the telescope by a specific amount
PHD2 handles these measurements automatically by sending guide pulses to the mount and "watching" how far
and in which direction the star moves between guide camera images. This process begins after you have
selected a star and then clicked on the Guide icon button. Yellow cross-hairs will appear over the original
location of your guide star and PHD2 will start to move the mount in various directions, tracking how the star
moves as a function of what move commands were sent to the mount. The status bar will display the
commands as they are sent to the mount, along with the incremental movements of the guide star relative to its
starting position. PHD2 will do this on both axes, first moving east and west, then north and south. PHD2
wants to move the star up to 25 pixels in each direction in order to get an accurate calibration. Once this is
complete, the crosshairs will turn green and guiding will start automatically...
In most cases, calibration will complete automatically without any user involvement. Obviously, this assumes
that the mount and all the cable connections are working correctly. If they are not, the calibration is likely to
fail with a message that the "star did not move enough"; and you'll need to do some trouble-shooting. If the star
has basically not moved at all in one or more directions, you should look first at the cable connections and
mount behavior. The "manual guide" function under the 'Tools' menu can help with this as can the troubleshooting section of this document. But if the star has moved by some amount in each direction, you may need
to adjust a calibration setting called the calibration step-size. PHD2 uses this parameter during calibration as a
fixed-size duration of movement. The default value has been chosen to cover a wide range of typical guide
scope set-ups, but you may need to adjust it for your situation. For example, an off-axis-guider on a long focal
length scope will probably need a setting that's different from the default value. To adjust it, go into the "brain
dialog" (Advanced Settings) and click on the 'Mount' tab. On that tab, you'll find a setting for 'Calibration step
(ms)', and that's where you change the value. There's even a 'Calculate' button that will help you choose an
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optimal value for the parameter. Once you've set this value, it will be remembered as part of your equipment
profile and you probably won't have to adjust it again.
You may also see a calibration failure if you're using a star too close to the celestial pole. In those locations,
fixed-length movements in right ascension often move the star only a very small distance. In that case, move to
a star location closer to the equator, ideally somewhere in the range of -30 to +30 degrees declination, and rerun the calibration. But if this is not feasible, you can adjust the calibration step-size upward until calibration
succeeds. You can always review the results of your last calibration by using the 'Tools' menu and clicking on
'Review Calibration Data.' That will open a dialog that shows a graphical representation of the mount's
movements along with the values that were computed for guiding your mount. This window is described
elsewhere in the "Calibration Details" section of the help file. As a quick quality check, you can open this
window and confirm that 1) the RA and Dec lines are roughly perpendicular and 2) the plotted points are
roughly linear with no significant curves, bends, or reversals in direction. If you do see these kinds of odd
patterns in the graph, you should probably re-do the calibration. Even with high-end mounts, calibrations can
occasionally go awry because of environmental conditions, especially wind and bad seeing.
Like all other guide settings, the calibration data is automatically saved as part of your current equipment
profile. If nothing has changed in your configuration from one session to the next - even over an extended time
period - you can restore the previous calibration data and start guiding immediately. This function is located
under the 'Tools' menu and is labelled 'Restore Calibration Data.' Remember, the data are saved whenever a full
calibration is completed or when you use the 'flip calibration data' command under the 'Tools' menu (see
below). If you're using a German equatorial mount and the ASCOM interface or you have an 'aux' mount
connected, the saved calibration data will include the side of pier where you were last guiding; so PHD2 will
adjust automatically for your current pointing position. But if you are not using mount interfaces that can return
pointing information, you'll need to remember which side of the pier was being used the last time you were
guiding. Clicking on 'Restore Calibration Data' will open a window that shows the results from your last
calibration, essentially identical to what is shown using 'Review Calibration Data.' You can quickly check the
information in this window to be sure the calibration you are restoring is sufficiently accurate and relevant to
your current set-up. If it looks good, click on 'Restore' to re-use it.
If you're using an ASCOM mount (or 'aux' mount) connection, PHD2 will also include the calibration and target
declinations as part of its calculations. In that case, guiding moves in right ascension will be automatically
adjusted to account for the declination of the current guide star, as opposed to the star you used for
calibration. This is really just a refinement and isn't critical in most situations, so absence of pointing
information is not a big problem. However, if you are concerned about it and are imaging near the celestial
pole, you should consider calibrating using a star that's also at high declination. Finally, if you have a rotator
configured as part of your equipment profile, PHD2 will use its angle position property to adjust the calibration
for changes in camera angle, thus eliminating another need to re-calibrate.
After a calibration is completed, PHD2 will "sanity check" the results to be sure the calculations at least look
reasonable. If they don't, you will see an 'alert' message at the top of the main window that describes the
calibration result that looks questionable. You can choose to ignore the alert or click on 'Details' to get more
information. It is generally advisable to pay attention to these alerts because there is no point in trying to
proceed using calibration results that are significantly in error.
Adaptive Optics Devices
If you are using an adaptive optics device, there are actually two calibration processes that must complete. The
first handles calibration of the tip/tilt mirror in the AO and calculates the magnitude and direction of the
adjustments as they relate to displacements of the guide star. The second calibration is the one described above,
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dealing with guide commands that need to be sent to the mount. Known as "bump" commands, these will be
issued when the guide star has moved beyond the range of corrections that can be achieved with the AO alone.
Once guiding has begun, diagnostic messages will be displayed in the status bar to show what guide commands
are being sent to the mount. PHD2 will continue guiding until you click on the 'stop' icon. To resume guiding,
simply start looping exposures again, select your star, and click on the 'Guide' button. You will not need to
repeat the calibration in order to resume guiding. In some cases, PHD2 may lose the guide star and you'll be
alerted by an audible beep and flashing orange crosshairs. There are several reasons this might occur:
1. Something may be obscuring the star - clouds, the observatory roof, a tree, etc.
2. The star may have abruptly moved out of the tracking rectangle because something shifted in the
mount/camera/cabling infrastructure - cable snags can cause this
3. The star may have "faded" for some other reason, perhaps because it is overly faint
Obviously, you'll need to identify the source of the problem and fix it. However, it's important to understand
that PHD2 will not start moving the telescope around in an attempt to relocate the guide star. It will simply
continue to take exposures and look for the guide star to reappear within the bounds of the current tracking
rectangle. When you first start guiding, you may see an 'alert' dialog at the top of the window if no dark library
or bad-pixel map is being used. You can choose to ignore this warning and continue with guiding, but you are
likely to get better results if you spend the few minutes needed to construct a dark library for future use.
If you are using a German equatorial mount (GEM), you will usually have to do a "meridian flip" around the
time your image target crosses the meridian. This means you will move the telescope around to the opposite
side of the pier and then resume imaging. Doing this invalidates the original calibration, typically because the
declination directions are now reversed. If you are using an ASCOM (or ‘aux’) mount interface, your
calibration will be adjusted automatically and you can simply resume guiding (assuming you haven't also
rotated the camera or focuser). If you aren't using an interface that returns pointing position, you will need to
take action to adjust the guider calibration. You can, of course, simply do another calibration on the "new" side
of the pier, a process that will typically take only a couple of minutes. Or, you can use the pull-down menu
item under 'Tools' to "flip calibration data" and then resume guiding immediately.
In some cases, you may want to force a re-calibration. For example, you may have rotated the guide camera as
part of resolving a cable problem. You can do this by clicking on the 'Brain button', moving to the 'Mount' tab,
and clicking the 'Clear Calibration' checkbox. Or, you can simply do a <shift>click on the ’Guide’ button on
the main screen and PHD2 will start a calibration run.
Once you have started guiding, you will almost certainly want to know how things are going. You can of
course watch the star in the guide camera display but in many cases you won't be able to see all the small
adjustments that are taking place. But PHD2 provides many tools for measuring and displaying your
performance, as described in the Visualization section. Several of the guiding algorithms have limit settings for
the maximum guide correction that can be issued with a single command. If these values are smaller than what
is needed to correct the mount's position, you will see an alert dialog at the top of the main window advising
you of the situation. If this is a recurring problem, you may want to increase the values for these settings or
otherwise solve the underlying problem.
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Dark Frames and Bad-pixel Maps
Cameras used for guiding are typically not temperature-regulated and may produce images that appear
quite noisy. As a result, guide exposures frequently show obvious defects in the form of hot ("stuck") pixels or
regions with spurious brightness levels. If there are too many of these defects, you may have trouble identifying
and selecting a good guide star - trying to calibrate on a hot pixel is a common problem for beginners. Even
after guiding has begun, a spurious hot pixel close to the guide star can disrupt the calculations needed for
smooth guiding and may case the software to "jump" between the real star and the hot pixel. These sorts of
problems can be mitigated by using either of two approaches in PHD2: dark frames and bad pixel maps. All
functions related to dark frames and bad-pixel maps are located under the top-level 'Darks' menu.
