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

hha wrote:

Alen K wrote:

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 2.5 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 2.5 has always worked for me.


Thank you for the detailed response.

The reliable determination of the standard deviation for each pixel is the trickiest part of the clipping algorithm. Pretty reliable with 100 frames, nearly impossible with 10 frames. I select the mildest filter (3.5) because I wanted to avoid clipping out faint star signals, which happens frequently with the 2.5 threshold.

Actually, I checked the limits in the software and it ranges from 1.4 to 3.3 (even in old versions). And I misspoke: the default is 2.0.

As long as I have at least eight images 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 activate "select best pixels."

With only eight frames the determination of sigma and what to clip out becomes very hazy. Faint stars disappear. I never use clipping with less than 60 frames.

I haven't actually used it with less than 16 frames, such as with this image. It might have clipped faint stars in that but there are so many of them there that it would be very hard to tell.

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.

The walking noise ( due to gaussian noise spikes) is also virtually eliminated by sigma clipping.

I have my doubts about that, especially if you are limiting the clipping so much. Can you provide a comparison of pure accumulate vs. kappa-sigma clipping that demonstrates that? If you don't feel like proving it to an old doubting Thomas like me, that's okay.

Have you tried the worm error and flexing with your setup?

I don't use a tracker (anymore). My current, highly-portable setup depends on Astrotracer. Random dithering is a natural consequence of how that must be used when creating large stacks (much more work than saner people than I will do). No worm error (no worm gear) and no flexing problems. The camera or lens is mounted directly to a tripod, like this. My other setup uses a Vixen GP-DX Goto mount, so no flexing of that sort even when it is supporting one of my telescopes. Unguided PE is less than 5 arcseconds (when the PPEC is properly trained).

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