G2 CCD Camera - Moravian Instruments

G2 CCD Camera - Moravian Instruments
G2 CCD
Camera
User's Guide
Version 2.6
Modified on July 15th, 2015
All information furnished by Moravian Instruments is believed to be accurate.
Moravian Instruments reserves the right to change any information contained
herein without notice.
G2 CCD devices are not authorized for and should not be used within Life
Support Systems without the specific written consent of the Moravian
Instruments. Product warranty is limited to repair or replacement of defective
components and does not cover injury or property or other consequential
damages.
Copyright © 2000-2015, Moravian Instruments
Moravian Instruments
Masarykova 1148
763 02 Zlín
Czech Republic
tel./fax: +420 577 107 171
www:
http://www.gxccd.com/
e-mail: [email protected]
Table of Contents
Introduction....................................................................................................5
G2 Camera Overview.....................................................................................7
CCD and Camera Electronics.......................................................................10
CCD Chip...............................................................................................13
Model G2-0402.................................................................................14
Model G2-1600.................................................................................14
Model G2-3200.................................................................................15
Model G2-8300.................................................................................15
Model G2-2000.................................................................................15
Model G2-4000.................................................................................16
Camera Electronics.................................................................................16
Model G2-0402.................................................................................17
Model G2-1600.................................................................................17
Model G2-3200.................................................................................18
Model G2-8300.................................................................................18
Model G2-2000.................................................................................18
Model G2-4000.................................................................................18
CCD Cooling and Power Supply..................................................................19
Power Supply..........................................................................................19
Mechanical Specifications............................................................................22
Telescope adapters..................................................................................24
Getting Started..............................................................................................27
Camera System Driver Installation.........................................................27
Windows 7 and 8 System Driver Installation...................................28
Windows XP and Windows Vista System Driver Installation.........29
SIPS Software Installation......................................................................29
SIPS configuration files....................................................................30
G2 CCD Camera Driver for SIPS...........................................................32
Using of multiple configuration files for different cameras.............34
Cropping of the CCD area................................................................34
Camera Connection................................................................................35
Camera LED state indicator..............................................................36
Working with Multiple Cameras............................................................37
Camera Operation.........................................................................................39
Camera and the Telescope......................................................................40
Temperature Control...............................................................................41
First Images............................................................................................43
Brightness and Contrast – Image Stretching..........................................44
Calibration..............................................................................................45
Color Images with monochrome camera and filters...............................48
Color images with color camera.............................................................51
Balancing colors.....................................................................................54
Some General Rules for Successful Imaging...............................................55
Camera Maintenance....................................................................................58
Desiccant exchange................................................................................58
Changing the silica-gel......................................................................59
Changing Filters......................................................................................60
Opening the camera head..................................................................60
Changing the Whole Filter Wheel..........................................................61
Changing the Telescope Adapter............................................................61
Power Supply Fuse.................................................................................61
G2 Camera Revisions...................................................................................62
Revision 1...............................................................................................62
Revision 2...............................................................................................62
Revision 3...............................................................................................63
Revision 4...............................................................................................64
Introduction
Thank you for choosing the G2 CCD camera. The cooled, slow-scan series of
G2 CCD cameras were developed for imaging under extremely low-light
conditions in astronomy, microscopy and similar areas. The development team
focused to every detail of camera mechanics, cooling, electronics and software
to create state-of-the-art product. G2 CCD cameras feature compact and robust
construction, rich features, sophisticated software support and easy operation.
G2 cameras can contain filter wheel with 5 positions for 1.25“ filters . Camera
variants without internal filter wheel can control external filter wheel with 12
positions for the same filter s or with 10 positions for D36 mm filters.
Please note the G2 CCD cameras are designed to work in cooperation with a
host Personal Computer (PC). As opposite to digital still cameras, which are
operated independently on the computer, the scientific slow-scan, cooled
cameras usually require computer for operation control, image download,
processing and storage etc. To operate G2 CCD camera, you need a computer
which:
1.
Is compatible with a PC standard.
2.
Runs a modern 32-bit or 64-bit Windows operating system.
Drivers for 32-bit and 64-bit Linux systems are also provided, but
camera control and image processing software, supplied with the
camera, requires Windows operating system.
3.
Provides at last one free USB port.
The current series of G2 CCD cameras are designed to operate with
USB 2.0 high-speed (480 Mbps) hosts. Although they are fully
backward compatible with USB 1.1 full-speed (12 Mbps) hosts, image
download time can be somewhat longer if USB 1.1 connection is used.
A simple and cheap device called USB hub can expand number of
available USB port. Typical USB hub occupies one computer USB
port and offers four free ports. Make sure the USB hub is USB 2.0
high-speed compatible.
5
But keep on mind that if more USB devices connected to one hub need
to communicate with a host PC, USB hub shares its single up link line
to the host PC. Although G2 CCD cameras can operate through a USB
hub, it can negatively affect the camera performance, like download
time etc. It is recommended to connect other USB devices through
USB hub (e.g. the mouse) and to provide the camera a direct USB
connection to the host PC.
4.
Alternatively it is possible to use the Gx Camera Ethernet Adapter.
This device can connect up to four Gx cameras of any type (not only
G3 and G4, but also G0, G1 and G2) and offers 1 Gbps and
10/100 Mbps Ethernet interface for direct connection to the host PC.
Because the PC then uses TCP/IP protocol to communicate with the
cameras, it is possible to insert e.g. WiFi bridge or other networking
device to the communication path.
The G2 CCD camera needs an external power supply to operate. It is not
possible to run the camera from the power lines provided by the USB cable,
which is common for webcams or very simple imagers. G2 CCD cameras
integrate highly efficient CCD chip cooling, shutter and filter wheel, so their
power requirements significantly exceed USB line power capabilities. On the
other side separate power source eliminates problems with voltage drop on long
USB cables or with drawing of laptop batteries etc.
Also note the camera must be connected to some optical system (e.g. the
telescope) to capture images. The camera is designed for long exposures,
necessary to acquire the light from faint objects. If you plan to use the camera
with the telescope, make sure the whole telescope/mount setup is capable to
track the target object smoothly during the exposure.
6
G2 Camera Overview
G2 camera head is designed to be easily used with a set of accessories to fulfill
various observing needs. Camera head itself is manufactured in two different
variants:
●
Camera with internal filter wheel.
●
Camera with control port for external filter wheel. This model allows
attachment of several variants of external filter wheels with various
number of filter positions and sizes.
Illustration 1: G2 camera without filter wheel (left), with internal filter wheel (middle)
and with attached external filter wheel (right).
7
Illustration 2: Schematic diagram of G2 camera system components
8
Components of G2 Camera system include:
1.
G2 camera head with internal filter wheel.
2.
G2 camera head without internal filter wheel, ready for attaching of
external filter wheel.
3.
G0 Guider camera.
4.
G1 Guider camera.
G0 and G1 cameras are completely independent devices with their
own USB connection to the host PC. They can be used on G2 OAG,
on standalone guiding telescope or for any other imaging purpose, like
Moon or planetary imaging etc.
Both G0 and G1 camera can share the Gx Camera Ethernet Adapter
with up to 3 other Gx cameras to be accessed over network.
5.
External filter wheel.
6.
Off-axis guider adapter, optionally with M42×0.75 thread (T-thread)
or M48×0.75 thread.
7.
Thin spacer. Camera with internal filter wheel and this spacer has the
same back focal distance as camera with external filter wheel.
8.
Thick spacer. Camera without internal filter wheel and this spacer has
the same back focal distance as camera with external filter wheel.
9.
Nikon bayonet adapter for Nikon compatible lenses.
10. Canon EOS bayonet adapter for Canon compatible lenses.
11. T-thread (M42×0.75) adapter.
12. 2-inch barrel adapter.
Other available adapters are missing from this illustration, e.g.
M48×0.75 thread adapter, M42×1 Pentax/Parktica lens adapter etc.
13. Gx Camera Ethernet Adapter allows connection of up to 4 Gx cameras
of any type on the one side and 1 Gbps Ethernet on the other side. This
adapter allows access to connected Gx cameras using routable TCP/IP
protocol over practically unlimited distance.
14. The whole system is controlled from a host PC.
9
CCD and Camera Electronics
G2 series of CCD cameras are manufactured with two kinds of CCD detectors:
●
G2 cameras with Kodak KAF Full Frame (FF) CCD architecture.
Almost all Full Frame CCD detector area is exposed to light. This is
why these detectors provide very high quantum efficiency. FF CCD
detectors, intended for research applications, are not equipped with socalled Anti Blooming Gate (ABG – a gate, which prohibits blooming
of the charge to neighboring pixels when image is over-exposed) to
ensure linear response to light through the whole dynamic range. FF
CCD detectors used for astrophotography are equipped with ABG to
eliminate disrupting blooming streaks within field of view.