Dark Frames
PHD2 will build and use a library of dark frames that match the range of exposures you use for guiding. Once
the library is built, it will be saved automatically and will be available for use across multiple PHD2
sessions. As a result, you can spend a modest amount of time to build a good dark library, then use that library
for an extended period of time. Once you have connected to your camera, you can build a dark library from the
'Dark Library...' item under the top-level 'Darks' menu. That will start a dialog that looks like this:
You use the two controls at the top to specify the minimum and maximum exposure times that will be used to
acquire dark frames. The starting, ending, and intermediate values match the exposure times used in the main
PHD2 window, so you can acquire dark frames that will match any exposure time you choose for guiding. The
third control specifies the number of dark frames that will be acquired and averaged for each exposure
time. The averaged image is referred to as a "master dark frame." Historically, PHD has used 5 dark frames for
this purpose, but you may want to increase that number to improve the quality of the master dark frame. You
can also add a note or comment if you wish - this will be embedded in the header of the master dark frames for
later reference.
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One you've set your parameters, click on 'Start' to begin the process. If your guide camera does not have a
shutter - most do not - you'll be prompted by PHD2 to cover the guide scope. To get the best results, be sure
there is no light leakage into the guide camera - doing this in daylight is not likely to work well. PHD2 will
systematically work through the range of exposure times you've chosen, taking the specified number of frames
for each exposure time. Progress will be displayed on the status bar at the bottom of the window, so you can
see where you are in the overall process. Once you've started the process, the 'Cancel' button above will change
to a 'Stop' button. You can click on this if something goes wrong or you want to change the parameters before
the entire sequence completes. Stopping in this way will discard whatever data has already been collected, so
you'll need to make your corrections and then restart the process. Once all the frames have been collected,
PHD2 will compute the master dark frames, store them in a dark library data file, then show a message box
summarizing the results. If your camera has no shutter, you'll also be prompted to uncover the guide scope so
you can return to normal imaging.
Once your dark library has been built, you control its use by the 'Use Dark Library’ item under the 'Darks'
menu. The checkbox on the menu item will toggle on or off each time you click on it. The setting of the item is
retained across program executions, so if you choose to leave the menu item checked, PHD2 will automatically
load the dark library and resume its use the next time you run the application. The dark library itself is retained
on disk until you build a new library, so you can freely change the setting on the 'Use Dark Library' menu item
without loss of any data. If you are using a dark library and there is no master dark frame that exactly matches
your guide exposure time, PHD2 will use the nearest fit. However, you are encouraged to obtain matching
master dark frames for best results. If you have a dark library that has missing exposure times, you can simply
acquire the missing data and it will be added to the existing dark library - there is no need to start over. By
changing the setting of the 'Use Dark Library' menu item, you'll be able to see the effect of using the dark
library and determine whether your guider images are sufficiently improved.
Remember that a dark library is associated with a particular camera. PHD2 will check to be sure that the dark
library matches the camera you are currently using. If it does not, you will see an alert message telling you the
dark library can't be used and must be rebuilt. This can happen when you've changed cameras inside an existing
equipment profile, something you shouldn't do unless you have upgraded your guide camera and have no plan
to revert to use of the old camera.
Bad-pixel Maps (Defect Maps)
For some guide cameras, dark frames don't do an adequate job of removing the defective pixels that are visible
in the guide frame. In those situations, you can probably get better results by building and using a bad-pixel
map. This approach directly measures and compensates for specific areas of the sensor that produce false signal
(hot/stuck pixels) or don't respond correctly to incoming light (cold pixels). Such a "map" is created by taking a
sequence of comparatively long dark exposures (e.g. 15 seconds), averaging them, and then statistically
analyzing the resultant frame to identify the locations of defective pixels. These pixel locations are saved for
future use. During normal guiding, each of these pixel locations on the guide image is replaced by a statistical
sample of the surrounding pixels, thus eliminating all or most of the effect of the "bad" pixel. The final result is
usually an image with a smoother background and fewer obvious defects. For any defects that remain, PHD2
also provides a way for you to manually click on bad pixel locations and add them to the map. This entire
process of obtaining and analyzing dark frames is handled for you by PHD2, so it's easy to build a bad-pixel
Building a bad-pixel map is done by clicking on the 'Bad Pixel Map...' item under the top-level 'Darks' menu. If
you are doing this for the first time, you will be prompted to obtain a sequence of dark frames for analyzing
your camera sensor and building the map:
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This is a slightly different version of the dialog used for obtaining dark frames, described in the previous
section. Because the analysis is based on statistics, you should use relatively long exposure times (> 10 sec)
and at least 10 frames. Since the bad-pixel map can be re-used for fairly long time periods, you won't have to
repeat this operation very often, and it's worth spending some time to get higher quality data.
Once the dark frames have been captured, PHD2 will compute the statistics and identify an initial set of
defective or suspect pixel locations. After a short delay, you'll then see a dialog that looks something like this:
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The 'General Information' section shows a summary of the statistics computed by PHD2 during the
identification of bad pixel locations. Normally, you won't need to look at these, and you can hide this portion of
the display by clearing the 'Show Master Dark Details' checkbox. The "Results" group shows the counts for hot
and cold pixels based on the current settings of the two "Aggressiveness" sliders below them. If you're doing
this for the first time, the aggressiveness sliders will be set at their default values, 75 within the range of 0 to
100. You'll need to experiment or make some judgment about whether the counts look reasonable based on
what you see on your normal guide frames. If you adjust the aggressiveness sliders left and right, you'll see the
hot and cold pixel counts change. The sliders control how "aggressive" PHD2 should be in identifying suspect
pixels and flagging them as being defective - so higher aggressiveness settings will result in higher pixel
counts. Once the settings are where you want them, click on the 'Generate' button to compute and load the new
defect map.
At this point, you'll probably want to examine the results. The main window of PHD2 is still active, so you can
take a normal guide exposure to see how things look. If you want to quickly see the result of using the defect
map, just toggle the 'Use Bad-pixel Map' menu item under the 'Darks' menu. Keep in mind that you don't need
to achieve a perfectly smooth, black background in the guider image - you just need to have a sufficiently small
number of remaining hot/cold pixels that neither you nor the PHD2 guiding algorithms will mistake a bad pixel
for a star. If you over-correct with very aggressive settings, you may create so many bad pixel areas that they
interfere with detection of usable guide stars. It's easy to make adjustments with the sliders - just change the
slider settings, click on 'Generate' again, and look at the results in the main PHD2 window.
You may find that this statistical approach still leaves a few hot pixels that you'd like to eliminate. You can do
this quickly and easily by manually adding the bad pixels to the map. Just use the main PHD2 window to take a
guide exposure, then click on the hot pixel you want to remove. PHD2 will highlight that area with the green
tracking box, at which time you can click on the 'Add Bad Pixel' button in the dialog box shown above. This
automatically updates the map with the pixel location you just identified and immediately reloads the map, so
you should see the result in the next guide exposure you take. You can continue with this process until you are
satisfied with the results. Remember, you don't need "perfect", just "good enough."
Once your bad-pixel map has been built, you control its use by the 'Use Bad-pixel Map’ item under the 'Darks'
menu. This setting is retained across program executions, so leaving it checked will tell PHD2 to automatically
load the defect map and use it for all guide exposures. The settings for 'Use Dark Library' and 'Use Bad-pixel
Map' are mutually exclusive - you can use one or neither, but not both at the same time. As with the dark
library, the bad-pixel map data file is stored permanently, so you can disable its use without losing any
data. Both of these data structures can be used for extended time periods, but it's worth remembering that
camera sensors do change over time. As a result, you may want to rebuild the dark library or bad-pixel maps at
periodic intervals or when you start to see a degradation in the appearance of your normal guide images. In
these cases, it is also advisable to click on the checkbox for 'Rebuild Master Dark Frame', which will tell PHD2
to reacquire the underlying dark frames and recompute a baseline bad-pixel map. You'll then need to refine the
map as you did before until you're happy with the results. There is no reason you should need to interact with
either the dark library or bad-pixel map data files, but you can find them located in the 'AppData\Local' logical
directory used by your operating system.
Like dark libraries, bad-pixel maps are associated with a particular camera. PHD2 will check to be sure that the
bad-pixel map matches the camera you are currently using. If it does not, you will see an alert message telling
you the bad-pixel map can't be used and must be rebuilt. This can happen when you've changed cameras inside
an existing equipment profile, something you shouldn't do unless you have upgraded your guide camera and
have no plan to revert to use of the old camera.
Reusing Dark Frames and Bad-pixel Maps
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If you're using the same camera in multiple profiles, you may want to re-use the dark libraries or bad-pixel maps
you built for that camera. This can be accomplished by importing the camera-related data files into a profile
that doesn't already have those files. For example, suppose you built an original profile - call it Profile1 - that
uses your Lodestar guide camera, and you built both a dark library and bad-pixel map for it. Sometime later,
you create a new profile, Profile2 that has different mount or focal length properties but still uses the original
Lodestar camera. In that case, you would connect your gear using Profile2, then use the 'Import from Profile...'
menu item under the top-level 'Darks' menu. You would select Profile1 as the source of the import function for
the dark library, bad-pixel map, or both. You will be shown only those profiles that have a camera with
compatible sensor geometry (size and pixel size). Clicking on 'Ok' will copy the dark/bad-pixel map files and
will associated them with your new profile, Profile2? Since they are copies, changing the data files in one
profile will not affect other profiles. Keeping them synchronized, if that is what you want to do, will require a
subsequent 'import' operation.
