FW8S-STT / FW8G-STT Filter Wheel Installation Instructions

FW8S-STT / FW8G-STT Filter Wheel Installation Instructions
Installation Instructions
Filter Wheel
SBIG Astronomical Instruments, A Division of Diffraction Limited.
59 Grenfell Crescent, Unit B, Ottawa, ON Canada, k2G 0G3
Tel: 613.225.2732 | Fax: 225.225.9688| E-mail: [email protected] | www.sbig.com
© 2015 Diffraction Limited. All rights reserved. The SBIG wordmark and logo are trademarks of
Diffraction Limited, All other trademarks, service marks and tradenames appearing in this brochure are
the property of their respective owners.
SBIG's STT series cameras contain several innovative features that can
be found in no other astro-imaging camera. This innovation extends to
STT accessories as well, including the integrated design of the FW8SSTT and FW8G-STT filter wheels. These filter wheels share common
mechanical and electronic design, the only difference being the type of
housing that is used as a front cover and some included items (see page
6). The FW8S-STT standard filter wheel can therefore be upgraded to an
FW8G-STT self-guiding filter wheel by simply adding a replacement
self-guiding front cover. Conversely, if you have the self-guiding
version and want to use a camera lens adapter you can simply add the
standard front cover which accepts Canon EOS or Nikon lens adapters.
Micron-Precision Filter Positioning
When taking flat field frames of filtered
images, the flat field frame must show
exactly the same optical characteristics to be
most effective. If there is dust on the filter,
or uneven illumination caused by the filter,
this must be represented precisely as seen in
the light image to be corrected by the flat field frame. The STT filter
wheels are designed to provide a new level of accuracy when taking flats
through filters by repositioning filters to within a few microns of the
same position every time they are centered over the imaging CCD. This
degree of precision allows the imager to take extremely accurate flat field
frames even after rotating the filter out of the field of view and even after
a loss of power. [Note: The filters offered by SBIG with the STT series
wheels are 36mm round unmounted filters needed by the largest sensor
in the STT camera series, the KAF8300. However, the STT-3200ME
and STT-1603ME can also use 1.25” filters without vignetting. If you
already have 1.25" filters and wish to use them with the STT, inserts are
available that reduce the filter slots to 1.25".]
Self-Guiding In Front of the Filters
Self-guiding has long been acknowledged as
the best, most accurate way to guide long
exposure astrophotos, particularly with
commercial SCTs. The single most common
complaint, however is the difficulty of finding
bright stars when guiding through dark or
narrowband filters. The FW8G filter wheel incorporates the self-guiding
CCD inside the front cover of the filter wheel so that the guider picks off
light from the guide stars before passing through the filters. There are
two adjustment knobs on the filter wheel base: one for moving the
guiding CCD to achieve good focus and one for moving the pick-off
mirror. A third knob on the side locks the adjustments in place.
Attaching the Filter Wheel to the
1. Remove the Front Cover Plate of the
STT camera by unscrewing the eight 440 x1.25" hex head screws around the
perimeter of the face plate (Fig. 1). You
can use the 3/32" hex wrench that came
with your STT camera to loosen the
front plate screws. You will reuse these
screws to attach the filter wheel base
plate to the STT camera body.
2. Remove the Front Cover Plate of the
Filter Wheel in the same manner,
unscrewing the four screws (4-40
socket head cap screws) that that attach
the cover to the base plate (Fig 2
3. Separate the filter retainer from the
carousel by unscrewing the 8 Philips
Flat Head screws that attach the filter
retainer to the carousel. The filter
retainer is the same size and shape as
the carousel and simply attaches to the
carousel over the filters to hold them in
place. (Fig. 3).
4. Attach the filter wheel base
plate to the STT camera body with
the eight x 4-40 x 1.25" hex head
screws that you previously
removed from the camera front
cover plate in Step 1. You may
have to rotate the filter carousel
slightly to see all eight of the holes
5. Determine the location you would
like for each filter. The carousel slots
are numbered 1-8. You can put any
filter in any numbered position and
then tell the software which filter is
number 1,2,... etc. In CCDOps,
starting with number 1, the default
order is Red, Green, Blue, Clear. If
you are using filters that are 3mm thick,
put one of the supplied o-rings into the
filter slots before dropping in the filter.