Cameras with Full Frame, non-ABG detectors are suitable for
scientific applications, where linear response is necessary for
photometric applications in astronomy, microscopy etc. High quantum
efficiency could be used also for narrow-band imaging, where
overexposure is a rare exception, and for imaging of small objects
without a bright star in the field of view.
Illustration 3: “Full Frame” CCD schematic diagram
10
●
G2 cameras with Kodak KAI Interline Transfer (IT) architecture.
There is a shielded column of pixels just beside each column of active
pixels on these detectors. The shielded columns are called Vertical
registers. One pulse moves charge from exposed pixels to shielded
pixels on the end of each exposure. The the charge is moved from
vertical registers to horizontal register and digitized in the same way
like in the case of Full Frame detectors. This mechanism is also known
as “electronic shuttering”, because it allows very short exposures and
also digitization of the image without mechanically shielding of the
detector from incoming light.
Also G2 cameras with IT CCDs are equipped with mechanical shutter,
because electronic shutter does not allow dark-frame exposures,
necessary for proper image calibration etc.
The price for electronic shutter if lower quantum efficiency
(sensitivity) of IT detectors compared to FF ones. Also all IT detectors
are equipped with ABG, so they can acquire images of very bright
objects without charge blooming to neighboring pixels.
Illustration 4: “Interline Transfer” CCD schematic diagram
11
G2 camera models with Full Frame CCD detectors:
Model
G2-0402
G2-1600
G2-3200
G2-8300
CCD chip
KAF-0402ME
KAF-1603ME
KAF-3200ME
KAF-8300
Resolution
768×512
1536×1024
2184×1472
3358×2536
Pixel size
9×9 µm
9×9 µm
6.8×6.8 µm
5,4×5,4 µm
CCD area
6.9×4.6 mm
13.8×9.2 mm
14.9×10.0 mm
18,1×13,7 mm
ABG
No
No
No
Yes
Color mask
No
No
No
No (see Note)
G2-8300 camera is available in the G2-8300C version with color CCD detector
(with Bayer mask), capable of single-shot color images.
G2 camera models with Interline Transfer CCD detectors::
Model
G2-2000
G2-2000C
G2-4000
G2-4000C
CCD chip
KAI-2020
KAI-2020
KAI-4022
KAI-4022
Resolution
1604×1204
1604×1204
2056×2062
2056×2062
Pixel size
7,4×7,4 µm
7,4×7,4 µm
7,4×7,4 µm
7,4×7,4 µm
CCD area
11,8×9,0 mm
11,8×9,0 mm
15,2×15,2 mm
15,2×15,2 mm
ABG
Yes
Yes
Yes
Yes
Color mask
No
Yes
No
Yes
Cameras with “C” suffix contains CCD detector covered with so-called Bayer
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mask. Color filters of three basic colors (red, green, blue) cover all pixels, so
every pixels detects only light of particular color.
These cameras are able to acquire color image in single exposure, without the
necessity to change color filters. On the other side color mask brings lower
sensitivity and limits the capability to perform exposures using narrow-band
filters etc.
Because each pixel is covered by one of three basic color filters, it is necessary
to compute (interpolate) remaining two colors for each pixel, which of course
limits resolution of color image. Imaging using color detectors is described in
the “Color images” chapter.
CCD Chip
Quantum efficiency (sensitivity) of CCD detectors used in G2 cameras depends
on the particular camera model.
Illustration 5: Quantum efficiency of Kodak CCD detectors used in G2 cameras
Inherent dark current of these detectors is quite low compared to other CCD
detectors, suitable for scientific applications, which results into very good
signal/noise ratio.
13
Illustration 6: Dark current of Kodak CCD detectors, used in G2 cameras
Model G2-0402
G2-0402 model uses 0.4 MPx Kodak KAF-0402ME.
Resolution
768×512 pixels
Pixel size
9×9 µm
Imaging area
6.9×4.6 mm
Full well capacity
Approx. 100 000 e-
Output node capacity
Approx. 220 000 e-
Dark current
1 e-/s/pixel at 0°C
Dark signal doubling
6.3 °C
Model G2-1600
G2-1600 model uses 1.6 MPx Kodak KAF-1603ME.
14
Resolution
1536×1024 pixels
Pixel size
9×9 µm
Imaging area
13.8×9.2 mm
Full well capacity
Approx. 100 000 e-
Output node capacity
Approx. 220 000 e-
Dark current
1 e-/s/pixel at 0°C
Dark signal doubling
6.3 °C
Model G2-3200
G2-3200 model uses 3.2 MPx Kodak KAF-3200ME.
Resolution
2184×1472 pixels
Pixel size
6.8×6.8 µm
Imaging area
14.9×10.0 mm
Full well capacity
Approx. 55 000 e-
Output node capacity
Approx. 110 000 e-
Dark current
0.8 e-/s/pixel at 0°C
Dark signal doubling
6 °C
Model G2-8300
G2-8300 model uses 8 MPx Kodak KAF-8300.
Resolution
3358×2536 pixels
Pixel size
5,4×5,4 µm
Imaging area
18,1×13,7 mm
Full well capacity
Approx. 25 000 e-
Output node capacity
Approx. 55 000 e-
Dark current
0.15 e-/s/pixel at 0°C
Dark signal doubling
5.8 °C
KAF-8300 CCD detector with color (Bayer) mask can be used in the G2-8300C
camera.
Model G2-2000
G2-2000 uses 2 MPx CCD Kodak KAI-2020.
Resolution
1604×1204 pixels
Pixel size
7.4×7.4 µm
Imaging area
11.9×8.9 mm
15
Full well capacity
Approx. 40 000 e-
Output node capacity
Approx. 80 000 e-
Dark current
0.3 e-/s/pixel at 0°C
Dark signal doubling
7 °C
KAI-2020 CCD detector with color (Bayer) mask can be used in the G2-2000C
camera.
Model G2-4000
G2-2000 uses 4 MPx CCD Kodak KAI-4022.
Resolution
2056×2062 pixels
Pixel size
7.4×7.4 µm
Imaging area
15.2×15.2 mm
Full well capacity
Approx. 40 000 e-
Output node capacity
Approx. 80 000 e-
Dark current
0.3 e-/s/pixel at 0°C
Dark signal doubling
7 °C
KAI-4022 CCD detector with color (Bayer) mask can be used in the G2-4000C
camera.
Camera Electronics
Remark
Stated values are valid for G2 cameras revision 4. Previous revisions could
differ in some parameters. Refer to chapter “G2 Camera Revisions” for
differences among individual revisions.
Some parameters (e.g. camera gain) are defined by the used system driver, so
they depend on the version of actually used driver. If the Gx Camera Ethernet
Adapter is used, system driver is defined by the firmware version of the
Ethernet Adapter device.
16-bit A/D converter with correlated double sampling ensures high dynamic
range and CCD chip-limited readout noise. Fast USB interface ensures image
download time within seconds.
16
ADC resolution
16 bits
Sampling method
Correlated double sampling
Read modes
Preview
Low-noise
Horizontal binning
1 to 4 pixels
Vertical binning
1 to 4 pixels
Sub-frame readout
Arbitrary sub-frame
Computer interface
USB 2.0 high-speed
USB 1.1 full-speed compatible
Binning can be combined independently on both axes.
Image download time and system read noise depends on the CCD chip used in
particular camera model.
Model G2-0402
Gain
1.5e-/ADU (1×1 binning)
2.0e-/ADU (other binnings)
System read noise
15 e- (Low Noise mode)
17 e- (Preview mode)
Full frame download
0.7 s (Low Noise mode)
0.5 s (Preview mode)
Model G2-1600
Gain
1.5e-/ADU (1×1 binning)
2.0e-/ADU (other binnings)
System read noise
15 e- (Low Noise mode)
17 e- (Preview mode)
Full frame download
2.6 s (Low Noise mode)
1.8 s (Preview mode)
17
Model G2-3200
Gain
0.8 e-/ADU (1×1 binning)
1.3 e-/ADU (other binnings)
System read noise
7 e- (Low Noise mode)
10 e- (Preview mode)
Full frame download
5.5 s (Low Noise mode)
3.8 s (Preview mode)
Model G2-8300
Gain
0.4 e-/ADU (1×1 binning)
0.8 e-/ADU (other binnings)
System read noise
8 e- (Low Noise mode)
9 e- ( Preview mode)
Full frame download
14.2 s (Low Noise mode)
9.8 s (Preview mode)
Model G2-2000
Gain
0.4 e-/ADU (1×1 binning)
0.8 e-/ADU (other binnings)
System read noise
7 e- (Low Noise mode)
9 e- ( Preview mode)
Full frame download
3.1 s (Low Noise mode)
2.1 s ( Preview mode)
Model G2-4000
Gain
0.4 e-/ADU (1×1 binning)
0.8 e-/ADU (other binnings)
System read noise
7 e- (Low Noise mode)
9 e- (Preview mode)
Full frame download
6.7 s (Low Noise mode)
4.5 s (Preview mode)
18
CCD Cooling and Power Supply
Regulated two-stage thermo-electric cooling is capable to cool the CCD chip up
to 50 °C below ambient temperature. The Peltier hot side is cooled by a fan.