Visualization Tools
PHD2 provides numerous visualization and display tools to help you see how your guider is performing. All of
these tools are accessed under the 'View' pull-down menu and are described below.
Graphical Display
Stats Display
Star Profile and Target Displays
AO Graph
Dockable Windows
The simplest display tools are grid overlays superimposed over the main guider display window. These are
quite straightforward and include the following choices:
Bullseye target
Fine grid
Coarse grid
RA/Dec - this shows how the telescope axes are aligned relative to the axes of the camera sensor
You can just click on the various overlay options under the 'View' menu and choose one that suits you.
Graphical Display
The graphical display window is one of the more powerful tools for judging guiding performance, and you will
probably learn to rely on it. A typical example is shown below:
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The major portion of the window shows the detailed displacements of the guide star for each guide exposure,
plotted left-to-right. Normally, one line shows displacements in right ascension while the second line shows
declination displacements. However, you can use the 'Settings' button to the left of the graph to switch to
camera (X/Y) axes if you prefer. You can also use the 'Settings' button to switch between display units of arcseconds vs. camera pixels or to change the colors of the two graph lines. The range of the vertical axis is
controlled by the second button from the top, labelled y: +/-4" in this example. The range of the horizontal axis
- the number of guide exposures being plotted - is controlled by the topmost button, labelled x: 100 in this
example. This scale also controls the sample size used for calculating the statistics you see in the lower left part
of the graph window. These values show the root-mean-square (RMS or standard deviation) of the motions in
each axis along with the total for both axes. These are probably your best estimators of guiding performance
because they can be directly compared to star sizes and seeing conditions. The 'RA Osc' value shows the odds
that the current RA move is in the opposite direction as the last RA move. If you are too aggressive in your
guiding and over-shooting the mark each time, this number will head toward 1.0. If you were perfect and not
over- or under-shooting and your mount had no periodic error, the score would be 0.5 Taking periodic error into
account, the ideal value would be closer to 0.3. If this score gets very low (e.g. 0.1), you may want to increase
the RA aggressiveness or decrease the hysteresis. If it gets quite high (e.g. 0.8), you may want adjust
aggressiveness/hysteresis in the opposite direction. There are two other checkboxes to the left that can help you
evaluate guider performance. Clicking on the 'Corrections' box results in an overlay showing when guide
commands are actually sent to the mount, along with their direction and magnitude. In this example, these are
shown as the vertical red and green lines appearing at irregular intervals along the horizontal axis. This shows
you how "busy" the guiding is - under optimal conditions, you should expect to see extended intervals when no
guide commands are sent at all. The other checkbox, labelled 'Trendlines', will superimpose trend lines in both
axes to show if there is a consistent overall drift in the star position. This is primarily useful for drift aligning
where the declination trendline is used extensively. But the RA trendline can show if your mount is tracking
systematically slow or fast (or is seeing the effects of flexure) and can help if you are trying to set up custom
tracking rates. If dithering commands are issued, usually by an external imaging application, a 'dithering' label
will be superimposed on the graph in the appropriate time interval. This tells you that the star displacements
being graphed are being influenced by the dithering operation.
The recommended way to look at guiding performance is to use units of arc-seconds rather than pixels. Doing
this allows an equipment-independent way of evaluating performance because it transcends questions of focal
length and image scale. To do this, you need to provide PHD2 with sufficient information to determine your
guider image scale: namely, the focal length of the guide scope and the size of the guide camera pixels. These
parameters are set in the 'Brain' dialog, on the 'Global' and 'Camera' tabs, respectively. If they are not specified,
PHD2 will use default values of 1.0, and the guiding performance numbers will effectively be reported in units
of pixels.
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At the bottom of the graph window are active controls for adjusting guiding parameters "on the fly". The
guiding algorithm selections you've made will control which controls are shown. These controls have the same
effect as those in the 'Brain' dialog, and they eliminate the need to stop guiding and navigate to another window
to adjust guiding parameters.
If you want to monitor guiding performance without necessarily having the graph window open, you can click
on the "Stats" menu item. That will display the salient statistics with controls for clearing the data or changing
the number of guide exposures used to compute the statistics.
Star Profile and Target Displays
The star profile display shows the cross-section of the guide star along with a measurement of its full-widthhalf-maximum (FWHM). If you see substantial fluctuations in this parameter, it may be an indication that the
star is too faint or the exposure time is too short.
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The target display is another useful way to visualize overall guider performance. The red 'X' shows the star
displacement for the most recent guide exposure, while the blue dots show the recent history. You can zoom in
or out with the controls at the upper left of the window, as well as change the number of points shown in the
Adaptive Optics (AO) Graph
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The AO graph is equivalent to the 'target' display, but shows the history of corrections relative to the axes of the
adaptive optics device. The red rectangle indicates the outer edges of the AO device, while the interior yellow
rectangle shows the "bump" region. If the star moves outside the yellow rectangle, PHD2 will send a sequence
of move commands to the mount - the "bump" - to smoothly place the guide star back near the center
position. When this occurs, green and blue lines will show the incremental bump and the remaining bump
respectively. The white dot on the display shows the current AO position, and the green circle (red when a
bump is in progress) shows the averaged AO position. The button in the upper left controls how many points
will be plotted in the history.
Dockable/Moveable Graphical Windows
When the various performance windows are initially displayed, they are "docked" in the main window. This
means they are sized in a particular way and are aligned with two edges of the window - they are entirely
contained within the bounds of the main PHD2 window. However, you can move them around and resize them
by clicking and dragging on the title bar of the window you want to examine. This will often let you get a better
view of the details being shown in the graphs. They can be re-docked by dragging the title bar to the general
region in which you want them docked - bottom, right, etc. With just a bit of practice, it's easy to place them
where they are most convenient.
There is also a menu item under the 'View" pulldown menu labeled 'Restore window positions.' Clicking on
this menu item will automatically restore all of the dock able/moveable windows to their default, docked
positions. This can be useful, for example, if you are switching between screens with different resolutions and
one or more of the dockable windows has been "lost." This function also restores the main PHD2 window to its
default size, with a position near the upper lefthand corner of the screen.
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Advanced Settings
Advanced settings are accessed by clicking on the 'Brain button', a feature well-known to users of the original
PHD. PHD2 has a considerably larger set of parameters that can be adjusted to optimize your guiding
performance. Although these are called "advanced" settings, they are not particularly difficult to understand,
and you shouldn't hesitate to explore them. All of the fields on these forms include "tool tips", small message
windows that describe each field in some detail. Simply "hover" the cursor over the field to see the tool-tip. In
many cases, this will provide all the information you need. Because there are many more settings available, the
Advanced Dialog in PHD2 is organized into tabs, and the documentation will follow that model as well.
Global Tab
Guiding Tab
Camera Tab
Mount Tab
AO Tab
Rotator Tab
Global Tab
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Most of the fields on the global tab deal with overall program functions and behavior. Several of them deal
with logging and debugging functions, which will be discussed first.
General Parameters
The remaining controls on the 'Global' tab are well-described by their respective tool-tips, but they are
summarized here for completeness:
'Reset Configuration' - restores all settings to their initial values as if PHD2 had been freshly installed
'Image format' - specifies the file format if star-image logging is enabled
'Dither RA only' - for companion apps that use the PHD2 server interface, specifies that dithering should
be done only on the RA axis
'Dither scale' - controls the magnitude of the dithering operation
'Noise reduction' - specifies the algorithm to use for handling noisy guide camera images - those for
which dark frames are not sufficient. Choices include None, 2x2 mean, and 3x3 median. Both 2x2 mean
and 3x3 median will reduce the noise considerably. 3x3 median is especially effective at removing hot
pixels and neither will significantly affect guiding accuracy. However, creating a bad-pixel map is
likely to be a better solution with less impact on your ability to detect faint stars.
'Time lapse' - imposes a fixed delay between guide exposures. This can be useful if the guide exposures
are very short and you don't want to overload either the mount or the camera link with very high traffic
'Focal length' - the focal length of the guide scope (millimeters). This provides one of two parameters
needed by PHD2 to compute the image scale and thus report guiding performance in units of arcseconds. The other parameter required for this is the guide camera pixel size, located on the 'Camera'
'Language' - determines the language used in the PHD2 user interface, subject to available
localization. Changing this will usually require a program restart
'Auto restore calibration' - tells PHD2 to automatically reload the most recent calibration data as soon as
the equipment is connected. Be sure to read the Auto calibration section to understand the implications
and potential risks of doing this.
'Auto Exposure' - these are the settings that control Auto exposure time.
o Min Exposure - the minimum exposure time. PHD2 will not set the exposure time less than this
value, even if the guide star SNR is higher than the target SNR value. If the min exposure time is
set too low, you could end up sending excessive small corrections to the mount which may not
give the best guiding results. Users with AO units would want to set this to a lower value, since
rapid small corrections are often desirable with an AO.
o Max Exposure - the maximum exposure time. Before a guide star is selected, PHD2 will set the
exposure time to the maximum value. Once a guide star is selected, PHD2 will then
incrementally decrease the exposure time until the desired SNR is reached.
o Target SNR - this is the average SNR value that PHD2 will attempt to achieve by adjusting the
exposure time. SNR can fluctuate from frame to frame even with a fixed exposure duration, so
be sure to account for that when choosing a target SNR value. PHD2 will reject frames when
SNR drops below 3.0. The default value of 6.0 should provide enough of a cushion to prevent
fluctuations from causing the SNR to go below 3.0.
