SBIG CFW-9 User manual

SBIG - Santa Barbara Instrument Group
CFW-9
Color Filter Wheel
Manual

©2006 Santa Barbara Instrument Group
147-A Castilian Drive
Santa Barbara, CA 93117
Telephone [phone]
FAX [phone]
Copyright Notice
This manual is copyrighted by Santa Barbara Instrument Group
(SBIG). This manual may not be copied in whole or in part.
CFW-9 Manual
Version 1.1
Printed 2006

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TABLE OF CONTENTS
Introduction ....................................................................................... 1
Overview of Taking and Processing Color Images...................... 2
Telescope Work.......................................................................... 2
Color Image Processing............................................................. 3
Hardware Reference......................................................................... 4
Making the Electrical Connection............................................ 7
Maintenance of the Color Filter Wheel ................................... 12

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The CFW-9 Color Filter Wheel
INTRODUCTION
This manual describes Santa Barbara Instrument Group's CFW-9
Color Filter Wheel. This product can be used with CCDOPS, CCDSoftV5,
and other software to facilitate taking and processing of color images with
USB versions of the ST-7/8/9/10/2000 cameras. This list may be expanded
as we add new models. Many third party software packages also support
SBIG products.
The Color Filter Wheel is an accessory that mounts on the front of the
camera. The filter wheel has five filter positions, typically for Red, Green,
Blue, and Clear Filters, plus 1 Clear Aperture position for additional filters
of your choice. "Primary" color images are taken through the Red, Green
and Blue Filters and the Clear Filter allows ordinary observing without
having to refocus.

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Overview of Taking and Processing RGB Color Images
This section provides an overview of the steps necessary, both at
the telescope and in front of the computer, to take and process
tri-color images.
Telescope Work
At the telescope you use camera control software such as
CCDOPS to take sets of three images for each object. The images
represent the Red, Green and Blue primary colors (For LRGB
imaging an additional grayscale image is captured. This technique
is discussed elsewhere). Take these images as you would for
monochromatic images except use the filter wheel to select each of
the three primary spectral bands.
You can take equal duration exposures in the Red, Green, or
Blue or match the exposure duration of each primary image to the
brightness of the object. Typical ratios of Red : Green : Blue are 1 : 1 :
1.6 to match the exposure to the CCD's sensitivity using SBIG Filters,
although accurate color balance should be done for your system
with neutral gray objects or "white" G5 type stars.
Each of these monochrome gray scale images should be dark
subtracted1and should be saved with a name that indicates the color
of the filter (e.g., M51R, M51G, M51B). Note, however, that different
software programs may handle tri-color files differently and the
naming convention you use is dictated by your software. If you are
using a fast system (low F number) and your images show signs of
1You may also want to use the other image enhancement features
of CCDOPS such as removing cool pixels. In general the three
image set should be made to look their best prior to merging the
RGB image.

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vignetting (such as a variation in the sky background level) then you
should also take flat field images and flat field correct your primary
images using CCDOPS. Whether you need to take different flat field
images for each primary color or not will depend on how different
the vignetting pattern is for the different primary images. Typically
a single flat field image will suffice.
Color Image Processing
A complete description of color processing is beyond the scope
of this manual. A complete description and step-by-step
instructions may be found in the CCDSoftV5 manual. Different
software packages may handle the combining differently, but in
general, you register or co-align the primary color images. You
select the Red image from the set of three primary images you want
to register and the software then loads and displays it. In that image
you select a reference star (or pixel) and this reference star (or pixel)
is used to register the Red, Green and Blue images to one another.
Once the images have been registered, they are merged together to
form a tri-color image and minor adjustments are made to achieve
color balance.
Third party software packages such as Maxim DL and Adobe
Photoshop are popular programs for combining the three primary
images. While these packages tend to be expensive, often times you
can find last year's versions at a sharp discount and they really are
powerful image processing packages producing the most
professional results.
Note: In most cases, before doing a color merge, attempt to have
all RGB images saved with a similar range or contrast setting. This
will make the initial result closer to what you desire.

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Hardware Reference
This section describes the CFW-9 Color Filter Wheel accessory
for the ST-7/8/9/10/2000 USB cameras. It includes instructions on
how to install and operate the filter wheel.
Attaching to the CCD Camera
The CFW-9 is installed onto the camera and sits between the
Camera Head and the Telescope.
The filter wheel contains a magazine with five filter positions
capable of holding standard threaded 1.25 inch eyepiece filters.
SBIG provides optional filters for use with the CFW-9. The filter
wheel attaches to other accessories through the universal T-Thread.
The front and back of the CFW-9 have apertures with female T-
threads. Using T-Thread makes the CFW-9 adaptable to a variety of
telescopes. You can attach a 1.25 inch or 2 inch nosepiece into the
front or you can attach an adapter that mates directly to the back of
SCTs, replacing the standard Visual Back.