The CCD chip temperature is regulated with ±0.1 °C precision. High
temperature drop and precision regulation ensure very low dark current for long
exposures and allow image proper calibration.
The camera head contains two temperature sensors – the first sensor measures
directly the temperature of the CCD chip. The second one measures the
temperature of the air cooling the Peltier hot side.
The cooling performance depends on the environmental conditions and also on
the power supply. If the power supply voltage drops below 12 V, the maximum
temperature drop is lower.
CCD chip cooling
Thermoelectric (Peltier modules)
TEC modules
Two stages
Maximal ∆T
>50 °C below ambient
Regulated ∆T
48 °C below ambient (85% cooling)
Regulation precision ±0.1 °C
Hot side cooling
Forced air cooling (fan)
Optional heat exchanger for liquid coolant
Maximum temperature difference between CCD and ambient air may exceed
50 °C when the cooling runs at 100% power. However, temperature cannot be
regulated in such case, camera has no room for lowering the CCD temperature
when the ambient temperature rises. The 45 °C temperature drop can be
achieved with cooling running at approx. 85% power, which provides enough
room for regulation.
Power Supply
The 12 V DC power supply enables camera operation from arbitrary power
19
source including batteries, wall adapters etc. Universal 100-240 V AC/5060 Hz, 60 W “brick” adapter is supplied with the camera. Although the camera
power consumption does not exceed 30 W, the 60 W power supply ensures
noise-free operation.
Camera head supply
12 V DC
Camera head power consumption 30 W
Adapter input voltage
100-240 V AC/50-60 Hz
Adapter output voltage
12 V DC/5 A
Adapter maximum power
60 W
Power consumption is measured on the AC side of the supplied 12 V AC/DC
power supply. Camera consumes less energy from 12 V power supply than
state here.
The camera contains its own power supplies inside, so it can be powered by
unregulated 12 V DC power source – the input voltage can be anywhere
between 10 and 14 V. However, some parameters (like cooling efficiency) can
degrade if the supply drops below 12 V.
G2 CCD camera measures its input voltage and provides it to the control
software. Input voltage is displayed in the Cooling tab of the CCD Camera
control tool in the SIPS. This feature is important especially if you power the
camera from batteries.
Illustration 7: 12 V DC/5 A power supply adapter for
G2 CCD Camera
20
Warning:
The power connector on the camera head uses center-plus pin. Although all
modern power supplies use this configuration, always make sure the polarity is
correct if you use own power source.
21
Mechanical Specifications
Compact and robust camera head measures only 114×114×65 mm (approx.
4.5×4.5×2.6 inches). The head is CNC-machined from high-quality aluminum
and black anodized. The head itself contains USB-B (device) connector and
12 V DC power plug. Integrated mechanical shutter allows streak-free image
readout, as well as automatic dark frame exposures, which are necessary for
unattended, robotic setups. Integrated filter wheel contains 5 positions for
standard 1.25-inch threaded filter cells. A variant of filter wheel with
6 positions for the same filters without cells (only a glass) is also available.
Internal mechanical shutter Yes, blade shutter
Shortest exposure time
0.1 s
Longest exposure time
Limited by chip saturation only
Internal filter wheel
5 positions for 1.25" threaded filter cells or
for 31 mm glass-only filters
6 positions for 26.5 mm glass-only filters
Head dimensions
114×114×77.5 mm (with internal filter wheel)
114×114×65 mm (without filter wheel)
Back focal distance
29 mm (with internal filter wheel)
16.5 mm (without filter wheel)
33.5 mm (with external filter wheel)
Camera head weight
1.15 kg (with internal filter wheel)
1.05 kg (without filter wheel)
1.95 kg (with external filter wheel)
Filter wheel with 6 positions cause vignetting (shielding of the detector corners)
if large CCD detector is used.
22
Illustration 8: G2 camera head front view dimensions
Illustration 9: G2 camera head with internal filter wheel
side view dimensions
23
Illustration 10: G2 camera head with external filter wheel side view dimensions
Telescope adapters
The camera is supplied with standard 2" barrel adapter by default, but the user
can choose any other adapter he/she prefers. Another adapters can be ordered
separately.
It is possible to choose among various telescope/lens adapters:
24
2" barrel adapter
Adapter for 2" focusers.
T-thread short
M42×0.75 mm inner thread,
7.5 mm thick.
T-thread with
55 mm BFD
M42×0.75 mm inner thread,
preserves 55 mm back focal
distance.
M48×0.75 thread
short
Adapter with inner thread
M48×0.75, 7.5mm thick
M48×0.75 thread
with 55 mm BFD
Adapter with inner thread
M48×0.75, preserves 55 mm
back focal distance.
Pentax (Praktica)
lens adapter
M42×1 mm inner thread,
preserves 45.5 mm back
focal distance.
M68×1 thread
adapter
Adapter with inner thread
M68×1
Canon EOS lens
adapter
Standard Canon EOS
bayonet adapter.
Canon EOS clip lens
adapter
Canon EOS bayonet adapter
with the possibility to insert
“clip” filter. Can be used on
cameras with internal filter
wheel only.
25
Nikon F lens
adapter
Standard Nikon F bayonet
adapter.
If the mounting standard defines also back focal distance (distance from adapter
front plane to detector), the particular adapter is constructed to preserve defined
distance (for instance T-thread defines back focal distance to 55 mm, but
certain distance is defined also for Pentax (Praktica) thread, for Canon EOS and
Nikon bayonets etc.).
Adapters are attached to the camera body using four M3 (3 mm metric) screws,
placed on the corners of 44 mm square. Custom adapters can be made upon
request.
26
Getting Started
Although the camera is intended for operation at night (or for very low-light
conditions at day), it is always better (and highly recommended) to install
software and to make sure everything is working OK during day, before the
first night under the stars.
The G2 CCD cameras can be in principle operated under various CCD control
software packages (refer to our web site for available drivers), this manual
demonstrates camera operation under the SIPS (Scientific Image Processing
System) – camera control and image processing software suite supplied with
the camera.
Camera System Driver Installation
Every USB device requires so-called “system driver”, incorporated directly into
the operating system kernel. Some devices (for instance USB Flash Disk
dongles) conform to some predefined class (USB mass-storage device class in
this case), so they can use the driver already present in the operating system.
But this is not the case of the G2 CCD camera – it requires its own system
driver to be installed.
Although 64 bit operating system can run 32 bit application without any
problems, it is basically impossible to combine e.g. 64 bit process with 32 bit
dynamic link library. The same is valid for operating system kernel - 64 bit
kernel cannot use 32 bit system driver. This is why all G2 camera drivers are
supplied in two versions, one for 32 bit systems (marked x86 according to Intel
386, 486 CPUs) another for 64 bit systems, marked x64 (according to CPUs
supporting 64 bit instructions marked x86-64 or only x64).
The simplest way to install Gx CCD camera system driver is to run driver preinstallation package (“GxCam Drivers 32bit EN.exe“ or “GxCam Drivers 64bit
EN.exe”, provided with the camera or downloaded from a web site) on the
target computer. This package installs the driver for all Gx cameras on the
particular computer. Then it is enough to plug in the camera and the operating
system already knows which driver to use.
27
Pre-installation is the recommended way how to install system drivers. It is not
necessary to deal with differences among individual versions of operating
systems, described in subsequent chapters. If the user decides not to use preinstallation, it is necessary to take into account different implementation of the
„Plug-and-play“ mechanisms of driver installation in different Windows
versions.
Due to differences in KAF and KAI CCD handling, there are two drivers for
cameras utilizing the respective detectors. Driver names are distinguished by
the last letter 'F' and 'I'.
Also individual camera revisions may require different drivers depending on
the used digital electronics. Older cameras use “g3ccdF.sys” and “g3ccdI.sys”
drivers, while newer use “gXccdF.sys” a “gXccdI.sys” drivers.
Windows 7 and 8 System Driver Installation
Windows 7 and 8 do not offer users the possibility to install system drivers
using Plug-and-Play, like in the case of older Windows 2000, Windows XP and
Windows Vista. It is necessary to pre-install all drivers, else the operating
system only informs user that it cannot find appropriate driver for newly
connected device.