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Guiding Tab
There are relatively few parameters found on the 'Guiding' tab. Their use is as follows:
'Always Scale Images' - tells PHD2 to always rescale the guider image to fit the display window. PHD2
does this automatically in almost all cases, so you'll rarely need to do this.
'Fast re-center after calibration or dither' - during calibration or dithering, the mount may be moved a
significant distance from the initial "lock" position. If you click this checkbox, PHD2 will move the
mount back to the lock position as quickly as possible, using the largest guide commands permitted by
the 'Max Duration' settings of your guide algorithms. This is only an optimization, so the use of this
checkbox is completely optional.
'Search region' - specifies the size of the "tracking rectangle", in units of pixels. You may need to
increase this value if your mount does not perform well or, more commonly, if it's not well-aligned on
the celestial pole.
'Star mass change detection' - tells PHD2 to monitor the brightness and size of the guide star compared
to the sky background.
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'Star mass tolerance' - if 'star mass change detection' is checked, PHD2 will trigger a 'lost star' error if
the measured brightness and size vary by more than this percentage. This might be useful if you have
two stars inside the tracking rectangle and you want to be sure that PHD2 doesn't mistakenly switch
stars. It can also prevent errors caused by thin clouds, high camera noise, or alpha particle "hits"; but it
may be unreliable if you are guiding on a faint star. If you are getting too many 'lost star' errors when
the star is plainly visible on the display, try increasing the value of this setting. Setting the value to 100
effectively disables the warnings entirely.
Camera Tab
The controls on the 'Camera' tab are used as follows:
'Use subframes' - For cameras that support this feature, PHD2 will download only a subframe of each
guide exposure. This is very useful for cameras with slow download times and can mean the difference
between the camera being too slow to effectively guide and being quite responsive. When this feature is
enabled, only a small subframe (100x100 pixels) will be downloaded once a star is selected. This feature
applies to both calibration and guiding. During initial looping without a selected star, the full frame is
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downloaded, but once a star is selected, only this small subframe is downloaded. If you are using
subframes but want to see the full frame to select a different star, just click anywhere outside the
'Pixel size' - The guide camera pixel size in microns. This is the second of two parameters needed by
PHD2 to compute the guider image scale and thus report guider statistics in units of arc-seconds. Refer
to your camera documentation to determine the correct value.
'Camera gain' - Sets the gain level for the many cameras that support this feature. If you really want to
use a bright star and a longer exposure duration or if your camera makes a very noisy image, try
reducing this parameter.
'Disconnect nonresponsive camera after (seconds) - Camera malfunctions will sometimes occur, often
because of faulty USB connections. In many cases, the camera will not return the requested image data,
and PHD2 will appear to "hang." This parameter determines how long PHD2 should wait for a response
after the expected exposure time has expired. For example, a timeout value of 5 seconds in conjunction
with an exposure time of 2 seconds will tell PHD2 to wait up to 7 seconds for a response. If the data are
not received within that period, PHD2 will attempt to halt the operation, disconnect the camera, and
display an alert message in the main window. Since a hardware problem is likely the underlying issue,
this recovery attempt won't always succeed. It is advisable to be generous with these timeout values to
avoid spurious recovery actions. Also, if you are using a guide camera that shares electronics with the
main imaging camera, you should set this timeout to a large value, well above the maximum expected
time for a full-frame download from the main imager. At this time, this affects users of the SBIG driver
that is packaged with Sequence Generator Pro. Regardless of whether PHD2 is able to handle the
situation gracefully, the underlying problem is almost certainly in the hardware or the camera driver and
will need to be resolved before guiding is continued.
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Mount Tab
The mount tab is the most complex area of Advanced Settings largely because it has so many parameters and UI
controls. Most of these settings are closely tied to the various guiding algorithms, and the contents of the dialog
will change significantly if you change the algorithm selections. For that reason, all the parameters related to
guide algorithms will be treated together, in a separate section.
The remaining controls, the ones that are independent of the guiding algorithm selections, are described below.
'Clear calibration' - tells PHD2 you want to clear the calibration data currently being used and recalibrate before guiding is restarted. You might do this for a variety of reasons - you might have moved
the guide camera, you might have done a meridian flip and want to recalibrate on the "new" side of the
pier, etc. You can also accomplish the same result by doing a Shift-Click on the guiding icon on the
main page, which will force a re-calibration.
'Enable guide output' - this is normally checked because it tells PHD2 to send guide commands to the
mount. But there are some circumstances where you might want to disable this, usually because you
want to observe the uncorrected behavior of the mount. For example, you can disable guider output in
order to see the general shape and amplitude of your mount's periodic error or to check the amount of
drift from polar mis-alignment.
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'Calibration step-size' - specifes the duration of the guide pulse that PHD2 will use during
calibration. Its use is described in the 'Auto Calibration' section of the 'Basic Use' help page. You can
adjust the value depending on whether the guide star is moving too quickly or too slowly during
calibration. As a general guideline, it is good to calibrate within about 30 degrees of the celestial
equator (declination = 0), and to use a calibration step size that will result in 8-14 steps in each
direction. The 'calculate...' button to the right of this control will launch a dialog that can help you
compute an appropriate value:
To use the calculator, be sure the topmost three edit controls are correctly filled in. If you have already
specified the focal length and the camera pixel size in the 'Global' and 'Camera' tabs respectively, those fields
will already be populated in this form. If you are using an ASCOM connection to your mount, the fields for
"Guide speed" and "Calibration declination" will also have the correct values. Otherwise, you'll need to supply
them yourself. The guide speed is specified as a multiple of sidereal speed - most mounts will use something
like 1X or 0.5X sidereal, but you can choose something else. You can leave the 'calibration steps' field at the
default value of 12, which is likely to result in a good calibration. Use of a significantly smaller value raises the
likelihood that seeing errors or small mount errors will cause calibration errors. As you change the values in
these fields, PHD2 will recalculate your current image scale and a recommended value for the calibration stepsize. If you then click on 'Ok', that value will be inserted into the calibration step-size field of the 'Mount'
dialog. Clicking 'Ok' will also populate the focal length and camera pixel size fields in the 'Global' and 'Camera'
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tabs, so any changes you made in the calculator will be reflected there as well. However, this will not be done
if you click on 'Cancel' in the calculator dialog.
'Max Duration RA' - specifies the maximum allowed guide pulse duration for right ascension. You
might reduce this below the default value if you want to avoid "chasing" a large deflection that could be
caused by a spurious event (e.g. a sudden 'hot pixel').’
'Max Duration Declination' - specifies the maximum allowed guide pulse duration for declination (same
as above but for declination). This can be useful to avoid over-correction and subsequent direction
reversal in declination guiding.
'Reverse Dec output after meridian flip' - tells PHD2 how to adjust the calibration data when you use the
function to "flip" it. Some mounts track their 'side of pier' state and automatically reverse the direction
of guide commands in declination. Others, perhaps most mounts, do not do this. In either case, PHD2
needs to know if the mount will automatically change its behavior based on side-of-pier. You may have
difficulty finding information about how your mount behaves in this respect, so it's probably easiest to
just run a quick experiment. With the checkbox disabled, calibrate on one side of the pier, then move
the mount to the other side. Select ‘Flip Calibration' under the 'Tools' menu, and start guiding. If the
guiding works normally, leave the box un-checked; but if you see run-away in declination, check the
box and repeat the experiment.
'Assume Dec orthogonal to RA' - Normally, the calibration process independently computes the camera
angles for both right ascension and declination. There is no need for great precision on these values, and
the default behavior normally works well. However, if your mount has very high periodic error or you
are dealing with very bad seeing conditions, you may want to force the RA and Declination angles to be
perpendicular. If you choose that option, PHD2 will compute the camera angle for RA, then assert a
declination angle that is orthogonal to it.
All of the other controls in this dialog are related to the guiding algorithms you have chosen and are described
here: Guiding Algorithms and Settings
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AO Tab
Most of the edit controls at the top of the AO tab have the same meaning as their counterparts on the 'Mount'
tab. The guiding algorithms apply to the control of the tip/tilt optical element in the AO device itself, not to the
"bump" commands sent to the mount. Since the AO device is not trying to move a heavy piece of equipment,
you can afford to be more aggressive in your guide algorithm parameters. If you use a hysteresis-based
algorithm, for example, you should probably start with a high level of aggressiveness, perhaps 100%. Or you
can choose the 'None' algorithm, which means there will be no damping or history-based algorithm applied at
all. In that case, each correction will be based only on the most recent guide frame and will make a 100%
correction of the most recent deflection.