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A permanent mounting of the CFW-9 to the ST-
7/8/9/10/2000 cameras is accomplished by attaching the CFW-9
directly to the face plate of the camera by the following procedure:
1. Remove the ST-7/8/9/10/2000's "D-block" and T-Thread ring
held in place with 4 phillips head screws and set the screws aside.
[The T-Thread mount is where the nose piece screws into the camera
head. It is a block approximately ¼” thick and shaped like the letter
“D” attached to the face plate of the camera. It has three set screws
holding an internal ring that has female T-Threads].
2. Remove the cover of the CFW-9 and pull the filter magazine
out of the housing.
3. Remove the four rubber hole plugs from the rear of the CFW-
9by pushing them through the housing with a small allen tool. Save

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these in case you decide to later use the CFW-9 with the T-Thread
coupler or with a different camera.
4. Using the four 4-40 by 3/16 socket screws provided, attach
the CFW-9 cover to the front of the ST-7/8/9/10/2000. Make sure
you use the provided screws whose length of 3/16 is required to
thread into the ST-7/8/9/10/2000 front plate without breaking
through the blind tapped holes. Then re-assemble the carousel and
housing.

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Making the Electrical Connection
The CFW-9 has a cable that comes out the side that needs to
connect to your camera electronics. The CFW-9 gets it power and
positioning information from the camera thru this connector. This
cable plugs into the I2C-AUX port on the ST-7/8/9/10/2000 USB
camera. Note: This is different from the earlier CFW8A which
used the AO/CFW/SCOPE port on both parallel port cameras and
USB cameras. You cannot use the CFW9 with our earlier parallel
port cameras. You will still plug the autoguider cable into the
AO/CFW/SCOPE connector using the ST-7-RC Adapter plug.

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Attaching to the Telescope
At the front of the CFW-9 you can attach any T-Thread based
adapter that will connect to your telescope, including the nosepiece
from your camera. When using a nosepiece to slip the camera and
filter wheel assembly into an eyepiece holder, you need to pay
careful attention to insure the weight of the filter wheel with the
camera attached is held in place securely with the eyepiece holder's
retaining screws. A safety strap is recommended. SBIG also sells an
optional T-Thread Visual Back Adapter which screws directly onto
the back end of the popular Schmidt-Cassegrain telescopes. This
makes for a very rigid mounting.
After attaching the camera/filter wheel combination to the
telescope, you should then drape the Camera's head cable over the
fork of the telescope and clip it in place so that the cable doesn't drag
on the ground as the telescope rotates. Also avoid hanging the
camera head and filter wheel by the cables as this could overly strain
the cables.
Positioning the Filter Wheel
The motorized CFW-9 filter wheel is positioned using
commands in the CCDOPS software's Filter menu. The unit does
not require calibration like older models and is auto-homing at
power-up. You need to tell the CCDOPS software that you are using
the CFW-9 and this is accomplished by selecting the CFW-9 as the
filter wheel type in the Setup command of the Filter menu. If you
have an older version of CCDOPS that does not show the CFW-9 you
should update your software and drivers by downloading the latest
version of CCDOPS and Driver Checker Utility from the SBIG web
site.

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There are individual commands in the Filter menu with hot keys
for positioning the filter wheel. Also the Camera Status window
shows which filter is currently in position, and this information is
recorded with images when they are captured. You can rename the
filters using the Filter Setup command. If you have removed the
filter wheel from your camera or removed the filter from the
magazine you can select the None command in the Filter menu. The
None command doesn't position the filter wheel, it just tells the
software that you aren't using a filter at the moment. If you run
without filters for a period of time you should use the Filter Setup
command to select the "None" type of filter wheel. Use the Setup
command in the Filter menu to tell the CCDOPS software which
filter position contains which filter. You do not have to follow the
default RGB positions.
Vignetting
The increased length of the filter wheel assembly will cause
vignetting for fast systems (those with low F numbers such as
F/3.3). The vignetting will be worse for the larger ST-8, ST-9, ST-10
with their relatively large CCDs versus the ST-7. Vignetting will
show up as a fall off in the sky background at the edges of the field
of view. Flat fielding greatly reduces the troublesome effects of
vignetting. If a nosepiece is used to attach the camera and filter
wheel combination to the telescope, the 2" nosepiece should be used
if possible. SCT users should use the optional T-thread Visual Back
to make a more secure attachment.
First Use
Before starting an observing session, particularly after
equipment changes / modifications, test the filter wheel and camera

