This is a bit late for this discussion, but I believe I have the same issue - I started to notice a lot more light noise in my longer exposures (concentrated in the center). I initially just chalked it up to me being a beginner at this. But after going thru the forums, it was noted that this is an issue with this filter drawer. Out of curiosity, I took 2 x 300s darks for comparison. 1 with the drawer in the train (no filter in the drawer) and one without the drawer. Camera is 294MC pro. See the images for the results, but it was a drastic difference. It at the least explains why I'm having such a hard time with post processing anyway. I'm going to try to add some sort of 'seal' around the outside of the main ring (plumbers tape maybe) to see if that helps.