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Introduction
SBIG's STXL series cameras contain several innovative features that can be found in no other astro-
imaging camera. This innovation extends to STXL accessories as well, including the integrated
design of the FW8S-STXL and FW8G-STXL 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. The FW8S-STXL standard filter wheel can therefore be upgraded to
an FW8G-STXL 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 STXL 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.
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.
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