Here are some additional data and discussion points. In answer to the specific questions above, focus can be set on any available object, but what really matters is if your mirror or focuser moves around as you maneuver the scope. My scope has mirror locks, and so changes in focus are unlikely, although my main concern is collimation, which doesn't hold very well in my scope. The best you can do is pick a region of the sky to collimate, and then keep your fingers crossed.
I haven't attempted the "gain sweep" feature in FC yet, although for reasons discussed below concerned stacking, I don't feel it will be helpful to me. Perhaps if you have a mount that automatically tracks the ISS it would be beneficial. The main problem is that getting sets of consecutive frames of equal quality is somewhat rare, and I don't want to waste any on suboptimal exposures. I feel very comfortable in the exposure settings I used here for any future passes.
The image presented at the top of the thread is a stack of two consecutive frames, although the benefit over either individual frame is minimal at best (perhaps nonexistent with some tinkering). There is no doubt, however, that frame stacking is beneficial to ISS imaging. Many of the best ISS images currently produced in the world are made from stacks containing up to 100-200 frames. In order to stack that number of frames, it is almost mandatory to have automated tracking, although I have seen some examples of people using dobsonians with manual tracking who are getting many more frames than I am. Manual tracking on an EQ mount is suboptimal.
Some raw data on the pass: I recorded for about 130s and used PIPP to isolate the frames with the spacecraft. This produced 300 frames out of 8196 that were recorded during that period, for a hit rate of 3.7%. My frame rate was only 63fps because I recorded in 12 bits. Had I recorded in 8 bits I would have achieved 150fps, and more than doubled my frames. Whether or not this would have been beneficial is hard to say, as the extra bit depth may have helped with dynamic range issues.
Here is an example of a raw frame without any changes, and the associated histogram.
If we apply a gamma of 2.2 (our monitors are not designed to display linear data so it looks abnormal) we get the following.
If we apply some deconvolution and make the background black, we get this:
The above image is very similar to the one I originally posted, with the original image being slightly sharper because it could accept a bit more deconvolution.
Here is an animated gif showing 5 consecutive frames that occurred a few seconds later in the pass.
And here is the result if we stack these five frames and apply moderate deconvolution.
Here is another example of 4 consecutive frames stacked and mildly sharpened.
The recurring theme here is attempting to squeeze as much information as possible out of a bare minimum of frames. Out of my 300 frames collected that contain the ISS, only about 50% of them occur during the optimal portion of the pass during which the angular size is near maximum. Of those, they tend to occur in groups of 3-8 consecutive frames. Unfortunately, most of these sets are unstackable because of inconsistency between adjacent frames. And because of the fast motion of the ISS at zenith (1deg/s) the change in perspective of the spacecraft is fast enough that anything beyond perhaps a 1s interval is non-stackable. And even for consecutive frames, the two main causes of image distortion are 1)atmospheric effects and 2)rolling shutter artifacts. In fact, rolling shutter artifacts cause a small percentage of the frames to be completely unusable, but more common is that differential distortion between two frames causes an apparent shift in perspective when none should occur. In this respect, cameras with global shutters will yield a higher percentage of usable frames. But the most obvious factor that would improve the final result of an ISS image is the use of a tracking system that continuously follows the ISS, resulting in the possibility of stacking 100+ frames.
Edited by Tom Glenn, 09 August 2021 - 12:52 AM.