Hello, I purchased an STXL-6303 and FW8S-STXL filter wheel at the last AIC. I've used it a few times and noticed there was always stray light in the images, which would vary from target to target but almost always would leave a pattern in the lower left corner area of the image. And the pattern would be different for each target. I initially thought it was an internal reflection from a bright star near the field, but targets with this issue are not near a bright star. And my old STL never had this problem using the same OTA (Celestron HD 11). This week I decided to dive into this to see what is happening. As part of my testing I shot a simple 1 frame through each filter LRGB of M33, and the pattern was seen in all filters at the same location on the chip: http://www.sbastro.net/Astroimages/M33-LRGB.jpg I took the camera assembly off the scope and sealed the entry port of the filter wheel, and took some 5 minute images all binned 2x2 to try and see where the light is getting in. The 5 minute dark frame looked normal, but does appear slightly brighter on the left side: http://www.sbastro.net/Astroimages/5_minute_Dark.jpg Next I shot a 5 minute light, using Lum filter, with the sealed camera, and got this: http://www.sbastro.net/Astroimages/5_minute_light_Lum_filter.jpg I took another 5 minute shot through an empty filter slot, and the pattern was much stronger: http://www.sbastro.net/Astroimages/5_minute_light_empty_filter.jpg Next I remove the filter wheel from the camera and put the red camera top back on, and sealed the entry port, and all the lights I took looked just like darks, there was no evidence of any light leak at all. And I did some test images using just the camera on the scope last night, and none show this issue, they are all clean. So the issue is with the filter wheel. I have checked all the seems and joints and there are no gaps. Any idea what is causing this? If you want to see the corresponding FITs files, just edit the links above and change the file type to .fit.