We can only estimate reasons for this limitation of system functionality,
probably it has something common with the inability of many hardware
vendors to provide drivers complying to Plug-and-play standards (notification
requiring installation of the software first and plugging of the device later was
present on many devices).
Although the Plug-and-Play mechanism is hidden in Windows 7/8, it is
possible to use it. Newly connected device appears in the “Device Manager” as
“Unknown Device” (such device is usually marked by a question mark on
yellow background icon). It is enough to click on such device by right mouse
button to invoke pop-up menu and choose “Update Driver...” menu item.
Operating system then opens driver installation wizard, basically identical to
the one in Windows XP and Windows Vista.
Let us note that 64 bit versions of Windows 8, Windows 7 and Windows Vista
require digitally signed drivers. Drivers without digital signature cannot be
installed on these systems.
All Gx camera drivers supplied by Moravian Instruments are digitally signed
28
from the beginning of the year 2010.
Windows XP and Windows Vista System Driver Installation
The operating system notifies the new USB device was plugged in the “Found
new hardware bubble”. The system then opens the “Found New Hardware”
Wizard.
1.
The wizard offers searching for suitable driver on Windows Update
site. Reject this offer (choose “No, not this time”) and click “Next”
button.
2.
Choose the “Install the software automatically” in the next step.
Insert the USB Flash Drive into the drive and the wizard will continue
by the next step.
It is not necessary to install files from USB Flash Drive. It is possible
to copy the folder containing driver files e.g. to shared network
volume etc. Then it is necessary to choose the “Install from a list or
specific location” and to define the path to driver files.
3.
The wizard starts to copy files. But Windows XP checks for driver file
digital signature. If it cannot find the signature, it notifies the user by a
message box. Click “Continue Anyway”, the digital signature is only
an administrative step and does not influence the proper functionality.
4.
The wizard then finishes the installation and the camera is ready to
work.
Please note the Windows XP system keeps the information about installed
devices separately for each USB port. If you later connect camera to a different
USB port (different USB connector on the PC or through the USB hub),
Windows reports “found new hardware” again and asks you to install the
software. Repeating the installation again brings no problem, just choose
“Install automatically” option and Windows will reuse already installed drivers.
SIPS Software Installation
The Scientific Image Processing System (SIPS) software package is designed
to operate without the necessity to be installed in any particular folder. The
package can be even run directly from USB Flash Drive.
29
SIPS needs the Microsoft Visual C++ 2008 libraries to work. These libraries
are already installed on many Windows PCs, because they are used by a lot of
other applications. But if they are not present, it is necessary to install them
first. The best way how to do it is to run the “Microsoft Visual C++ 2008 SP1
Redistributable Setup” package (executable file 'vcredist_x86.exe'). This
package can be downloaded from the Microsoft web site and it is also supplied
on the USB Flash Drive shipped with the camera,
SIPS package is distributed in the two forms:
1.
In the form of the executable installation package 'SIPS_EN.exe'.
Running of this package installs SIPS similarly to any other Windows
application. The user does not need to care whether other libraries or
packages are installed, the setup process installs everything necessary.
If the SIPS is installed this way, then it can be easily uninstalled from
the application management of the Windows operating system.
2.
In the form of so-called “portable version” on the USB Flash Drive.
The directory called “SIPS” contains SIPS image (set of EXE and
DLL files, as well as auxiliary INI files etc.), which can be directly
executed. The image can be copied to computer local drive into the
(possibly newly created) directory chosen by the user.
The portable version can be also downloaded in the form of ZIP
archive (file 'sips.zip') from the web site. Again it is enough to unzip
the archive into chosen directory.
Uninstalling of the SIPS portable version is also quite easy – just
delete the SIPS folder.
No matter how is the package installed, the software is run by launching the
'SIPS.exe' main program file.
SIPS configuration files
The software package distinguishes two types of configuration:
●
Global configuration, common for all users.
●
User-specific configuration.
Global configuration defines which hardware is used and which drivers
30
controls it. The configuration is stored in the simple text file “sips.ini”, which
must be placed in the same folder as the “sips.exe” main executable. The file
may look for example like this:
[Camera]
Gx Camera on USB = gxusb.dll
Gx Camera on Ethernet = gxeth.dll
Legacy G2 camera = g2ccd2.dll
ASCOM Camera = ascom_camera.dll
[GPS]
GarminUSB = gps18.dll
NMEA = nmea.dll
[Telescope]
NexStar = nexstar.dll
Meade = meade.dll
ASCOM = ascom_tele.dll
[Focuser]
ASCOM = ascom_focuser.dll
[Dome]
ASCOM = ascom_dome.dll
Individual sections define which driver would be loaded and asked to
enumerate all connected devices of particular type (CCD cameras, GPS
receivers, telescope mounts).
SIPS package already contains this file containing all included drivers. This file
is not modified programmatically, it is necessary to edit it manually if new
device driver, not included into basic package, is installed.
User-specific configuration is stored in the file named also “sips.ini”, but this
file is placed in the “\Documents and Settings\%user_name%\Application
Data\SIPS\” folder. Number of setting is stored in this text file, beginning from
the position and open state of individual tool windows, to the preferred
astrometry catalog and parameters for searching stars in images.
31
G2 CCD Camera Driver for SIPS
SIPS is designed to work with any CCD camera, providing the driver for the
particular camera is installed. The driver for G2 CCD camera is included into
the basic SIPS package and is not necessary to install it separately.
All Gx cameras use common driver 'gxusb.dll' when connected directly to the
host computer or 'gxeth.dll' when connected trough the Gx Camera Ethernet
Adapter.
Common drivers for all Gx cameras were introduced in SIPS version 2.3,
previous SIPS versions used different drivers for G0/G1 and for G2/G3/G4
cameras. G2 cameras used the 'g3ccd.dll' driver (g3 name prefix had historical
reasons, G2 cameras inherited electronics originally developed for G3 series
and because thy were software compatible, they used the same driver), but it
was replaced with the 'gxusb.dll' common driver.
Every CCD camera driver for SIPS (including the G2 CCD drivers) is required
to provide information about available filters (if the particular camera has the
integrated filter wheel, of course). But the user can order camera with various
filters, or he or she can change individual filters or the whole filter wheel etc.
There is no way how to determine the actual filters in the filter wheel
automatically. This is why the G2 CCD camera driver for SIPS reads the
'gxusb.ini' file to determine actual configuration of filters, which will be then
reported to SIPS.
The 'gxusb.ini' file is placed in the same directory where the camera driver and
the SIPS itself is installed. This file is ordinary text file following the .INI files
conventions. Here is the example of the 'gxusb.ini':
[filters]
Luminance, Gray, 0
Red, Lred, 330
Green, Lgreen, 330
Blue, Lblue, 330
Clear, 0, 330
Filters are described in the [filters] Section. Every line in this section
describes one filter position. Filter description is a comma-separated list of
three values:
●
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Filter name: This name is returned to the client application, which
can use it to list available filters in the filter wheel.
●
Filter color: This color can be used by client application to display the
filter name with a color, hinting the filter type. The color can be
expressed by a name (White, Red, LRed, etc.) or directly by number
representing the particular color (0 represents black).
●
Filter offset: Distance to move the focuser to refocus upon filter
change. Plan-parallel glass shifts the actual focus position back for 1/3
of the glass thickness (exact value depends on the glass refraction
index, but for almost all glasses 1/3 is very close to exact value).
Refocusing is useful when changing filters of different thickness
among exposures or when some exposures are performed through
filters and other without filters at all.
Filter offsets can be defined in focuser dependent units (steps) or in
micrometers (μm). If the micrometers are used, it is necessary to inform driver
by the “MicrometerFilterOffsets” parameter in the “[driver]” section of the ini
file.
[driver]
MicrometerFilterOffsets = true
[filters]
Luminance, Gray, 660
...
Value of the “MicrometerFilterOffsets” parameter can be expressed as
keywords “true” or “false” as well as numbers “0” (for false) or “1” (for true).
The above mentioned information will be displayed e.g. in the filter-choosing
combo-box this way:
33
Illustration 11: Filters
offered by the CCD
Camera tool
If there are more filters in the camera than the configuration file describes,
another filters will be added with undefined name. And if the configuration file
describes more filters than the number of filter in the camera, last descriptions
will be omitted.
Using of multiple configuration files for different cameras
It is sometimes necessary to work with multiple cameras, sharing single driver
on the computer (whole series of Gx cameras share 'gusb.dll' or 'gxeth.dll'
drivers). If multiple cameras have different filter wheels with different filters, it
is rather complicated to adopt the 'gxusb.ini' configuration file to currently
connected camera. If there are multiple cameras connected at once, adopting of
configuration file is not possible.
