Okay so I started my astroimaging journey with a Nikon D600, 180mm f/2.8 lens stopped down to f/5.6 and imaged the Pleiades with a cheap iOptron Skytracker (this is obviously unguided). I got a pretty good result. Later on I tried the Sony A7S, AT60ED f/6 and Orion Sirius. I started seeing some ugly noise because it was unguided and I think fixed pattern noise started to show. More recently, I used a cooled ASI1600MC, Hyperstar on a C8 and Orion Sirius guided but no dithering. The fixed pattern noise became horrendous.
See below for a comparison of the same area between the Nikon D600 unguided (left), Sony A7S unguided (center), ASI1600MC guided (right).
In all three cases I stacked using Deep Sky Stacker and did a screen transfer function stretch in Pixinsight, nothing else.
Why did the fixed pattern noise go from non-existent to somewhat annoying to extremely horrendous?!? In all cases I had at least 30 minutes of exposure in fairly dark skies. In all 3 cases I did not dither. First two were unguided. Third was guided but no dither (ASIAir doesn't support this yet)
Edited by joelin, 12 December 2018 - 02:35 AM.