satellite and hot pixel track suppression in software

Started 2 months ago | Discussions thread
Alen K
Alen K Senior Member • Posts: 1,298
Re: satellite and hot pixel track suppression in software

Sequator uses what is often called kappa-sigma clipping or rejection in its "select best pixels" accumulation mode, in three passes according to the manual. The rejection setting (related to standard deviation, I believe) is 2.0 by default. By calling it "accumulation 3.5" I assume you are changing the setting to 3.5. Different images can need different settings but so far 2.0 has always worked for me. (Actually, I think the scale only goes up to 3.3 from a minimum of 1.4.)

As long as I have at least eight light frames in my stack, I never use anything else. (My largest stack so far was nearly 400 frames.) If I do have less than eight frames, I also select "reduce dynamic noises." If I have less than four frames, which Sequator's manual says is the minimum required for that accumulation mode, I do not even try to activate "select best pixels." (I'm not sure the program will let you.)

Your worm track is one of the clearest I have yet seen. As you have discovered, those can be invaluable for evaluating sources of tracking error. Although intentionally offsetting the polar alignment to induce declination drift combined with kappa-sigma stacking is one way to solve those problems, I would encourage you to try to reduce them at the source.

And instead of offsetting the polar alignment, I recommend doing random dithering. You may find, as many others have experienced, that PA offset alone will cause walking noise in the background. Random dithering significantly reduces that as well as satisfying the conditions for kappa-sigma clipping. I always do random dithering but it does take more than eight frames; at least a dozen different, random positions and the more the better.

BTW, "select best pixels" is slower both because of the multiple iterations and because it writes temporary files to your computer's drive. If you have an SSD, be sure to specify a directory on that as the location for your output image rather than a hard-drive. SSD's are, of course, much faster for both reads and writes.

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