The 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.
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 using the STT-8300.