The SBIG FW8S-STXL is identical to the FW8G-STXL filter wheel except that the Standard version does not have a self-guiding CCD built-in to the front cover. The filter mechanism is the same, however. In fact the wheels share the same base, and the covers interchangeable. If you have the Self-Guiding version you can add just the Standard cover if you wish to use the STXL camera lens adapters for Nikon or Canon EOS lenses.
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.