Over-correcting flats question

I’m sure it would help if I understood the math behind preprocessing with the master flat - but I don’t and I have run into a perplexing case.

Ever since obtaining my QHY183C I have taken flats with a target ADU of 19,000, because I read somewhere that it was a good value for this camera. And I never had any reason to doubt it, I never saw any issues in my images that seemed like a problem with the flats.

Last weekend I was at the club dark(ish) site. While waiting for the moon to set, I took an hour’s worth of 30-second exposures of Caroline’s Rose cluster at gain 5, offset 8. I had not disassembled the imaging train since my previous session, so I have that master flat and I took a new set that evening anyway before it was dark enough to image.

When I stacked the images, I got this (with the histogram screen stretch):

I processed again with the earlier master flat (same result) then took a new set of flats (still had not disassembled) - same result.

Then I processed the 5-minute subs I took later that evening (same gain / offset) - and they looked normal with the normal master flat workflow.

So I did what I thought one was to never, ever do - I processed the 30s subs without flat-field correction:

Next I ran a series of flats with lower and lower ADU values, all the way down to just 1k, which seemed to be getting pretty close to the limit. Less reverse vignetting, but still obvious:

So - is this normal behavior? Did I manage to repeatedly do something wrong this time?

Not sure I understand exactely the extent of your tests but I’m sure there’s a tutorial explaining the maths at play behind flat correction on Siril website :wink:

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Thank you.

Reading that, the only way to over-correct for vignetting is to omit dark subtraction (from the lights) - is that correct?
If so - that points me in a useful direction - that there is likely something wrong with my 30s master dark. Lucky for me, it is cloudy tonight so I can re-shoot those darks :laughing:
That would explain also why the other lights from the same night are fine. I shall report back later.

It’s not necessarily an omission altogether. Just having the level wrong is enough to get this overcorrection. So either a wrong duration if you have significant dark current or the wrong offset setting with the camera could also give you some inverse vignetting.

Let me know when you find the culprit


True - I should have said that the only example shown was for omitting darks.

Made new darks - no change. Which means most likely that my lights aren’t what I intended them to be. Time to go dig through the Ekos logs from the session.

Whew - fixed! I use either gain 5, offset 8 or gain 11, offset 8. Somehow I had a brain fart out in the field and entered gain 5 offset 11 when I set up the sequence for Caroline’s Cluster. One tiny moment of carelessness turned into several hours of troubleshooting (but I learned something, so next time it should only be about 1 hour worth of troubleshooting :grinning:).

Thanks for your help, @cissou8 !

Yes indeed you’re right. I did not write all the possibilities (there are many!) just to try to keep it as “simple” as possible…

Glad you found the solution! I think in most cases, inspecting the different levels in your lights/masters using stat or seqstat commands will point you in the right direction.

Clear skies,