If the filters are less than 3mm thick
(e.g., Baader) put the filter in first, then
the o-ring. For filters over 3 mm thick,
do not use the o-ring (Fig. 4).
6. Replace the filter retainer plate over the filters and fasten it to the
carousel using the eight flat head screws and washers that you removed
in step 3, above.
7. Replace the filter wheel cover being careful not to pinch any wires or
the I2C cable. Fasten the cover to the base plate using the four 4-40
socket head cap screws that you removed in step 2, above.
10. The filter wheel has a
short cable extending from it
with a 9-pin (DB9) connector.
Plug this connector into the
I2C port on the camera. This
cable is the same for both
standard and self-guiding filter
wheels and supplies power and
control signals to the filter
wheel carousel. If you have
the FW8G-STT Self-Guiding
filter wheel, also attach the
supplied HDMI cable between
the HDMI port on the self-guiding filter wheel cover and the Remote
Guide Head port on the camera. (Fig. 5).
If not already attached, install the t-thread adapter plate in the FW8SSTT standard filter Wheel, or the STL thread (2.156" opening) adapter
plate in the FW8G-STT self-guiding filter wheel.
Adjusting the Pick-Off Mirror (FW8G-STT)
The STT guider assembly has three black knobs that the user
can adjust externally without opening the cover. The knob by itself on
one side is a lock knob that, when tightened clockwise, will press against
the Mirror and CCD slides and lock them both down. Of the pair of
knobs on the other side of the assembly, the knob closest to the centerline
adjusts the CCD focus slide, and the one farther to the side adjusts the
mirror slide (Figure 6). They are easily set for optimal operation as
First, mount the camera/wheel/guider assembly to the telescope. Point
the telescope at a nearby wall or flat field screen illuminated by ambient
light or a penlight of some kind. Total darkness is not necessary. Start
up the imaging CCD in focus mode
with an exposure adequate to get a
uniform, not saturated field. A count
level of 5,000 to 50,000 counts
should be fine. The CCD does not
need to be focused on a star field
first. Loosen the lock knob on the
side of the guider assembly so the
slides are free to move (about a turn).
Next, while watching the image
updating on the computer screen,
rotate the mirror slide knob counterclockwise, which will push the mirror
closer to the optical axis. There is no danger of the screw falling out:
when you reach the limit of its travel it will jam up. You should
eventually see a shadow start to appear at the bottom of the image, which
is the shadow of the mirror vignetting the imaging CCD. This is
undesirable. Adjust the image brightness in focus mode so that the
contrast is not too stretched, and then back out the mirror by rotating the
knob clockwise until the shadow is fully off the screen. This adjustment
depends on telescope choice and F/number, which is why we cannot
preset it at the factory. Also, it is important to get the mirror as far in as
you can without vignetting – popular Schmidt-Cassegrain telescopes
have limited exit apertures and can barely illuminate a guide chip offaxis. [TIP: Once this knob has been positioned we recommend putting
some tape over it so you don’t accidentally grab the wrong knob in the
dark and change the adjustment – Its easy to do! Black electrical tape is
best since it can be removed in the future with the least mess.]
Adjusting the CCD Focus (FW8G-STT)
Once the mirror slide is adjusted, point the telescope at the sky and focus
the imaging CCD on a star field using the telescope focuser controls. If
you plan on using filters do this with a filter installed. Next, view the
field hitting the guide CCD by starting it up in focus mode with an
exposure of 1 to 3 seconds, and focus that image by turning the more
central knob on the assembly, the CCD slide, back and forth. Finding
stars initially can be a bit tough, so go to an open cluster or crowded area
to facilitate this. Once you see stars, focusing on them is easy due to the
action of the built-in focal reducer. A critical focus is not required for the
guider. If you change filter thicknesses over the imaging CCD the guider
focus may need to be adjusted, but with reasonably matched filters this is
not necessary. You will probably see a gradient in the sky background
due to the apertures upstream of the assembly limiting the field of view,
but this does not affect guiding. It merely causes stars farther off-axis to
be dimmer, if it is occurring.
