Hi, Robin,
Tell me please, when we go in "Alignment" tab of LiveStack and there check the box "Optimize for Faint Stars", then SC calculates FWHM already after (not before) image noise/brightness corrections applied (for the better Faint Stars Detection) and so we see other (larger) FWHM than when we don't check this option?
If I'm right, will it not better to change such strategy and to calculate real FWHM in all situations by one method - on clear (not corrected) image? In such case we always be seeing equal FWHM during equal seeing conditions.
Anton
Live stacking / Faint stars
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Re: Live stacking / Faint stars
Hi,
what's happening here is that when the option is checked, more faint stars get picked up and included in the average FWHM calculation. This can then affect the average value for a range of reasons - one of them is that for faint stars, the outer fringes of the star can be lost in the noise and therefore not included in the calculation of the FWHM, giving those faint stars a slightly smaller average FWHM than perhaps they should have.
The real aim of the FWHM measurement in this area of SharpCap is to measure how the quality of the captured frames varies from one to the next - for that it doesn't matter that the two values with and without the option are a bit different - as long as the option is left on a consistent value throughout the stack, the relative values from frame to frame (caused by seeing fluctuations, or perhaps focus drift) will be usable.
cheers,
Robin
what's happening here is that when the option is checked, more faint stars get picked up and included in the average FWHM calculation. This can then affect the average value for a range of reasons - one of them is that for faint stars, the outer fringes of the star can be lost in the noise and therefore not included in the calculation of the FWHM, giving those faint stars a slightly smaller average FWHM than perhaps they should have.
The real aim of the FWHM measurement in this area of SharpCap is to measure how the quality of the captured frames varies from one to the next - for that it doesn't matter that the two values with and without the option are a bit different - as long as the option is left on a consistent value throughout the stack, the relative values from frame to frame (caused by seeing fluctuations, or perhaps focus drift) will be usable.
cheers,
Robin
Re: Live stacking / Faint stars
Hm... I really observed a considerably larger average FWHM, when box "opt. for faint stars" was checked.admin wrote: Thu Jul 31, 2025 2:38 pm This can then affect the average value for a range of reasons - one of them is that for faint stars, the outer fringes of the star can be lost in the noise and therefore not included in the calculation of the FWHM, giving those faint stars a slightly smaller average FWHM than perhaps they should have.
Absolutely so, Robin. But I meant the case when the stacks from different nights collect together (as super-subs) for the future summing up of all stacks. In such case we naturally have to compare the stacks from different nights by some criterion of quality - and FWHM could be a such criterion, were it not so dependent on checking the box "opt. for faint stars".admin wrote: Thu Jul 31, 2025 2:38 pm The real aim of the FWHM measurement in this area of SharpCap is to measure how the quality of the captured frames varies from one to the next - for that it doesn't matter that the two values with and without the option are a bit different...
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Re: Live stacking / Faint stars
Hi,
how up to date is the version of SharpCap that you are using? I did some improvements maybe 2 months back (maybe 3?) that helped quite a lot in terms of reducing the sensitivity of the FWHM calculation to the star brightness. Any chance you could be using an older version from before that change?
Anyway, aside from that, if you are bringing together data from different nights, you have two ways of doing it...
1) Restack all the RAW frames from all sessions together - in this case you would leave the star detection settings alone during the restack, so the values measured from each frame would be consistent throughout the stack
or
2) Stack the saved, stacked, images from each night in a secondary stacking process. Here you would end up measuring the FWHM of the stacked images going in to the final stack to assess the quality of those images - again the measurement would happen with consistent star detection settings, so should be fine.
cheers,
Robin
how up to date is the version of SharpCap that you are using? I did some improvements maybe 2 months back (maybe 3?) that helped quite a lot in terms of reducing the sensitivity of the FWHM calculation to the star brightness. Any chance you could be using an older version from before that change?
Anyway, aside from that, if you are bringing together data from different nights, you have two ways of doing it...
1) Restack all the RAW frames from all sessions together - in this case you would leave the star detection settings alone during the restack, so the values measured from each frame would be consistent throughout the stack
or
2) Stack the saved, stacked, images from each night in a secondary stacking process. Here you would end up measuring the FWHM of the stacked images going in to the final stack to assess the quality of those images - again the measurement would happen with consistent star detection settings, so should be fine.
cheers,
Robin
Re: Live stacking / Faint stars
Yes, I am using not the latest version SC. My one is 4.1.13331.0-64bit. I'll upgrade it and observe its FHWM behavior.