Additive dark/flat application [SOLVED]

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gaar
Posts: 12
Joined: Sat Jul 04, 2020 8:42 pm

Re: Additive dark/flat application

Post by gaar »

Thanks for looking them over, Robin.

Gain was set to 121 (just above unity for my ASI294MC Pro) for all screenshots, as well as for dark and flat capture. If we grant that I was diligent about gain matching, then I'll point to the difference in histogram peaks (between A1 and A2/A3; and between B1 and B2) as evidence that something is amiss. I agree that mismatched gain could cause this, but I do experience the problem every time I use SharpCap, and restarting the application is the only way I've found to resolve it.

The same flat was selected for all screenshots. It was captured as monochrome at 3.5 seconds, 121 gain, using a dark captured at the same exposure and gain. The dark frame is about a year old, but given my woefully infrequent usage, I doubt the camera's dark profile has materially changed.

Next time I'm out, I'll make make more settings visible in screenshots, and I'll screenshot settings during flat capture as well. I'll also try matching the stretch to the different histogram shapes as you suggest. Skies are cloudy tonight of course!
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admin
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Re: Additive dark/flat application

Post by admin »

Hi,

that sounds a good plan - also please do save the following files from your next try if at all possible

1) some RAW frames from the live stacking (set the RAW FRAMES option to 'Save Stacked' or similar)
2) the dark frame you are using
3) the flat frame you are using
4) (if possible) some unprocessed flat and dark flat frames.

Ideally keep the 'capturesettings.txt' files that go along with these too so we can check the settings.

With the files listed above we can go back and 're run' the imaging session in the folder monitor camera or in other tools like DSS until we work out what is going on.

cheers,

Robin
gaar
Posts: 12
Joined: Sat Jul 04, 2020 8:42 pm

Re: Additive dark/flat application [SOLVED]

Post by gaar »

I think I've figured out my problem, and it's user error rather than a bug. I need to collect more evidence to be positive, but I'm confident enough to mark the topic SOLVED.

The short version is that my offset/brightness settings have been inconsistent.

Meno555 and Robin suggested this possibility earlier in the thread, but I was under the impression that my offset/brightness was constant because I wasn't changing it. I didn't realize Smart Histogram alters brightness as well as exposure and gain, and as you can see in the screenshots, I keep the Image Control panel closed. (To be fair, the Smart Histogram dialog uses the term "Black Level", but the Image Control panel calls it "Brightness". Confusing!) So every use of the Smart Histogram--even without applying the recommended settings--would alter brightness. As described in the repro steps, I knew to manually apply the exposure and gain settings, but not brightness.

In my tests last night, running the Smart Histogram changed the brightness value of 4 to 24 (without clicking Apply), resulting in a rightward histogram shift similar to the screenshots I've already posted. Manually restoring the brightness to 4 fixed the image.

That explains why not applying the settings caused problems for me. I think the problem would occur in reverse if I took flats first, then the Smart Histogram recommended a recommended a different brightness, and then I did click Apply. That could be why I've seen issues both with and without clicking Apply.

OK, now I need to dig into the other forums and learn more about offset/brightness. Thanks for the help, folks, even if it took months to penetrate :?
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Re: Additive dark/flat application [SOLVED]

Post by admin »

Hi,

certainly having a different offset/brightness/black level between the darks and lights could be a problem. Unfortunately different manufacturers have chosen different terms for this setting and SharpCap tries to go along with the choice that the manufacturers of the camera have made (for consistency with other software).

Smart histogram sets the black level to avoid histogram clipping at the zero level - basically you don't want any pixels with zero value (or very few - a few or a few tens in millions would not hurt). If a pixel has zero value you have no idea if it really should have had zero or it should have been lower and just got clipped to the minimum zero value.

cheers,

Robin
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