This is why SIPS camera drivers (and also camera drivers for other programs)
introduced enhanced naming convention of driver configuration file. Every Gx
series camera has unique identification number, stated on the camera shell (this
number is also displayed in the list of all connected cameras in the SIPS “CCD
Camera” tool). Camera driver tries to open configuration file, which name is
extended with the camera ID number. If for instance camera ID is 1234, driver
first tries to open configuration file named 'gxusb.1234.ini'. Only is such file
does not exists, general configuration file 'gxusb.ini' is used. So it is possible to
create separate configuration files describing filters in every connected camera.
Cropping of the CCD area
The Gx camera drivers is able to crop the image matrix even before the image
is passed to SIPS. Although it is possible to define sub-frames directly in SIPS
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camera control tool, limiting camera resolution this way is not very convenient
when multiple frames of different types (light, dark, flat) are acquired. If for
instance the user wants to use only center area of a large CCD because the
optics used cannot utilize such large CCD detector, it is possible to read only a
sub-frame (sub-frame 256, 0, 1024, 1024 converts 1.5Mpx G2-1600 camera
into 1MPx camera). But different sub-frame is used e.g. when focusing the
camera and it is necessary to properly restore above mentioned subframe before
each dark, light of flat field is acquired. And 1 pixel difference between light
and dark frame harms the possibility to properly calibrate images.
This is why the 'gxusb' and 'gxeth' drivers allow definition of sub frame in the
appropriate .ini file in the “[crop]” section:
[crop]
x = 256
y = 0
w = 1024
h = 1024
Such camera will report resolution 1024× 1024 pixels to SIPS and all other subframes, defined in the SIPS camera control tools, will be related to the above
defined subframe.
Camera Connection
Camera connection is pretty easy. Plug the power supply into the camera and
connect the camera to the computer USB port using the supplied USB cable.
Note the computer recognizes the camera only if it is also powered. Camera
without power act the same way as the unplugged one from the computer point
of view.
When the camera is powered and connected to the computer (with appropriate
drivers installed), it starts to initialize filter wheel. The internal filter wheel
starts to rotate and the camera control unit searches for the filter wheel home
position. This operation takes a few seconds, during which the camera does not
respond to computer commands. Camera indicates this state by flashing the
orange LED. See the “Camera LED state indicator” chapter for details.
35
Illustration 12: Camera without internal wheel on left, with internal filter wheel on
right. USB connector is on left side and power connector on right side of the camera
head.
The camera is fully powered by the external power supply, it does not use USB
cable power lines. This means it does not draw laptop batteries and long USB
cables with thin power lines (which can cause voltage drops and power-related
problems for USB-powered devices) does not affect the G2 CCD camera
operation.
Camera LED state indicator
There is a two-color LED on the camera body, close to the USB connector. The
LED is functional only upon camera startup not to influence observations.
The LED starts blinking orange when the camera starts to initialize filter wheel.
Orange blinks are not always the same – they depend on the filter position
when the camera is powered up.
If the case the camera control unit cannot find the filter wheel origin, the
camera notifies the user by 2 s long red flash immediately after filter
initialization failed (orange blinking terminates). Please note the while filter
wheel initialization is skipped by the firmware when the camera is supplied
without the filter wheel. So if you notice orange blinks followed by 2 s red
blink, the filter wheel failed to initialize. Although the camera continues
operation like the model without filter wheel, it is not recommended to start
36
work with such camera – it is not clear which filter is behind the CCD or the
wheel can be in the inter-filter position. Return the camera to manufacturer for
maintenance in such case.
Camera firmware finishes initialization by signaling the USB speed, on which
it is currently operating.
●
USB 2.0 High Speed (480 Mbps) is signalized by 4 short green blinks.
●
USB 1.1 Full Speed (12 Mbps) is signalized by 4 short red blinks.
Working with Multiple Cameras
It is possible to connect multiple CCD cameras to single computer, be it
directly to USB ports available on the computer I/O panel or through the USB
hub. The operating system assigns unique name to every connected USB
device. The name is rather complex string derived from the device driver
GUID, USB hub identifiers, USB port number on the particular hub etc. Simply
put, these identifiers are intended for distinguishing USB devices within
operating system, not to be used by computer users.
But the user always needs to distinguish individual cameras – for instance one
camera should be used for pointing, another for imaging. This is why every
camera has assigned unique identifier (ID number). This number is printed on
the sticker on camera body and it is also displayed in the list of all available
cameras in the CCD Camera tool in SIPS. This enables the user to select the
particular camera he or she needs.
37
Illustration 13: Camera Id number is displayed in brackets after
camera name in SIPS
38
Camera Operation
Camera operation depends on the software used. Scientific cameras usually
cannot be operated independently on the host computer and G2 CCD also needs
a host PC (with properly installed software) to work. Camera itself has no
displays, buttons or other controls. On the other side, every function can be
controlled programmatically, so the camera is suitable for unattended operation
in robotic setups.
Plug the camera into computer and power supply and run the SIPS program.
Open the “CCD Camera” tool (choose the “Tools” menu and click the “CCD
Camera...” item or click the
tool button). The camera name (e.g. “G21600”) should be displayed in the title bar of the tool window.
If you run the SIPS before the camera was plugged and powered, SIPS does not
know about it and it is necessary to scan for available cameras. Select the
“Camera” tab and press the “Scan Cameras” button. The G2 CCD camera
should appear in the displayed tree. Select it (click its name by mouse – its
name should be highlighted) and press “Select Camera” button.
If the G2 CCD does not appear in the tree of available cameras, check the
following items:
1.
Check the USB cable – make sure both connectors are properly
inserted to PC (or USB hub) and to camera head.
2.
Check the camera power – the power adapter should be plugged to AC
source (the green LED on the adapter should shine) and the power
output cable connector must be properly inserted to camera head
connector.
3.
Check if the camera system driver is properly installed. Refer to the
“Camera System Driver Installation” chapter for information about
system driver installation.
39
Camera and the Telescope
The camera needs some optical system to capture real images. It depends on the
telescope adapter to which telescopes (or lenses) the camera can be connected.
Standard 2" barrel adapter is recommended if your telescope is equipped with
2" focuser. But the best way to attach a camera to the telescope is threading the
camera to the focuser (be it T-thread, M48×0.75 thread or other standard).
Illustration 14: Complete system consisting of G2 camera, External Filter Wheel, OffAxis Guider adapter and G1 guider on the Newtonian reflector telescope
Photographic lens or some small refractor is the best optical system to start
experimenting with the camera. If you are using some bigger telescope at home
for the first experiments, make sure the telescope can be focused to relatively
nearby objects in the room.
It is better to start experimenting at night, because it is very easy to saturate the
camera at daylight. The shortest exposure can be around 0.1 s, which can be too
long at daylight conditions.
40
The following chapters provide only a brief description of camera operation
under SIPS (Scientific Image Processing System) program, supplied with the
camera. Refer to the SIPS User's Guide (click “Help” and “Contents” from the
SIPS main menu) for thorough description of all SIPS features.
Temperature Control
Active chip cooling is one of the basic features of scientific CCD cameras
(SIPS User's Guide explains why cooling is important to reduce thermal noise).
If you plug the G2 CCD to power supply, you may notice the fan on the back
side of the camera head starts operation. This fan take away the heat from the
hot side of the Peltier modules, which cool the CCD chip. Fan is running
continuously when the camera is plugged to power supply, independently on
the Peltier cooler (it is also used to cool down the camera power supplies etc.).
Peltier cooler can be controlled from the “Cooling” tab of the SIPS CCD
Camera tool.
Illustration 15: Cooling tab of the CCD Camera Control tool
41
Although the Cooling tab displays number of values and graphs, only two
values can be modified by the user. The “Set Temperature” count-box defines
required CCD chip temperature and the “Max. dT” count-box defines the
maximum speed, with which the temperature can change. If the required
temperature is greater or equal to the current CCD chip temperature, the Peltier
cooler is off. The “Cooling utilization” indicator displays 0% and the camera
consumes minimum energy.
To cool down the CCD chip, set the required temperature to target value.
Camera does not switch the Peltier cooler to 100% immediately, but starts
changing of the target temperature according to defined maximal speed. The
target temperature is displayed in cyan color on the graph. The current chip
temperature is displayed in red. Also notice the blue line, which displays the
cooling utilization – it starts to grow from 0% to higher values.
Also notice the yellow line in the graph – it displays camera internal
temperature. This temperature also somewhat grows as the cooling utilization
grows. The hot air from the Peltier hot side warms up the camera interior
slightly.
How fast can be the chip cooled? Can be the chip damaged, if it is cooled too
fast? Unfortunately the maximum speed of temperature change is not defined
for Kodak CCD chips (at last the author does not know about it). But in general
slow temperature changes cause less stress to electronic components than rapid
changes. The SIPS temperature change speed default value is 3 °C per minute.