You can use the four parameters at the bottom of the AO tab to control the calibration process and the manner
in which 'bump' operations are done. The 'calibration step' field tells PHD2 the amount to move the tip/tilt
element in each of the up/down/left/right directions, in units of AO steps, during calibration. The guide star
position is measured at the beginning and end of each leg of the calibration, and the 'samples to average'
parameter tells PHD2 how many samples to take at each of these points. Averaging images is important
because the seeing will always cause the guide star to "bounce around" a bit. As discussed earlier, the AO unit
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can make corrections only within a limited range of guide star movement. You will want to initiate mount
'bump' corrections before these limits are actually reached, and the 'bump percentage' field is used for that
purpose. To move the mount, the full bump correction is accomplished in steps - the 'bump step' field controls
the size of these increments. If the bump operation has begun and the guide star remains outside the "bump
percentage" area, PHD2 will increase the bump size until the guide star is back within that range. Additional
movement from that point to the center position will continue at the specified "bump step size". This
complexity is required in order to maintain good guiding, with no elongated stars, even as the mount is being
bumped. During the bump operation, the AO is continuing to make corrections, so the long "mount bump" is
continuously offset by adjustments in the AO.
Rotator Tab
Currently, the rotator tab has only one control, and this window is likely to change in the future. The
"Reversed" checkbox can be used for optical systems that reverse the image because they have an odd number
of mirrors. The direction and amount of rotation is used to adjust the calibration data, so PHD2 follows the
ASCOM standard: "the rotator position is expressed as an angle from 0 up to but not including 360 degrees,
counter-clockwise against the sky."
Guide Algorithms
Guiding Theory
Guide Algorithm Parameters
Guiding Theory
The default guiding algorithms in PHD2 are well-established and should work well for most users. Unless you
already have some experience with guiding and understand the basics, you should probably be cautious about
changing algorithms. However, you may have some special circumstances that require changes or you may
simply want to experiment with the different algorithm choices. The Advanced Dialog settings in PHD2 make
it easy to do that. Each algorithm has a set of parameters that controls how observed changes in guide star
position (star deflections) are translated into guiding commands that are most likely to restore the star to its
initial position.
Before discussing the details of these parameters, it is worth reviewing a little guiding theory and looking at
what these algorithms are trying to accomplish. Setting aside adaptive optics devices, which are entirely
different, conventional guiding faces enormous challenges. The problem at hand is how to move machinery
that weighs tens or even hundreds of pounds with a level of precision that will not cause streaked or oblong
stars. This type of guiding can only hope to deal with tracking errors that are "slow and steady", not "fast and
random." Sources of slow and steady (correctable) errors include the following:
Certain kinds of mechanical imperfections in right ascension gears, including those that cause periodic
Small errors in the sidereal tracking rate of the mount
Atmospheric refraction - stars appear to move more slowly as they near the horizon
Limited kinds of mechanical deflection and flexure - but not differential flexure
Mis-alignment of the right ascension axis on the celestial pole
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So what isn't included in the above and isn't correctable by conventional guiding? Unfortunately, it's a very
long list, of which a few are:
Atmospheric seeing ("turbulence")
Gear noise, roughness, and vibration
Differential flexure - relative movement between the imaging scope and the guide scope
Wind gusts, cable snags, grit in the drive gears
And lots more...
The common denominator shared by the guide algorithms is the need to somehow react to the slow and steady
deflections while ignoring the rest. This is a difficult problem at best because any given guide star deflection is
likely to have contributions from many of these sources. And if that isn't hard enough, remember that realworld mounts are never perfect - so the move you ask for will not be exactly the move you get. Usually, the
most important requirement for any algorithm is to avoid over-correcting, wherein the mount is being pushed
back and forth and the guiding never stabilizes. A typical approach in these algorithms is to apply "inertia" or
"impedance" to the guiding corrections. That means making corrections that follow a pattern and are generally
consistent with corrections that have been made before, while being reluctant to make corrections that require a
big change in direction or amplitude. Resistance to changes in direction is particularly important in declination,
where gear backlash is a common problem. Hopefully, this background will give you enough insight into the
basics of guiding so that the various guiding parameters used in PHD2 will make sense.
Guide Algorithm Parameters
In PHD2, the various guide algorithms can be applied to either the right ascension or declination axes. Most of
these algorithms include a minimum move parameter. This is used to avoid making guide corrections that are
overly small, are unlikely to have any effect on star shape, and are mostly due to transient seeing effects. These
values are entered in units of pixels, so you need to think about them in the context of how large your star
images are. The default values work well for short-to-medium focal length systems, but you may need to
increase them if you are working at long focal lengths and expect stars to have larger diameters.
The hysteresis algorithms keep a history of the guiding corrections that have been made in the recent past, and
these are used to help compute the next guide correction. The hysteresis parameter, expressed as a percentage,
specifies the "weight" that should be given to this history as opposed to looking only at the star deflection in the
current guide frame. Consider an example where the hysteresis parameter is 10%. In that case, the next guiding
correction will be 90% influenced by the star movement seen in the current guide frame and 10% by the
corrections that have been made in the recent past. Increasing the hysteresis value will smooth out the
corrections at the risk of being too slow to react to a legitimate change in direction. The hysteresis algorithms
also include an aggressiveness parameter, again expressed as a percentage that is used to reduce overcorrecting. On each frame, PHD2 computes how far it thinks the mount should move and in what direction(s)
it should move. The aggressiveness parameter scales this. For example, take a case where the star deflection has
been evaluated and a corrective move of 0.5 pixels is warranted. If the aggressiveness is set to 100%, a guider
command will be issued to move the mount the full 0.5 pixels. But if the aggressiveness is set to 60%, the
mount will be asked to move only 60% of that amount, or 0.3 pixels. If you find your mount is always
overshooting the star, decrease this value slightly (say, by 10% steps). If you find PHD2 always seems to be
lagging behind the star's motion, increase this by a little bit. A little can go a long way here.
The ResistSwitch algorithm behaves much as its name implies. Like the hysteresis algorithms, it also
maintains a history of past guide corrections, and any change of direction must be "compelling" in order to issue
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a reversing guide command. This is appropriate for declination guiding, where reversals in direction are
both suspect and likely to trigger backlash in the gears. For that reason, ResistSwitch is the default algorithm
for declination but not for right ascension, where valid direction reversals are expected. Starting with Release
2.4.1, two additional parameters are available for fine-tuning the ResistSwitch algorithm. The first is
"aggression", a percentage amount that controls how much of the computed guide correction will be
issued. Reducing this parameter can help to avoid over-shooting with mounts that have little or no backlash.
The second parameter is a checkbox labeled "Fast switch for large deflections." If this is checked, PHD2 will
react immediately to a large change of direction rather than waiting for three consecutive deflections in the new
direction, which is the normal behavior. This can help to more quickly recover from large excursions in Dec,
perhaps caused by wind, cable snags, or other mechanical shifts. The definition of a "large deflection" is 3x the
minimum-move value. So if PHD2 is over-reacting to direction changes, you can tune the behavior with the
min-move parameter or disable the "fast switch" option altogether. It is worth remembering that "less is usually
better" when it comes to Dec guiding, so don't try to over-tune these parameters.
The LowPass algorithms also employ a history of recent guiding corrections in order to compute the next
correction. The starting point for the computed move is the median value of the guide star deflections that have
occurred in recent history. This means that the star deflection seen in the current guide frame has relatively
little impact on calculating the next move and the algorithm is very resistant to quick changes. But the history
accumulation also includes a calculation to determine if deflections are moving in a consistent direction or
"getting worse." The slope weight parameter, expressed as a percentage, determines how much influence this
should have in calculating the actual guider movement - it is there to keep the algorithm from being overly
sluggish. Because the low-pass algorithm is so resistant to quick changes, it is probably most applicable to
declination guiding.
The LowPass2 algorithm is a variation of the original LowPass algorithm with somewhat different behavior. It
also maintains a history of guiding corrections, but the next correction is simply a linear extension of the
commands that have come before it (i.e. a slope calculation). This continues until a significant change in
direction is seen, at which point the history is cleared. The algorithm has two adjustable properties: minimummove and aggressiveness. Minimum-move has the same effect as it does in the other guide algorithms, and
aggressiveness (percentage) is a way of further dampening the size of the guide corrections. LowPass2 is a very
conservative, high-impedance algorithm that may be a good choice for users with good seeing conditions and
well-behaved mounts.
The declination guide mode gives you additional control for declination guiding. Declination guiding is not like
RA guiding because the errors are not caused by imperfections in your mount's gears. Instead, deflections in
declination are primarily the result of imperfect polar alignment. The result is an error that should be smooth
and should be in only one direction, assuming there is no over-shoot from an earlier correction. The default
value of 'auto' tells PHD2 that some reversals in direction are acceptable, subject to the behavior of the various
guiding algorithms. However, if your mount has severe declination backlash, you may want to
prevent direction reversal altogether. If so, you can select either 'north' or 'south' to restrict corrections to one
direction only. Keep in mind, however, that an over-shoot in correction with one of these modes will leave the
star positioned off-target for an extended period of time. So you'll probably want to use conservative parameters
for aggressiveness if you are disallowing direction reversals. Finally, a choice of 'none' here disables
declination guiding altogether.