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combination while watching the wheel to make sure it positions
properly. To verify the correct filter is in place, look through the
filter with a flashlight. The reflection off the filter is a different color!
Selection of Color Filters
SBIG color filters include an infrared blocking coating so that a
separate IR blocking filter is not needed. The filters used in the
SBIG filter wheel have been chosen to optimize the accuracy of the
colors produced by the tri-color software. The passbands of each
filter are summarized in the table below:
Color Filter Passbands
Filter Passband (nm)
Red 612-670
Green 488-574
Blue 392-508
The filters are custom filters designed specifically for SBIG
cameras. Dichroic filters were chosen over absorption filters for
three reasons. First of all, they have high transmission in the
passband region. Second, a sharp transition can be obtained in the
region around 496 nm. And last, they are all deposited on a sheet of
glass of the same thickness, eliminating focus shifts due to varying
filter thickness. An infrared blocking coating is applied to these
filters to eliminate spectral leaks in the infrared where the CCD has
high sensitivity. These bands would be severely contaminated if
these leaks were not blocked. Filters designed for visual use and
colored plastic filters also have severe red leaks beyond the visible
spectral range, so one should particularly avoid those.
The sharp transition between the blue and green region is
necessary for proper color rendition of emission sources. The 500nm

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oxygen line is blue-green in color, and so must pass through both
blue and green filters. The hydrogen-beta line at 486 nm is very
blue, and must be rejected by the green filter. The green filter will
pass less than 10% of this line.
Hints for the User
Be aware that color imaging is substantially more difficult than
using the CCD unfiltered. Whereas before you only needed to take
one image you now need to take three, and each one needs to be
about 4 times as long since only a portion of the total spectral range
is passed by each filter. This aggravates tracking problems, which
are also more pronounced due to the requirement of registering
three images. Hot pixels and cosmic ray hits are more objectionable
since they become strongly colored in the final image (the hot pixel
remove utility in CCDOPS should be used). In spite of all this, you
will find that even subtle colors present in galactic scenes enhance
the image noticeably, and emission nebulas take on beautiful hues of
blue, blue-green and red. Star colors are dramatically enhanced, and
impart a three dimensional aspect to the image.
We have endeavored to create a product that enables true color
pictures of deep sky objects to be produced. We have made certain
philosophical decisions with which some may disagree. For
example, if we traveled in space to where we were quite close to the
Dumbbell Nebula it would appear to be a large gray mass devoid of
color, buried in mostly white stars. If we traveled to other galaxies
they would be no brighter, in general, than our Milky Way. Many
extended objects, such as the Horsehead Nebula, have very low
contrast relative to the sky background, often less than 10%, and
would never stand out sharply. The images you see on the
computer screen are not true, in the general sense, being
considerably

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enhanced in brightness and contrast over the actual object. We have
carried this philosophy to the way our program displays star
images; stars saturate to the proper color, not white. Your eye
would see them differently but less accurately! We find the result
very pleasing, but please give us your impression.
Maintenance of the Color Filter Wheel
The surface of the filters may be safely cleaned with Isopropyl or
Methyl alcohol and cotton swabs. Use a gentle circular motion,
using several swabs until the surface is clean. The machined parts
may be cleaned by rubbing with a soft cloth. A small amount of
lubricant can be applied to the magazine axle should the unit start to
chatter but please do so sparingly. It really is a light duty interface.
If some part of the CFW-8A assembly is damaged, replacement parts
may be obtained from Santa Barbara Instrument Group.
Disassembly of the CFW-9 is rather straight-forward. You can
remove the front cover by removing the six screws that hold it in
place. Once removed, the front cover comes completely free of the
CFW-9 allowing access to the filter magazine.
The filter magazine can be removed by lifting the magazine
straight-up out of the back housing by one-quarter inch then tilting
away from the motor and removed completely from the housing.
Upon inspection you will notice that the filter magazine has a top
and a bottom. The filters screw in from the bottom.

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1
2
34
5
As viewed from the top the filter positions are shown in the
figure above, noting the position of the double-holed position
adjacent to position 3.
The filter wheel is reassembled in the reverse process. Ttilt the
magazine into place and drop it into the back of the housing. Be
careful to not bend the axle in the process and also make sure the
magazine is inserted with the filters down towards the camera.
The filter wheel will be held in place against the motor and can
be leveled to make installing the front cover easier. Though it
should not be necessary to do so you may loosen the two screws that
hold the motor in place. If you do so you'll need to remove the
motor cover from the font cover so that you can readjust the tension
of the motor against the magazine. Adjust the tension till the
magazine does not rotate freely but not so tight as to put strain on
the motor that would be accompanied by irregular movement of the
magazine or soft "clicking" sounds. You do not need to worry about
the angular orientation of the magazine as the unit will auto home
itself upon power-up.