It is usually no problem to switch the camera earlier and to provide time for
slow cooling. However, if it is necessary to cool the camera rapidly, alter the
“Max. dT” value.
It is also easier to achieve higher temperature differences if the temperature is
changed only slowly. Switching the Peltier cooler from zero to 100%
immediately provides a lot of heat and, especially in the case of air-cooled
Peltier, the overall camera temperature can raise more than necessary. The
result is the chip temperature is higher in absolute numbers, because the hotside temperature is also higher. It takes long time before the hot side slowly
settles.
What is the best temperature for the CCD chip? The answer is simple – the
lower the better. But the minimum temperature is limited by the camera
construction. The G2 CCD cameras are equipped with two-stage cooler, which
can cool the chip up to 50 °C below ambient temperature with air cooling. But
42
it is not recommended to use maximum possible cooling. If the environment
conditions change, the camera may be unable to regulate the temperature if the
environment air temperature rises. Set the target temperature, which requires
approx. 85% of the cooling utilization. This provides enough room for e.g.
environment temperature changes etc.
The power supply voltage is also displayed in the “Cooling” tab. Especially
when the camera is powered from 12 V battery, this information can be used to
estimate when the battery should be replaced and recharged. Note that working
with less intensive cooling can significantly prolong the battery life.
First Images
Actual exposure is performed from the “Exposure” tab of the CCD Camera
tool.
Illustration 16: Exposure tab of the CCD Camera Control tool
It is necessary to define few parameters before the first shot. First, it is
43
necessary to define the image type – choose “Light” from “Exposure” combo
box. Then choose the exposure time. If you experiment with exposures in the
dark room with a camera connected to some f/6 refractor, start with 1 second.
Do not forget to review the image handling options on the right side of the
“Exposure” tab. Let the “Open new Light image window” and “Overwrite
image in selected window” check-boxes checked, uncheck other options for
now (we do no plan to save our first images).
Then click the “Start Exp” button. Camera will open the shutter, perform 1 s
exposure, close the shutter and download the image. Image is then opened in
new image window. If this is the first shot, it will probably be far from sharp
focused image. Alter the focuser and try again.
Notice that options determining the new image handling on the right side of this
tab changes with every change of the exposure type. SIPS remembers these
options for every exposure type separately. So it is possible e.g. to define
separate folders for dark frames and for flat fields.
Always check whether new image processing options are defined properly
before you start any exposure.
If you choose “Dark” from “Exposure” combo box (remember the image
handling options on the right side changes – make sure they are properly
defined), image will be captured without opening the shutter. The captured
image will represent the thermal noise, generated by the CCD chip itself,
combined with the CCD chip and camera electronics read noise. Such images
are subtracted from normal images during image calibration to reduce the dark
current effects.
Brightness and Contrast – Image Stretching
The G2 CCD dynamic range spans 65 536 levels. But only imaging of perfectly
illuminated and perfectly exposed scenes can result in images with pixels
spanning this range. Usually only a fraction of this range is used, e.g. the black
background can have values around 500 counts and the brightest part of the
image can have around 10 000 counts. If we assign the black to white range to
the full possible range (0 to 65 535), the image with 500 to 10 000 counts will
be displayed only in dark gray tones. This is why image brightness scale should
be “stretched” before they are displayed.
Open the “Histogram and Stretch” tool
44
.
Illustration 17: Histogram and Stretch tool
The exact meaning of the histogram chart is explained in the SIPS software
documentation. Now only try to play with “Low” and “High” count-boxes or
better with the related horizontal sliders. Observe how the image view is
changed when you alter these values.
The best positions of Low and High control are as follows: the Low count
should be on the count value representing black on the image. Any pixel with
value lower than this count will be displayed black. The High count should be
on the count value representing white on the image. Any pixel with value
higher than this count will be displayed white.
Similar adjustments are usually called brightness and contrast adjustments.
●
Brightness is changed by moving both Low and High values together
up and down. Try to move both values using the second slider below
the histogram chart.
●
Contrast is changed if the relative distance between Low and High
values changes. Try to narrow or widen the distance between Low and
High values.
But astronomers often need precise control of Low and High values so the
terms brightness and contract are not used within SIPS.
Calibration
If you preform short exposure of bright object, the signal to noise ratio of the
image is very high. Image artifacts related to CCD chip (like hot/cold pixels or
45
thermal noise) almost do not affect the image. But all unwanted effects of
unevenly illuminated field, CCD thermal noise etc. significantly degrade image
quality when imaging dim deep-sky objects for many minutes.
This is why every CCD image should be calibrated. Image calibration basically
consists of two steps:
1.
Dark frame subtraction
2.
Applying flat field
Image calibration is supported by the “Calibration” tool in SIPS
Illustration 18: Calibration tool
46
.
The raw image downloaded from the camera contains not only the information
desired (the image of the target field), but also CCD chip thermal noise and
artifacts caused by unevenly illuminated field (vignetting), shadows of dust
particles on camera cover glass and filters etc.
Illustration 19: The raw image
downloaded from the camera
The Dark frame is taken with the same exposition time at the same CCD chip
temperature. Because hot pixels are less stable than normal pixels, it is always
better to take more dark frames (at last 5) and to create resulting dark frame as
their average or better median.
Illustration 20: The dark frame
corresponding to the above raw image
Illustration 21: The raw image with
subtracted dark frame
Subtraction of the dark frame eliminated majority of thermal noise, but
unevenly illuminated field is still obvious. Image center background is much
brighter than the border parts.
47
Illustration 22: Flat field represents the
telescope/camera response to uniformly
illuminated field
Illustration 23: Fully calibrated image
with dark frame subtracted and applied
flat field
CCD image calibration is described in detail in the SIPS User's Guide. Refer to
the “Introduction to CCD Imaging” and “Calibrate Tool” chapters for
calibration description in theory and in practice.
Color Images with monochrome camera and filters
Color images are definitely more appealing than black and white ones. It is also
easier to gather more information from color images – for instance it is possible
to distinguish which part of the nebula is emission (red) and which is reflection
(blue). But astronomical cameras are only rarely equipped with color CCD
chips from number of reasons. The color and monochrome chips are discussed
in the SIPS User's Guide – refer to the “Introduction to CCD Imaging” chapter.
Although the G2 CCD camera is equipped with monochrome CCD chip, it is
definitely capable to capture color images, at last when the internal filter wheel
contains RGB filters. Instead of shooting single color image, three images –
each for Red, Green and Blue colors, must be obtained and combined. This
process is not suitable for fast moving/changing objects, but astronomical
objects usually do not change so fast.
Taking three images and combining them is undoubtedly more complex
procedure than shooting simple color image. But using of monochrome chip
brings so important advantages for astronomical usage, that bothering with
multiple images is definitely worth the effort:
●
48
Color CCD chips have one fixed set of filters without the possibility to
exchange them or to completely remove them. Monochrome chip is
capable to take images with narrow-band filters like Hα, OIII, etc.
●
Color chips have less Quantum Efficiency (QE) then monochrome
ones. Limiting QE from around 80% to around 30% by color filters
only wastes light in number of applications.
●
Interpolation of pixel luminance from surrounding pixels, necessarily
performed when processing images from color chips, introduces
significant error and prohibits precise measurement of position
(astrometry) and brightness (photometry).
●
Color CCD chips do not allow reading of binned images.
●
Color CCD chips do not allow so-called Time Delay Integration (or
Drift-Scan Integration).
Another huge advantage of monochrome chip is the possibility to combine
color images from three color images and one luminance image. Luminance
image is captured without filter, using maximum chip sensitivity. This
technique is often called LRGB imaging.
Inserting the color filter into the light path significantly reduces the amount of
light captured by the chip. On the other side the human eye is much less
sensitive to changes of color than to changes of brightness. This is why the
CCD chip can be binned when capturing color images to 2×2 or 3×3 to
significantly increase its sensitivity. Luminance image is taken without binning
so the image resolution is not degraded.
Let us note that imaging through separate color filters is close to impossible in
some cases. For instance taking images of some fast evolving scenes, like
planet occultation by Moon, imaging of fast moving comet etc. There is no time
to take separate exposures through filters, because the scene changes between
individual exposures. Then it is not possible to combine red, green and blue
images into one image. In such cases using a single-shot color camera is
necessary.
The color images can be combined in the (L)RGB Add Tool
tool is thoroughly described in the SIPS User's Guide.
in SIPS. This
49
Illustration 24: “(L)RGB Image Add“ tool in SIPS...
50
Illustration 25: ...and a resulting image
If we take images for individual colors and also luminance image, possibly with
different binning and exposure times, the calibration starts to be relatively
complex. We need dark frame for every exposure time and binning. We need
flat field for every filter and binning. We need dark frames for every flat field.
