Integration of several LiveStack's results

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diant
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Integration of several LiveStack's results

Post by diant » Fri May 22, 2020 4:51 pm

Hi, Robin!
Please, advice me a convenient method of integration of several LiveStack's results.
For example. I've imaged some DSO during 7 nights and have now 7 FITs files (32-bit) - each is result of one night imaging with Live Stack.
Ok. All 7 FITs have
(1) different integration times
(2) different sky backgrounds (Moon phase etc...)
(3) fading out edges (because of constant drift of field of view, and manual returning of aiming and manual dithering...)

I like to sum in Fitswork. But I understand that such files should not be integrated straight (as is).
They have different SNR and should be weighted proportionally SNR^2. Am I right?
If so they should be weighted. But a fading out edges deceive many programs.
Robin what can you advise in such situation so that I can integrate them and get maximum SNR of sum?
Anton

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admin
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Re: Integration of several LiveStack's results

Post by admin » Fri May 22, 2020 6:12 pm

Hi Anton,

I think you should weight the frames proportional to the total integration time of each frame. Imagine that you have a 30 minute stack, a five minute stack and a 15 minute stack. That's equivalent to having 10 five-minute stacks where six of them have already been averaged together into one image, three have been averaged into a second image and one is left on its own. When you think about it that way it's fairly clear that weighting them proportional to the exposure time is the right way to go.

Most often I've heard of people using the auto save and reset functionality in the life stacking when they want to try this approach – for instance they set it up to auto reset the stack every 15 minutes and then over the course of the evening will get 12 separate stacks each of 15 minutes total integration time and then they will average those in postprocessing.

Hope that makes sense, Robin

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turfpit
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Re: Integration of several LiveStack's results

Post by turfpit » Fri May 22, 2020 6:27 pm

Anton

https://www.astropixelprocessor.com/com ... -exposure/

This was processed using sets of exposures of varying lengths - 120s, 60s, 30s 10s, 4s. I just loaded all the FITS frames as multi-session and Astro Pixel Processor dealt with it using default settings. Note APP has a trail version available.

Dave

diant
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Re: Integration of several LiveStack's results

Post by diant » Sat May 23, 2020 8:15 am

Thank you to all!
Robin, I understand that, in first approaching, the integration time of each frame should be proportional to its weights.
But it is only in first approaching, because a sky background may vary from night to night (for high-altitude haze, Moon etc.)
And even two equal frames (say, 15 and 15 mins in successive nights) may vary in its background noises.
I ask in view of this...
But all my question seem to fade out - for the last night I did some sums with different method of weighting of individual frames in Fitswork and saw (to my surprise) that result proved to be nearly the same. This means that Fitswork itself does "noise weighting" during integration, so it's no need to weight the frames manually.
Anton

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Re: Integration of several LiveStack's results

Post by admin » Sat May 23, 2020 7:53 pm

Hi,

Ah, interesting. If you want to take the noise in each image into account then it all becomes very complicated. In theory you should give the images with the lowest noise the most weight, but whatever weighting you apply it would not be correct across all parts of the image. If you have a particularly clear night then the sky background will drop and you will have much lower noise in the faintest regions, but in brighter areas of nebulousity the noise level will change much less. Maybe there is some software out there that does this automatically by doing local noise estimations as it stacks?

Cheers, Robin

diant
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Re: Integration of several LiveStack's results

Post by diant » Sat May 23, 2020 8:13 pm

admin wrote:
Sat May 23, 2020 7:53 pm
If you have a particularly clear night then the sky background will drop and you will have much lower noise in the faintest regions, but in brighter areas of nebulousity the noise level will change much less. Maybe there is some software out there that does this automatically by doing local noise estimations as it stacks?
Robin, of course I see the sky background on the most darken, free from stars, nebulae etc. regions of image.
I look as well after signal level on one or two unsaturated stars (because not only background noise give us correct weight but S/N).
And surely there are many such programs that can weighted frames during stacking (DSS, PI, Fitswork as it turned out...). But all they are failed when you slip into they input raw SharpCap's stacks - because of its fading out edges (it need to be cropped first).

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