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The SBIG FW8G-STXL is an 8-position self-guiding filter wheel for STXL series cameras.
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 Schmidt-Cassegrain-Optics. The single most common complaint however is finding bright stars when guiding through dark or narrowband filters. The STXL filter wheel incorporates a KAI-0340 CCD with 640 x 480 pixels @ 7.4u inside the front cover of the filter wheel so that the guider picks off light from the guide stars before passing through the filters. This guiding CCD is the same sensor used in the ST-I autoguider. Moreover, the optical elements that transfer the light from the pick-off mirror to the CCD act as a 0.7X focal reducer. The result is a doubling of the field of view of the guiding CCD with a correspondingly greater selection of guide stars.
There are two smaller adjustment knobs on the filter wheel base: one for focus and one for moving the pick-off mirror to avoid vignetting. A larger knob on the side of the filter wheel locks the adjustments in place once the best focus is achieved. A short cable is provided to connect the filter wheel guider to the remote guide head port on the camera. In the event that a Standard filter wheel cover is substituted for using Nikon or Canon EOS camera lens adapters, a remote guide head may be used for guiding instead of the built-in self-guiding CCD. The Standard and Self-Guiding Filter wheel covers are interchangeable.
Micron-Precision Filter Positioning: Both the Standard and Self-Guiding versions of the STXL filter wheels incorporate the same micron-precision filter positioning mechanism as the STT Series cameras. When taking flat field frames of filtered images, the flat field frame must show exactly the same optical characteristics to be the most effective. If there is dust on the filter, or uneven illumination caused by the filter, this must be positioned exactly the same every time or the flat-field will not be able to remove the image artifact. The STXL filter wheel is designed to provide a new level of accuracy when taking flats through filters by re-positioning filters to within a few microns every time. This allows the system to take extremely accurate flat field frames even after rotating the filter wheel several times, or after a loss of power.
The light frame at top, taken with the new filter wheel design, shows a large opaque spot caused by debris intentionally stuck on a clear filter. The filter wheel was rotated several times, then the clear filter was re-positioned, and a flat field image was taken through the same dirty filter. The image at bottom left shows the result after applying the flat field image to the original light frame. We measured this positional accuracy to be better than 5.4 microns, or better than a single pixel of the STT-8300.