This is the price for beautiful images of deep-sky wonders.
Color images with color camera
Single-shot color cameras use special CCD detectors with red, green and blue
color filters applied directly on individual pixels. G2 CCD cameras can be
51
equipped with such detectors (the name of the camera is then followed by the
letter “C” to indicate color CCD).
Illustration 26: Schematic diagram of color CCD detector
Every pixel receives light of particular color only (red, green or blue). But color
image consists of pixels with all three colors specified. So it is necessary to
calculate other color from the values of neighboring pixels.
Covering pixels with such color mask and subsequent calculations of remaining
colors was invented by Mr. Bayer, engineer working at Kodak company. This
is why this color mask is called Bayer mask and the process of calculation of
missing color is called Debayer processing.
There are several algorithms for calculating missing color components of
individual pixels – from simply using of color from neighboring pixels (this
method provides quite coarse images) to more accurate methods like bilinear or
bicubic interpolation. There are even more sophisticated algorithms like pixel
grouping etc.
No G1 camera performs the Debayer processing itself. The raw image is always
passed to the host PC and processed by control software. It is also possible not
to perform Debayer filtering and save images in the raw form for processing by
some other software packages.
52
SIPS software implements bilinear Debayer interpolation. It is possible to
perform Debayer processing immediately when the image is downloaded from
the camera (color image is then immediately displayed and/or saved and no raw
monochrome image is shown) or to perform this processing anytime later.
Debayer processing can be performed from “Image Transform” tool (to open
this tool click
button in the tool-bar or choose “Image Transform” from the
“Tools” menu). Check box “Debayer new images” allows immediate Debayer
processing of images downloaded from the camera. The
Debayer processing of currently selected image.
button performs
The Bayer mask displayed on the schematic image above begins with blue
pixel. But there are no rules specifying the color of the first pixel – in principle
there can be also green pixel from the blue-green line on the upper-left corner
as well as green pixel from the green-red line or red pixel.
There is no way how to determine the Bayer mask organization from the image.
This is why the “Image Transform” tool provides two check-boxes called
“Bayer X odd” and “Bayer Y odd”. Combination of these check-boxes allows
specification of Bayer mask organization on the particular CCD.
State of “Bayer X odd” and “Bayer Y odd” check-boxes are always updated
when you connect camera with color CCD according to the information
provided by the driver. Is is necessary to update them manually only if the raw
color image is loaded from the disk file and needs to be processed without
connected camera.
Wrong definition of these two flags results in wrong color calculation. Proper
settings can be easily determined by the try-and-error method. But Debayer
processing discards the original raw image so it is always necessary to backup
the original raw image.
Also please note the settings of the “Bayer X odd” and “Bayer Y odd” check
boxes must be altered when any geometric transformations are applied to the
raw image (e.g. mirroring, rotation, etc.). Some transformations (e.g. soft
binning or resampling) cannot be performed on raw image at all. It is always
better to Debayer images first and process them later.
Also note that stacking of raw color images results in loss of color information.
Stacking algorithms align images regardless if the particular pixel is red, green
or blue. SIPS allows also sub-pixel stacking, which can mix pixels of different
53
colors. Images must be Debayer processed first and then stacked.
Balancing colors
CCD chip sensitivity to red, green and blue light is different. This means the
exposure of uniformly illuminated white surface does not produce the same
signal in pixels covered with different color filters. Usually blue pixels gather
less light (they have less quantum efficiency) then green and red pixels. This
results into more or less yellowish images (yellow is a combination of red and
green colors).
The effect described above is compensated by so-called “white balancing”.
White balancing is performed by brightening of less intensive colors (or
darkening of more intensive colors) to achieve color-neutral appearance of
white and/or gray colors. Usually is one color considered reference (e.g. green)
and other colors (red and blue) is lightened or darkened to level with the green.
Automatic white balancing can be relatively easy on normal images, where all
colors are represented approximately uniformly. But this is almost impossible
on images of deep-space objects. For instance consider the image of emission
nebula, dominated by deep-red hydrogen alpha lines – any attempts to lighten
green and blue color to create color-neutral image result to totally wrong color
representation. Astronomical images are usually color balanced manually.
As already described in the “Brightness and Contrast – Image Stretching”
chapter, image can be visually brightened by altering its stretch limits. SIPS
“Histogram and Stretch” tool displays and also enables altering of stretching
curve limits and shape for red, green and blue color individually.
Illustration 27: Histogram and Stretch tool shows histograms of individual colors
54
Some General Rules for
Successful Imaging
Advanced CCD cameras caused a revolution in amateur astronomy. Amateurs
started to capture images of deep-sky objects similar or surpassing the ones
captured on film by multi-meter telescopes on professional observatories.
While the CCD technology allows capturing of beautiful images, doing so is
definitely not easy and straightforward as it may seems. It is necessary to gain
experience, to learn imaging and image processing techniques, to spend many
nights mastering the technology.
Although CCD camera can convert majority of incoming light into information,
it is not a miracle device. Keep on mind that laws of physics are sill valid.
●
CCD camera does nothing more than converting image created on the
chip by telescope (or objective lens) into information. A quality
telescope and quality “photographic-class” mount is absolute must for
successful imaging. If the mount cannot keep the telescope on track or
the telescope cannot create perfectly focused image, result is always
distorted and blurred.
●
Ideally the exposures should be automatically guided using guiding
CCD camera or at last webcam or similar device. Tracking errors
caused by drive periodic error, mount polar misalignment or other
mechanical issues (often unnoticeable by eyes) cause streaking of star
images. Note the exposure time for each color often reaches tens of
minutes or even hours if the really high quality images are taken.
The G1 series of CCD cameras are especially designed with guiding
on mind. G1 CCD cameras are equipped with “autoguider” connector,
which allows direct connection between the G1 camera head and
telescope mount. 16-bit digitization and using of sensitive Sony
EXview HAD CCDs provide higher sensitivity and dynamic range
compared to typical video or web cameras. The SIPS software
package supports both imaging and guiding cameras and implements
sophisticated guiding algorithms.
55
●
Focus image properly. Almost unnoticeable focuser shift affects star
diameter. Focusing, especially on fast telescopes, is critical for sharp
images. Electrical focuser is a huge advantage, because it allows
focusing without shaking the telescope by hand and with precision
surpassing the manual focusing.
Keep on mind that the star images are affected not only by focusing,
but also by seeing. Star images will be considerably bigger in the night
of poor seeing, no matter how carefully you focus.
●
Master image calibration (dark frame subtraction and flat fielding) and
carefully calibrate all images. Various artifacts (thermal noise, hot
pixels, gradients, telescope/lens vignettation, dust particles on filters
etc.) degrade the image and properly calibrated image always looks
better. Take care to obtain dark frames and flat fields for all filters
used, for all resolutions/binnings etc.
●
If the image is processed to be as aesthetic as possible, other
processing than basic calibration can significantly improve its
appearance. Nonlinear stretching (called “curves” in some imageediting packages), special filters (hot/cold pixels removal, noise
reduction etc.) and other processing (e.g. deconvolution) enhances the
image.
Never perform these enhancing filters on images intended for data
reduction processing. It is always good idea to store original image
and to enhance only its copy. Scientific information can be
significantly degraded by various noise filters, deconvolution etc. If
for instance the image of some galaxy contains newly discovered
supernova, photometric reduction of the original image can be
scientifically very important.
56
●
A common saying “there is a science in every astronomical picture” is
especially true for CCD images. Examine your images carefully, blink
them with older images of the same object or field. There is always a
chance some new variable star, new asteroid, new nova or supernova
appear in the image.
●
Be patient. Although many advertisements proclaim “capture images
like these your first night out”, they probably mean your first
successful night out. Nights can become cloudy or foggy, the full
Moon can shine too much, the seeing can be extremely bad… Number
of things can come wrong, but the bad luck never lasts forever. Start
with bright objects (globular clusters, planetary nebulae) and learn the
technique. Then proceed to more difficult dimmer objects.
If you are new to CCD imaging and terms like “dark frame”, “read noise” and
“image binning” sound strange to you, refer to the “Introduction to CCD
Imaging” chapter of the SIPS software documentation. This chapter explains
basic principles of CCD operation and their usage in astronomy, discusses color
imaging, CCD chip dark current and camera read noise, chip resolution and
pixel scales in relation to telescope focal length, explains basic image
calibration etc.
57
Camera Maintenance
The G2 CCD camera is a precision optical and mechanical instrument, so it
should be handled with care. Camera should be protected from moisture and
dust. Always cover the telescope adapter when the camera is removed from the
telescope or put the whole camera into protective plastic bag.