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Tools and Utilities
Manual Guide
Auto-Select Star
Calibration Details
PHD2 Server
Logging and Debug Output
Drift Alignment
Lock Positions
Comet Tracking
Guiding Assistant
Equipment Profiles
Simulator Parameters
Keyboard Shortcuts
Manual Guide
If you are connecting to a new mount and are encountering calibration problems, you will probably want to be
sure that PHD2's commands are actually getting to the mount. Or you may want to nudge the mount or
experiment with manual dithering. In the 'Tools' menu, click on 'Manual Guide' and a dialog will appear to let
you move the mount at guide speed in any direction. Each time you press the button, a pulse of the duration
specified in the 'Guide Pulse Duration' field will be sent. The default value is the 'calibration step-size' set in the
Advanced Options dialog. If you are debugging calibration problems, listen to (rather than watch) your mount
to determine if the mount is getting the commands from PHD2. The idea here is just to figure out if the mount is
responding to PHD2's signals. You won't be able to see the mount move (it's moving at guide speed) but you
may be able to hear it. Other options include watching the motors themselves or attaching a laser pointer to your
scope and aiming it at something fairly far away (to amplify your motions). If you have an adaptive optics
device attached, you'll see separate move buttons for both the AO and the secondary mount.
Dithering is used primarily with image capture or automation applications, usually through the PHD2 server
interface. However, you can do manual dithering or experiment with dither settings using the controls at the
bottom of the dialog. The 'dither' amount field at the left controls the amount the mount will be moved, in units
of pixels. You can scale this amount - i.e. multiply it by a constant - by using the 'scale' spin control to the
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right. These two controls establish a maximum amount of movement that will be used for dithering - the
product of 'scale' X 'dither'. When you click on the 'Dither' button, PHD2 will move the mount by a random
amount that is less than or equal to the limit you have set, in one of the north/south/east/west directions. The
'RA Only' checkbox will constrain the dither adjustments to only east or west. Obviously, if you are doing a
manual dither in this way, you'll want to be sure your imaging camera is not in the middle of an exposure.
Auto-Select Star
Clicking on 'Auto-select Star' under the 'Tools' menu, or using the keyboard shortcut of <Alt>S, tells PHD2 to
scan the current guide image and identify a star suitable for guiding. PHD2 will try to select a star of sufficient
brightness that is not near another star and not too close to the edge of the frame. The selected star may appear
overly dim on the screen, but this is usually not a problem. You can use the Star Profile tool to examine the
properties of the selected star - it should be non-saturated with a sharp profile, the same properties you should
look for when choosing a guide star manually. The auto-select process is not infallible, so you may need to
select a star yourself if you don't like the automated result. If you want to use Auto-Select, you should
definitely use either a bad-pixel map or dark library to reduce the likelihood of PHD2 mistakenly choosing a hot
Calibration Details
Calibration data is saved automatically each time a calibration sequence completes successfully. The use of the
calibration data has been described elsewhere (Using PHD Guiding), including options for restoring calibration
data from an earlier time or "flipping" it after a meridian flip. You access these functions using the 'Calibration'
sub-menu under the 'Tools' menu. Two other calibration-related items are shown there, namely the options to
clear the current data or to enter calibration data manually. The "clear" option accomplishes the same thing as
the 'Clear calibration' checkbox in the Advanced Dialog - it will force a recalibration whenever guiding is
resumed. The 'Enter calibration data' option should be used only under very unusual circumstances and only if
you're sure you know what you're doing; but it is available as a matter of completeness. If you click on the
'Enter calibration data' item, you'll see a dialog box that allows input of relatively low-level calibration
data. This data might come from a much earlier session, perhaps extracted from the PHD2 guiding log file.
Most of the calibration-related windows, including calibration sanity-checks, will open a window that looks
something like this:
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The first thing to look at is the graph to the left, which shows what star movements resulted from the guide
pulses that PHD2 sent during calibration. The lines represent the RA and Dec guide rates that were computed
as a result of the calibration, and these lines should be roughly perpendicular. The data points will never be
perfectly aligned, but they should not have major curves, sharp inflections, or reversals in direction. Particularly
with longer focal length scopes, the points will often show considerable scatter around the lines, but this is
normal. The solid points (west and north pulses) are used to compute the RA and declination rates, while the
hollow points show the "return" paths of the east and south moves. These can help you see how much
fluctuation occurred due to seeing and also whether there is a significant amount of backlash. If you are using
the "fast-recenter" option in the Advanced Settings, there may be many fewer points shown in the east and north
paths. The tabular information to the right shows what was known about the pointing position of the scope and
the various ASCOM settings that relate to guiding. If you are not using an ASCOM mount and don't have an
"Aux mount" specified, some of this information will be missing. The table will also show the expected guiding
rates for a "perfect" calibration using the same sky position and guide speed settings you used. You will almost
never achieve these ideal values, and you shouldn't worry about them unless your values are very different. If
you didn't see an alert message when the calibration completed, your results are probably good enough. If you
want to re-use a calibration for an extended time, it is probably worth a few extra minutes to check this
information and confirm that the calibration went reasonably well and produced sensible results. Bad
calibrations can occur even for very experienced imagers using high-end mounts, so it is good to check.
PHD2 Server
PHD2 supports third-party imaging and automation applications that need to control the guiding process. Stark
Labs' Nebulosity program was the first to do this, but other applications have subsequently been produced. By
using the PHD2 server process, image capture programs can control dithering between exposures or suspend
guide exposures while the primary imaging camera is downloading data. To use these capabilities with a
compatible application, you should click on the 'Enable Server' option under the 'Tools' menu. The server
interface has been reworked substantially in PHD2, and it's now possible for an application to control most
aspects of PHD2's guiding operations. Documentation for the server API is available on the PHD2 Wiki.
Logging and Debug Output
PHD2 can optionally produce two types of log files: a debug log and a guiding log. Both are highly useful for
different reasons. The guiding log is similar to the one produced by PHD, but with extended information. The
guide log is intentionally formatted to allow easy interpretation by either a human reader or an external
application. For example, the very capable PHDLogView application (not part of the PHD2 release) can
produce a variety of graphs and summary statistics based on data in the PHD2 guide log. But the log can also
be easily imported into Excel or other applications for analysis and graphing. When importing into Excel, just
specify that a comma should be used as a column separator. The debug log has a complete record of everything
that was done in the PHD2 session, so it is very helpful in isolating any problems you have. It also employs a
human-friendly (albeit verbose) text format, so it's not difficult to examine the debug log to see what
happened. If you need to report a problem with the software, you will almost certainly be asked to provide the
debug log file. If you have neither log file available, you are unlikely to get any help. Creation of these log
files is controlled by the two menu items under the 'Tools' menu, and these should be left "checked" as a normal
The location for the files is controlled by the 'Log File Location' field in the 'Global' tab of the 'Advanced
Settings' dialog. By default, log files are stored in the OS-specific default directory for application data files. In
Windows7, for example, the files will be stored in a 'PHD2' sub-folder in the "AppData\Local" location. This
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may not be a convenient location, so you can specify a different folder using this edit field.
In some unusual cases, you may need to capture guide camera images, usually to support debugging and
problem resolution. This can be done by clicking the 'Enable Star Imaging Logging' menu item under the
'Tools' menu. The resultant image files will be stored in the same location as the other log files. The format of
these image files is controlled from the 'Global' tab of the 'Advanced Settings' dialog. If you are trying to
document a problem you're having, you should choose the 'Raw Fits' format for maximum flexibility.
Drift Align
Drift alignment is a well-known technique for achieving polar alignment and is considered by many to be the
"gold standard". The Drift Alignment tool is a wizard-like sequence of dialogs that can help you work through
the drift alignment process and get quantifiable results. Once you've calibrated your guider, click on 'Drift
Align' under the 'Tools' menu. The first Drift Align dialog will appear to help you adjust the azimuth on your
mount. If you are using an ASCOM mount, you'll have the option of slewing to an area near the celestial
equator and the celestial meridian. If you're not using an ASCOM mount, you'll need to slew to that location
manually. Once the scope is positioned and you have a suitable star in the field of view, click on the 'Drift'
button to begin collecting data. You'll see the graph window with a display of star deflections and corrections
and, more importantly, two trendlines. When the mount is precisely polar aligned in azimuth, the Declination
trend line will be perfectly horizontal. Let the exposures continue until the declination trendline has stabilized
and is no longer jumping around with each new exposure. At the bottom of the graph window, you'll see a
measurement for the polar alignment error in azimuth. And, in the image window, you will see a magenta circle
around the guide star. The circle indicates an upper limit on how far the guide star needs to move when azimuth
is adjusted. (Initially, the circle may be too large to be visible on the screen, so you may not see it until your
alignment gets closer.)
Now click on the 'Adjust' button to halt guiding, then make a mechanical adjustment in azimuth. Watch the
guide star as you make the adjustment, moving the guide star towards the magenta circle, but not beyond
it. Once done, click on the 'drift' button again to repeat the measurement. If your adjustment was in the right
direction and did not over-shoot, the Declination trendline will be closer to horizontal. Continue iterating in this
way until you are satisfied with your azimuth accuracy. You can use the 'notes' field to record which way the
drift line moves depending on how you make the adjustment. For example, you might note that a counterclockwise turn of the mount azimuth knob moves the drift line "up." Since these notes are retained across
PHD2 sessions, subsequent drift alignments will probably proceed more quickly.