Once the focus is good, rotate the knob on the side of the
assembly clockwise to lock the two slides, which should not perturb the
focus. If it does, you may need to repeat the CCD focusing step. Do not
over-tighten the lock knob: just snugging it up is fine. You are now
ready for guided imaging with your STT camera.
Note: the star images on the tracking CCD may not be as good as your
imaging CCD since the focal reducer reduces the F/number by 0.7X, and
you are quite a ways off axis. This is not a problem for guiding.
Software Settings
If your software supports SBIG cameras and you have the latest drivers,
you should be able to select the FW8-STT filter wheel in the software
setup. If for some reason you do not see this filter wheel in the list of
supported SBIG filter wheels, try selecting select FW8-STL or CFW10
as these older models use the same control codes.
In the Box
There are different items included with the different models and
replacement covers. The self-guiding filter wheel requires a larger
aperture than the t-thread adapter so it includes an STL 2.156" thread
adapter along with a special STL to 2" nosepiece. The standard model
uses a t-thread adapter and can therefore accept the t-thread to 2"
nosepiece that comes with the STT camera.
Listed below are the items included with each model and replacement
cover of STT filter wheel:
FW8S-STT Standard Filter Wheel
Filter Wheel with I2C pigtail
Standard Cover
T-thread Adapter Plate
Eight O-rings
FW8G-STT Self-Guiding Filter Wheel
Filter Wheel with I2C pigtail
Self-Guiding Cover
Short HDMI Cable
STL Thread Adapter Plate and STL Thread to 2" Nosepiece
Eight O-rings
Replacement FW8S-STT Standard Front Cover
Standard Front Cover Housing
Four 4-40 x 3/8 inch socket head cap screws
Replacement FW8G-STT Self-Guiding Front Cover
Self-Guiding Cover Housing
1' HDMI Cable
STL Thread Adapter Plate and STL Thread to 2" Nosepiece
Four 4-40 x 1inch socket head cap screws
Adapter plates:
STL (2.156"), SCT (2.00"), T-thread (T-thread is not
recommended for self-guiding filter wheel).
AO-8T Adaptive Optics
The AO-8T Adaptive Optics accessory is compatible with the
FW8G-STT self-guiding filter wheel. The AO-8T is identical to
the AO-8 except for the bottom plate which has been modified
to match the hole pattern on the STT cameras. The AO-8T
cannot be used with the standard filter wheel cover because it
requires a guiding CCD to monitor the guide star.
Canon EOS and Nikon Lens Adapters
Canon EOS and Nikon lens adapters are compatible with the
FW8S standard filter wheel. They cannot be used with the selfguiding filter wheel because the backfocus requirements of the
lenses are too short for the self-guiding version. In this case,
one can use the self-guiding cover for longer focal length
telescopes and add the standard cover for use with camera
lenses where external guiding is quite acceptable. Guiding with
the standard filter wheel can be accomplished using the remote
guide head or separate guider like the ST-i.
Cleaning Filters
Filters can be cleaned with cotton swabs and isopropyl alcohol. A can of
compressed air is good for removing dust. Do NOT tip the can or shake
it when dusting - tilt the filters (or the whole camera if attached) up
slightly instead.
Mechanical Backfocus
Telescope Adapter
Filter Size
Filter Positions
FW8G-STT (Self-Guiding)
FW8S-STT (Standard)
8.3 x 6.3 x 1.5"
8.3 x 6.3 x 0.8"
(210 x 160 x 38mm)
(210 x 160 x 20mm)
39.7 oz (1150g)
28.5 oz (825g)
2.128" (54.06mm)
1.425" (36.2mm)
STL thread with 2" nosepiece
Suggested Focal Ratio
F/4 or slower
F/2 or slower
Camera Lens Adapters
Not Available
Canon EOS, Nikon
Guiding CCD
648 x 484
Pixel Size
7.4 u
A/D Resolution
Imaging Exposure Time
0.12 to 600 sec.
Guiding Exposure Time
0.001 to 600 sec.
Ver. 1.1 Aug. 8, 2012
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