Flat field problem with latest Beta

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PetePeterson
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Flat field problem with latest Beta

#1

Post by PetePeterson »

Image

Haven't experienced this problem before and have just changed to latest Beta last night.

RGB parts of flat will overlay properly below 9% saturation but increasing exposure even a small fraction blows things out fully saturated.

The ADUs will finally drop to the point where R, G & B are properly stacked but once again it's down in the 9% saturation level and not really usable.

Is it me or the Beta? I'd load the release 3 to see if it'll fix things but have already lost an observing night and am a bit discouraged.

Help!

Pete
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Re: Flat field problem with latest Beta

#2

Post by admin »

Hi Pete,

I haven't changed anything in the flat correction code for quite a long time (several weeks at least), so it's unlikely to be any change in the latest beta. In particular I checked the changes between 7806 and 7824 and there was nothing that would affect flat capture or flat correction.

What I can see is that you have a high level of vignetting in the corners/edges - probably nearly a factor of 10 dimmer than the frame center. This level of dimming means that flat correction is very sensitive to glitches - remember that the correction will be brightening the corners of the frame by a factor of nearly 10.

I'd suggest trying to use the 'monochrome flat' and 'subtract bias' options - the former tends to make flats a little more forgiving and the latter leads to increased accuracy of correction.

If you still have trouble, please post a screenshot showing the incorrect correction as well as the flat frame.

thanks,

Robin
PetePeterson
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Re: Flat field problem with latest Beta

#3

Post by PetePeterson »

Robin, I've been comparing camera settings good flats from May 15 to the problem flat attempt above. The only significant difference in the entire setup is that last night I had Background subtraction set to Blended Offset rather than Off, and Used Bias =False rather than Used Bias = True

Suspect this is the problem. Heading out to try another flat field with the May 15 settings.
PetePeterson
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Flat field problem due to use of Background Subtraction

#4

Post by PetePeterson »

Robin, the flats were bad because I'd had the Background Subtraction turned to Blended Offset. Turned that function off and flat capture was back to normal. The "What changed?" question was answered easily by comparing the imaging details. That function is a real winner. So many bells and whistles that this old man sometimes misses a setting and more often doesn't understand a setting.

Unrelated suggestion: Is there any reason that the centering reticles aren't horizontal/vertical rather than rotated 45°? Yes, the bullseye does have squared reticle but the others are clearer and would be easier to use when squaring up a camera.

Thanks for your support.
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Re: Flat field problem with latest Beta

#5

Post by admin »

Hi Pete,

thanks for checking and reporting back. I'm surprised that the background subtraction was the cause of the problem - it's supposed to be compatible with darks and flats (it is applied after dark and flat correction). Or do you mean that background subtraction might have been on when taking your darks or flats? That would be bad!

cheers,

Robin
PetePeterson
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Re: Flat field problem with latest Beta

#6

Post by PetePeterson »

Yup, the background subtraction was on while taking the flats. And it was indeed bad. You might consider dummying things up and adding an alert to this same as you do when darks and/or flats are selected when trying to run flats.
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Re: Flat field problem with latest Beta

#7

Post by admin »

Hi,

I have decided to automatically turn it off when capturing darks/flats and then restore the setting afterwards - best to just do the right thing :)

Thanks for reporting this - I hadn't spotted that potential problem.

cheers,

Robin
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