satellite and hot pixel track suppression in software

Started 2 months ago | Discussions thread
OP hha Contributing Member • Posts: 651
Re: satellite and hot pixel track suppression in software

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.

Alen:

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.

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.

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.

I agree. When Polaris is visible, my alignment with the Polaris scope alone is much better than 0.5 degree. I have taken the Skytracker to camp sites high in the mountains, where Polaris may be hidden by trees or mountains and I can only align with a magnetic compass.

I  did this test to evaluate

1. the SkyTracker tracking error and discovered that the flexing error in the tripod is significant. The camera with the lens weighs only 1.5 kg. No counterweights were used.

2. how much pole axis alignment error I could get away with.

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.

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.

Interesting, but random dithering is not an option with the SkyTracker.

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.

I write the huge scratch files from each frame into the SSH on the motherboard.

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

Cheers.

hha

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