Until you are experienced with drift aligning your particular mount, the 'adjustment' part of the process can be a
bit tedious. At first, you'll have to determine how to adjust a knob on the mount to achieve the desired effect:
"how much" and "what direction." To help with this, the PHD2 drift align tool supports "bookmarks". These
are a handy way to record the positions of the guide star before and after you've made an
adjustment. Bookmarks are accessed using the Bookmarks menu, or keyboard shortcuts, as follows:
b : toggle/show bookmarks
Shift-b : set a bookmark at the current guide star position (the "lock position")
Ctrl-b : clear all bookmarks
Ctrl-click somewhere on the image: set a bookmark at that position, or remove the bookmark that's
already there
By setting a bookmark before you make a mount adjustment, you can get a clear view of how the adjustment
has moved the star on the guide frame.
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Next, click on the 'Altitude' button. Then slew the scope to a position near the celestial equator and 25-30
degrees above the horizon. Click on the 'drift' button to begin collecting data for the altitude part of the
alignment process. As before, you will iterate between making adjustments and measuring your alignment until
you are satisfied with the result, keeping notes as you go about how mount adjustments affect the behavior of
the declination drift line. If you make substantial adjustments in altitude, you'll need to go back to the 'azimuth'
measurement and repeat that procedure. If you work through these procedures systematically, you'll converge
on a good polar alignment with a known degree of accuracy. A good polar alignment will help your guiding
performance, especially in declination.
The drift alignment tool is easiest to use when you are using an ASCOM connection to your mount (including
an 'Aux' connection). Even if you subsequently want to use ST-4 style guiding, you should use the ASCOM
connection for drift alignment. If you can't do that for some reason, the following features will be impaired:
Scope position data and slewing functions will not be available - you'll have to slew the scope yourself
The magenta circle that identifies the target for moving the star will be inaccurate and will be displayed
as a dashed line. This dashed circle will identify only an upper bound to the adjustment, so you will
probably want to make smaller adjustments to avoid over-shooting.
A very complete step-by-step tutorial for drift alignment is available on the Openphdguiding web site, and firsttime users are strongly encouraged to study it. (
Lock Positions
PHD2 normally sets a 'lock position' where the guide star is located at the end of calibration. Depending on the
details of the calibration sequence, this may not be exactly where the star was located at the start of calibration it could be off by a few pixels. If you are trying to precisely center your target, you may want to use a 'sticky
lock position.' You do this by clicking on your guide star before calibration, then setting the 'Sticky Lock
Position' under the 'Tools' menu. After calibration is complete, PHD2 will continue to move the mount until the
star is located at the sticky lock position. So you may see an additional delay after the calibration while PHD2
repositions the scope at guide speed. The sticky lock position will continue to be used even as guiding is
stopped and subsequently resumed. Again, this insures a rigorous positioning of the guide star (and presumably
your image target) at the expense of delays needed for PHD2 to reposition the mount.
Comet Tracking
One way to image a comet is to have PHD2 use the comet as the guide "star", but this approach may not always
work. For example, the head of the comet may not present a star-like center suitable for guiding. Or, when using
an off-axis guider, the comet may not even be visible in the guide camera.
PHD2 provides a Comet Tracking tool for use when guiding on the comet itself is not feasible. The idea is to
guide on an ordinary star, but to gradually shift the lock position to match the comet's motion, or tracking rate.
There are a three different ways to provide the comet tracking rate to PHD2.
Some planetarium applications, like Cartes du Ciel, can send the rate directly to PHD2;
You can enter the tracking rate manually, or,
You can train the rate in PHD2 by following the comet for a period of time in the imaging camera.
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To enter the rate manually, you would select "Arcsec/hr" for units and "RA/Dec" for axes, then enter the rates
from the comet's ephemeris.
Comet rate training works like this:
First, center the comet in your imaging camera. If your imaging application has some kind of reticle display,
you should use that to note the precise position of the comet on the imaging sensor. Once this is ready, select a
guide star in PHD2 and start guiding. Next click "Start" in the Comet Tracking tool to begin training.
Take a continuous series of short exposures in your imaging camera using your imaging application's Frame
and Focus feature. Over time, the comet will drift away from the starting location. Use PHD2's "Adjust Lock
Position" controls to move the comet back to the starting location. You may have to experiment a bit to
determine which way the comet moves on the imaging camera sensor in response to the Up/Down/Left/Right
controls in PHD2. You may find it useful to enable the "Always on top" button in the Adjust Lock Position
window so the controls stay visible on top of your imaging application.
PHD2 will quickly learn the comet tracking rate as you re-center the comet. Once you are satisfied that PHD2 is
tracking the comet, you can click Stop to end the training. PHD2 will continue shifting the lock position to track
the comet until you disable comet tracking by toggling the Enable/Disable button.
You can practice the comet training technique using the built-in camera simulator. Check the "Comet" option in
the Cam Dialog, and the simulator will display a comet. Use a bookmark to mark the comet's starting location,
and use the Adjust Lock Position controls to move the comet back to the bookmark location.
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Guiding Assistant
The Guiding Assistant is an instructional tool to help you measure current seeing conditions and the general
behavior of your guiding subsystem. When it is run, it temporarily disables guiding output and measures the
ensuing motion of the guide star. This can help you see the high-frequency motions caused by seeing
(atmospheric) conditions. These cannot usually be corrected by conventional guiding because they occur at a
much higher frequency than you can typically even measure. Trying to correct for them with conventional
guiding is often called "chasing the seeing" and is not recommended. Avoiding it is best accomplished by
setting a minimum-move level that will cause PHD2 to ignore most of this high-frequency behavior. The
Guiding Assistant can also show you other behavior of your system such as overall drift rates in right ascension
and declination as well as peak-to-peak and maximum-rate-of-change measurements in right ascension, While
these things can usually be "guided out", measuring them can be helpful if you want to improve the underlying
performance of the mount - for example, by improving your polar alignment if the declination drift rate is high.
When the Guiding Assistant is first started, you'll see a dialog box like this:
The upper message area in the Guiding Assistant dialog-box displays usage instructions, much like a wizard
interface. In order for the Guiding Assistant to start measurement, you first need to start guiding in the usual
way. This identifies the target star in the frame and enables (but does not start) the underlying data collection
mechanism. You then click 'Start' in the Guiding Assistant to begin the measurement process. Once you do
this, guiding commands will be disabled, so the star will appear to wander around on the display - this is
entirely normal. As guider images are acquired, statistics are computed and displayed in real-time in the user
interface. Of particular interest are the table entries in the "High-frequency Star Motion" section which show
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ongoing results of the averaging process. After about one minute of data collection, these numbers will usually
stabilize and you'll have a reasonable measurement of the high-frequency star movement caused by seeing
conditions. When you click the 'Stop' button, guiding commands will be re-enabled and the data collection
process will end. Other computed results will then be displayed in the lower area of the table showing overall
drift rates and various other measurements. All of these values are displayed in units of both arc-seconds and
pixels. The dialog box will look something like this:
The contents of the 'Recommendations' group at the bottom of the window reflect the results of the highfrequency measurements. Assuming your chosen guide algorithms support a minimum-move property, you
have the option of automatically setting those parameters based on the results. You can also decide to re-run the
measurements or close the dialog box altogether if you want to proceed with normal guiding operations. Over
time, we may be able to add recommendations regarding other guiding properties.
Managing Equipment Profiles
Equipment profiles were introduced in the section on Basic Use where they are used as part of the 'Connect
Equipment' dialog. If you want to manage multiple profiles, you will probably want to use the 'Manage
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Profiles' button in the 'Connect Equipment' dialog. Using the menu items there, you can create a new profile
or edit/rename/delete an existing one. Each profile holds all the settings that were active at the time the profile
was last used. If you create a new profile, you can import these settings from either the PHD2 generic defaults
or from an existing profile. You can also use the 'Wizard' option to have PHD2 establish settings that are
specific to your equipment configuration. To edit the settings in an existing profile, you first select it in the
equipment profile drop-down list, then click on 'Settings' under the 'Manage Profiles' pull-down. This will take
you to the 'Brain' dialog, where you can make whatever changes you want. Remember than profiles are
automatically updated anytime settings are changed during a PHD2 session. Finally, you can import and export
profiles for purposes of debugging, backup, or even exchange with other PHD2 users.
Advanced Settings for the Simulators
The device simulators were introduced in the Basic Use section as useful tools for you to experiment with
PHD2 and become familiar with its features. Remember that you must choose 'Simulator' as the camera type
and 'On-camera' as the mount type in order to get the benefits of simulation. As you become more interested in
the details of the simulation, you can use the 'Cam Dialog' button on the main display to adjust the simulation
You can adjust simulated mount behaviors for declination backlash, drift due to polar mis-alignment, and
periodic error. You can also adjust the 'seeing' level, which will create fairly realistic guide star deflections that
look like seeing effects. If you adjust these parameters one-by-one, you'll see how they affect star deflections
and how the different guide algorithms react to those movements. Of course, you're dealing with a "nearly
perfect" mount in these scenarios (except for backlash), so the simulation can't be entirely realistic.
