I took the images with a Celestron NexStar 6SE, the 6.3 Focal Reducer, and a Nikon D7100. This was my first time to use the Focal Reducer.
I stacked thirty two, 25 second subs of M33 in DeepSkyStacker and out popped this when I was processing in Gimp. I see a Weird Semi Circle Shadow along the Bottom.
I never got a shadow like like until I tried the Focal Reducer. I have tried to use my Colombo like investigation skills to try and figure out want is causing the shadow, but I can't.
Could it be collimation and the semi circle line is the secondary mirror? Any help would be much appreciated.