Desiccant exchange
The G2 CCD cooling is designed to be resistant to humidity inside the CCD
chamber. When the temperature decreases, the copper cold finger crosses
freezing point earlier than the CCD chip itself, so the water vapor inside the
CCD chamber freezes on the cold finger surface first. Although this mechanism
works very reliably in majority of cases, it has some limitations, especially
when the humidity level inside the CCD chamber is high or the chip is cooled
to very low temperatures.
This is why a cylindrical chamber, filled with silica-gel desiccant, is placed
inside the G2 CCD camera head. This cylindrical chamber is connected with
the insulated cooled CCD chamber itself.
Warning:
High level of moisture in the CCD chip chamber can cause camera malfunction
or even damage to the CCD chip. Even if the frost does not create on the
detector when the CCD is cooled below freezing point, the moisture can be still
present. It is necessary to keep the CCD chamber interior dry by the regular
exchange of the silica-gel. The frequency of necessary silica-gel exchanges
depends on the camera usage. If the camera is used regularly, it is necessary to
dry the CCD chamber every few months.
It is possible bake the wet silica-gel in the oven (not the microvawe one!) to dry
it again. Dry the silica-gel for at last one hour at 150 to 160 °C. Exceeding the
170 °C can damage the silica-gel and its ability to absorb moisture will be
limited.
The silica-gel used in G2 cameras changes its color according to amount of
58
water absorbed – it is bright yellow or orange when it is dry and turns to
transparent without any color hue when it becomes wet.
Changing the silica-gel
G2 cameras have the container accessible from the back side of the camera
head.
Illustration 28: Silica-gel container is under the screwed cap with slot, right of the fan
vents
The slotted desiccant chamber cap can be unscrewed e.g. by a coin. Pour out
wet silica-gel and fill the chamber with a dry one. The desiccant chamber can
be filled with a hot silica-gel without a danger of damaging of the container.
The desiccant container can be left open without the fear from contamination of
CCD chamber interior by dust. There is a very faint stainless steel grid between
the CCD chamber and the desiccant container, so dust particles cannot enter the
chamber itself. It is even recommended to keep the desiccant container cap off
for a couple of hours when the camera is in the room with low humidity. This
helps drying the CCD chamber interior and prolongs the silica-gel exchange
interval.
G2 cameras revisions 1 and 2 employed desiccant container accessible only
from inside of the camera. It was necessary to open the camera head before the
desiccant could be replaced.
59
Changing Filters
It is necessary to open the camera head to change filters or the whole filter
wheel. To open the head unscrew the six bolts holding camera head together.
Opening the camera head
The blade shutter rotates 180° between individual snapshots. Camera cover
could be opened only when the shutter fully closed (covers the CCD). If for
instance the camera is unplugged from power adapter while exposing, the
shutter remains open. Camera cannot be opened in such case.
Warning:
Shutter can be damaged while removing the camera cover if not in proper
position.
After removing the screws carefully turn the camera head by the telescope
adapter upward. Gently pull the front part of the case. Notice there are two
cables, connecting the filter wheel motor and the filter position optical bar,
plugged into the electronics board. It is not necessary to unplug these cables to
change filters, but if you unplug them, take care to connect them again in the
proper orientation!
Illustration 29: Filter positions are marked on the filter wheel
60
Illustration 30: Filters cane be exchanged after removing of the camera front cover
Changing the Whole Filter Wheel
The whole filter wheel can be changed at once. It is necessary to remove the
front part of the camera case the same way as in the case of changing filters.
The filter wheel can be removed when you unscrew the bolt on the center of the
front part of camera case. Take care not to damage the horseshoe-shaped
optical bar when replacing the filter wheel.
Changing the Telescope Adapter
The camera head contains bolt square. The telescope adapter is attached by four
bolts. If you want to change the adapter, simply unscrew these bolts and replace
the adapter with the new one.
Power Supply Fuse
The power supply inside the camera is protected against connecting of invertedpolarity power plug or against connecting of too-high DC voltage (above 15 V)
by a fuse. If such event happens and the cooling fans on the back side of the
camera do not work when the camera is connected to proper power supply,
return the camera to the service center for repair.
61
G2 Camera Revisions
G2 series of CCD cameras underwent several revisions, each implementing
various enhancements, new printed circuits boards with latest electronics
components and utilizing all gained experience and new ideas.
Revision 1
The first introduced G2 camera. Only G2-0400 model with KAF0402ME CCD
was available in this revision.
●
Cameras used USB 1.1 interface working at 12 Mbps transfer speed.
●
Camera driver was called “g2ccd”.
●
Image download time was approx. 3-times longer compared to
revision 4.
●
Peltier hot side was cooled by two fans.
●
Desiccant container was hidden inside the camera head, it was
necessary to open the head to exchange it.
●
Internal filter wheel has 98 mm diameter.
Revision 2
The second revision of G2 cameras added models G2-1600 and G2-3200 and
62
the original model changed name to G2-0402.
●
Digital electronics was completely overhauled and upgraded to USB
2.0 interface with 480 Mbps transfer speed.
●
The air intake vents shaping was slightly changed to make the camera
somewhat quieter. This revision still used two fans.
●
Camera driver was named “g2ccd2”, the digit “2” in the suffix
signalized USB interface version.
Revision 3
Yet another version of the digital electronics was developed for the new G3
series of CCD cameras with detectors up to 36 × 24 mm (digital electronics
board is the same for various models of the same series, individual models
differ in analog electronics, designed for every particular type of CCD). The
same electronic module was later reused also in G2 cameras and even later in
the even bigger G4 series. The third revision of G2 cameras was introduced.
●
The most prominent visual difference of revision three is the usage of
a single and slightly larger (and again slightly more quiet) cooling fan
63
instead of two smaller fans employed by previous revisions.
Information on the camera head (Camera Id, Serial number etc.) was
placed on the sticker at first, but later it was laser engraved directly to
the aluminum shell. But this difference is only visual and does not
define new revision.
●
The enlarged desiccant barrel allowed much easier desiccant exchange
without opening of the camera head (accessible from the outside).
●
Number of models of G2 cameras were offered – KAF detector
variants (G2-0402, G2-1600, G2-3200), KAI detector variants (G22000, G2-4000) and also “astrophotographic” model with ABG KAF
detector (G2-8300).
●
Internal filter wheel diameter was shrunk to 95.5 mm.
●
Camera driver was named “g3ccd”. The slightly confusing naming of
G2 and G4 camera drivers originated here – all three series (including
the G3 one) used the same, software compatible electronics and thus
also the same driver.
More exactly two drivers for two versions of these cameras were
introduced, one for Full-Frame KAF CCD based cameras (g3ccdF)
and the second for Interline-Transfer detectors KAI CCD based
cameras (g3ccdI). Differentiation is necessary because of fundamental
differences in driving of CCD chips of the above mentioned
architectures.
Number of internal updates were performed on G2 cameras revision 3, for
instance variants for external filter wheel were developed, additional heating of
the cold CCD chamber front optical window was added, near-IR preflash
capability was introduced for KAF based models etc. But cameras were the
same from the outside and they were software compatible and used the same
driver.
Revision 4
Development of another version of digital electronics, using modern electronics
circuits, required introduction of new revisions of G2, G3 and G4 cameras. G2
series advanced from revision 3 to 4 and G3 and G4 series are now in
revision 2.
64
●
The G2 revision 4 shell underwent a slight facelift. Camera is less
“boxy” and more similar to bigger G3 and G4 models. Air intake vents
are better protected against unwanted interference of rotating fans with
either fingers of various wires (hitting a rotating fan with finger is not
dangerous, only somewhat unpleasant).
Basic mechanical dimensions (width, height, depth) of the camera
heads did not change. Only the distance of the optical axis relative to
camera body center axis is slightly shifted by 2 mm.
●
Desiccant container of the G2 cameras was enlarged, it is now
identical to the container used on G3 and G4 models.
●
CCD cold chambers of all models were significantly redesigned. New
chambers bring better CCD insulation from the environment and thus
require less frequent desiccant exchanges. They also allow usage of
CCDs without cover glass, which are much more demanding to
insulation, absence of moisture etc.
●
New digital electronics uses 480 Mbps USB 2.0 interface (Gx cameras
use only a small fraction the transfer capacity of this interface, so
moving to 4.8 Gbps USB 3.0 brings no advantages, only problems
with bigger connector, shorter and less flexible cables etc.), still it
brings numerous important improvements, like faster image download
(especially in the Preview mode), much bigger internal memory
buffers or more precise temperature regulation.
●
New drivers were unified and named “gXccdF” and “gXccdI” (for FF
and IT detectors). G2 cameras employing Back-Illuminated E2V
detectors use new driver “gXccdBI”.
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