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Keyboard Shortcuts
Keyboard shortcuts are available for many of the more commonly used tools and functions in PHD2. These are
enumerated in the Keyboard Shortcuts section.
Table of PHD2 keyboard shortcuts
Open Connect equipment window
Shift-Ctrl-C Connect all equipment
Auto-select Star
Dismiss alert
Review calibration
Clear calibration (force re-calibration)
Open Advanced settings
Toggle Bookmarks shown/hidden
Delete all Bookmarks
Bookmark lock position
Toggle Guide Log enabled/disabled
Shift-Ctrl-M Enter Manual calibration
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Trouble-shooting and Analysis
Calibration and "First Use" Problems
If you are just starting to use PHD2 or are connecting to new equipment for the first time, you may have trouble
getting the guider calibration done. This problem usually takes one of two forms, each requiring different
1. The star moves during calibration but it moves "too far" or "too little." This is easy to fix - simply adjust
the 'calibration step-size' parameter in the 'Mount' tab of Advanced Settings. The help content there
describes how this parameter is used, and you should be able to resolve the problem quickly.
2. The star doesn't move at all or it moves only very small amounts and in unpredictable ways. Actually,
the small movements in this scenario are often the result of seeing deflections and the mount isn't really
moving at all. Dealing with this sort of problem is described next.
In nearly all cases, the "no movement" problem is caused by failures in the hardware or, even more likely,
problems in the cabling and connections. The best tool for trouble-shooting this is the 'Manual Guide' option
under the 'Tools' menu, as described in the Tools section of this help document. Simply use the directional
controls in the 'Manual Guide' window to send commands directly to the mount. If the mount does not respond,
you know you have either hardware or connectivity problems to resolve - nothing to do with PHD2. If you're
using a Shoestring device to connect to the mount, watch its indicator lights to see if the commands are reaching
it. Similarly, your ST-4 compatible guide camera may have indicator lights to show when guide commands are
being received. If you're using an ASCOM connection to the mount, be sure the COM port assignments are
correct. You can also use some of the ASCOM-supplied tools like POTH to be sure the ASCOM driver is
communicating correctly with the mount.
Calibration Sanity-Checks
It is also possible that the calibration process will complete but PHD2 will post a "calibration sanity-check" alert
message saying that some of the results are questionable. This "sanity check" dialog will show an explanation
of the issue and some details of the calibration results:
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In the initial release (2.4.0), there are four things checked by PHD2:
Too few steps (shown above) - resolving this issue is quite easy. Just adjust the calibration step-size
downward until you get at least 8 steps in both the west and north calibrations.
Non-orthogonal camera axes - the camera axes are normally computed independently even though
they should be perpendicular. The angle calculations do not require great precision, but if they are
significantly non-orthogonal, you should repeat the calibration. Alternatively, you can set the option to
‘Assume Dec orthogonal to RA' in the 'Mount' tab of the Advanced Setup dialog. The graph at the left
shows the RA and Dec axis orientation as measured by PHD2...
Suspicious RA and Dec rates - the guide rate for right ascension should be related to the declination
guiding rate by approximately a factor of cosine (Declination). In other words, the RA rate gets smaller
as you move the scope further away from the celestial equator (Dec=0). In this initial release, PHD2
won't try to identify which rate is incorrect - it is simply alerting you that something looks wrong with
the rates. You can sanity check these rates yourself quite simply. If you are guiding at 1X sidereal rate,
your declination guide rate should be approximately 15 arc-sec/sec; with a guide rate of 0.5X sidereal,
the declination rate would be 7.5 arc-sec/sec, etc.
Inconsistent results - if the calibration results are significantly different from your last-used calibration,
an alert message will be generated. This may happen because you've made a change in your
configuration. That doesn't imply a real problem, but you should probably consider creating a separate
profile for the new configuration. By doing so, PHD2 will remember settings for each of your profiles,
letting you switch between them easily. If you haven't made a configuration change, you will probably
want to determine why the results are so different.
With any of these alerts, the relevant data field will be highlighted based on the type of message. You can
choose to ignore the warning ('Accept calibration'), re-run the calibration ('Discard calibration'), or restore your
last good calibration ('Restore old calibration'). With the third option, you can defer calibration until later and
start guiding with your last good calibration data. If you see repeated alerts on the same topic and are
convinced there really isn't a problem, you can use the 'don't show' checkbox to block future alerts of that type.
Program "Hangs" and Camera Download Problems
In some cases, you may experience problems where guider images aren't downloaded or displayed. In extreme
cases, this may even cause PHD2 or other camera-related applications to be non-responsive (i.e. to
"hang"). Again, this is almost always due to hardware, camera driver, or connectivity issues, with one of the
most common culprits being a faulty USB cable or device. It is highly unlikely to be caused by an application
like PHD2, so you should begin your investigation at the lower levels of the system. You can start by
confirming that the guide camera is working - try using a short, direct cable from the camera to the computer
and taking exposures with the "native" or test application that came with the camera. If the camera is
functional, you can start moving "upstream" by looking at USB hubs and cables, swapping them one at a time
to see if you can isolate the problem. It's worth remembering that we work in a hostile environment while doing
our imaging, and many of the components we use are not designed for cold, outdoor conditions. So something
that worked just last week or last month may no longer be reliable.
Poor Guiding Performance
Once you've gotten everything running, you will probably get reasonably good guiding results almost
immediately. You will have to decide what "good enough" means, and everyone's standard is likely to be
different. But if you find your imaging results are not acceptable because the stars are streaked or elongated,
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you'll need to take a systematic approach to correcting the problems. This can be a complex task and not
something to be covered here. However, you can get help on the web from a variety of sources, with the
document by Craig Stark being one of the best:
Log Analysis
Any sort of problem isolation or tuning will invariably require use of the PHD2 log files. Both are formatted
for straightforward interpretation by a human reader, and the guide log is constructed to enable easy import into
other applications. Logging of debug and guiding data is controlled by two menu items under "Tools", and
these should be left checked so that diagnostic information is available if problems occur. As mentioned in the
'Tools' section, applications such as PHDLogViewer or Excel can be used to visualize overall performance,
compute performance statistics, and examine time periods when guiding was problematic. With Excel or
similar applications, simply specify that the guide log uses a comma as a column separator.
Guiding Log Contents
The contents of the guiding log will continue to evolve as new capabilities are added. But the basic content is
stable, and considerable care is taken to not "break" applications that parse it. If you wish to analyze the log
yourself, the following information will be helpful.
The PHD2 guide log will contain zero or more sequences of calibration and zero or more sequences of
guiding. Each of these sections has a header that provides most of the information about the guiding algorithms
being used and the internal parameters used by PHD2 for guiding. At the start of either a calibration run or a
guiding sequence, the last line of the header information defines a set of column headings. The meanings of
those columns are shown below:
Calibration columns:
dx, dy are offsets from the starting position, in pixels, in the camera coordinate system
x, y are the camera x/y coordinates of the guide star at the end of each calibration step
Dist is the total distance moved in the camera coordinate system (dist = sqrt (dx*dx + dy*dy). This is
the value used by PHD2 to compute the calibration parameters
Guiding columns:
dx, dy are the same as for calibration - offsets from the "lock position" of the guide star in the camera
coordinate system
RARawDistance and DECRawDistance - these are the transforms of dx and dy into the mount
coordinates - in other words, they use the arbitrary angle of the guide camera to map from X/Y on the
camera to RA/Dec on the mount
RAGuideDistance and DECGuideDistance - these are the outputs from the various guiding
algorithms. The guide algorithms operate on the "raw" distances and decide how far, if any, the
telescope position should be adjusted in each axis. For example, with a "minimum move" parameter set,
the "guide" distances can be zero even when the "raw" distances are non-zero.
RADuration, RADirection, DECDuration, DECDirection - these are the values determined by the two
"guide" distances above. The "durations" are the lengths of the guide pulses, in milliseconds, needed to
move the mount by the distances specified by RAGuideDistance and DECGuideDistance
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XStep, YStep - step-adjustment durations for the adaptive optics device if one is being used
StarMass - a brightness measure of the guide star image
SNR - an internal "signal-to-noise" ratio used by PHD2
ErrorCode - an indication that some error occurred, typically a "guide star faded" error
All distance values are in units of pixels. The header for the guiding section will show the image scale as it is
known by PHD2, and that can be used to scale the pixel distance values into units of arc-seconds if desired.
Problem Reporting
If you encounter application problems that are specific to PHD2, you are encouraged to report them to the openphd-guiding Google group:!forum/open-phdguiding. Obviously, the more information you can provide, the more likely we will be able to resolve the
problem. Using the following guidelines will help in that regard:
1. Try to reproduce the problem - if we have a clear set of steps to follow, we are more likely to find a
solution quickly. If you can reproduce it, try to reduce things to the minimum number of
steps. Remember, we won't have your hardware or computer environment when we try to reproduce it
2. Try to be complete about describing your configuration - operating system, equipment types, PHD2
version, etc.
3. Attach the PHD2 debug log from the session in which you encountered the problem. You can find the
debug log in the PHD2 folder in your Documents folder. If you can't reproduce the problem, try to
estimate the time of day when you first saw it - this could help us find evidence in the debug log without
having to sift through hundreds of lines of output.
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