Joe, The ASI 1600 is an FPN machine! You are pretty much GUARANTEED to get fixed pattern noise if you don't dither frequently and aggressively. FPN and the ASI 1600 has been discussed ad nauseam here on the forum for the last two years. You must dither aggressively and frequently with this camera. One reason is that this camera has ridiculously low thermal noise and read noise. This is a good thing! But unlike your DSLR's which "swamp" the building blocks of FPN with deeper exposure and thermal noise, the ASI 1600 requires active management of collection techniques (i.e. Dithering) to prevent the issues you are having.
... movements of the scope are not dithering. If the scope moved enough to "dither" you would have stars trailing all over the place in the image....
... the drift of the stars across the frame due to imperfect polar alignment or differential flexure, which are also often called "dithering", are not only most definitely NOT dithering...they are the direct cause of the issue the OP is having: Correlated noise! (Walking noise, raining noise, etc.)
...It IS the drift that ultimately allows the FPN to correlate. Once the frames are registered, the stars stop drifting...and now the pattern drifts. When stacked, it is the slight but consistent changes in the position of the pattern that ultimately "correlates" in the stack, giving rise to the streaking in the noise.
...The only way to dither such that it can help you randomize the FPN .... is to actually dither, using a program like PHD or a direct mount dither, and dither by 5-10 pixels every few frames for short (never less often than 3 frames, more often if the frames are longer), and dither RANDOMLY (do not use any kind of pattern dither, like box, or spiral, etc.)
Edited by 17.5Dob, 15 December 2018 - 04:19 PM.