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Vignetting I have not seen before.... [Deep Sky] Processing techniques · Christian Bennich · ... · 5 · 180 · 9

Bennich 5.02
Christian Bennich
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I am seeing some vignetting issues I have not experienced before. 

I was shooting some test shots on my lightweight rig - Samyang 135mm and ASI2600MM.
Shooting in Oiii. 

The raw frame - autostretched:
image.png

The raw frame Boosted Auto Stretch:
image.png

I redid my flats, as I believed they might have been the cause, but still I see the following. 

The registered frame, Autostretched:
image.png

The registered frame, Boosted Autostretch:
image.png

My masterflat autostretched:
image.png

Masterflat Boosted Autostretch:
image.png

The integrated stack - 16x5min exposures.

image.png

Flats are calibrated with Dark frames as well. 

What might I be missing ???

This is the first time I've ever seen something like this. 

I am shooting at F2 - can that be part of the reason?

Data was from last night in DK and I was testing on UMi
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danwatt 3.31
Dan Watt
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This looks like flat overcorrection due to something wrong in the calibration process. Have you tried with just bias frames on the lights and flats and omit the darks? Are you positive your offset values are the same on the flats and lights?
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Bennich 5.02
Christian Bennich
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offset - that's a good point!!!
Will check smile
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Bennich 5.02
Christian Bennich
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hmm, both the Light and Flat frames have an offset of 50.

Flat:
image.png

Light: 
image.png

Will try the Bias route.
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Bennich 5.02
Christian Bennich
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Hmm, have now tried redoing flats with same ADU value + Dark Flats. Tried redoing the Flats with lower ADU value + Dark Flats. Tried calibrating Flats with ONLY Bias frames. 

So far - exactly the same result. 

Any suggestions for why I seem to be unable to calibrate this vignetting away from my light frames?
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Bennich 5.02
Christian Bennich
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HA….Found the culprit.

I had started using an Offset with my camera - and the difference was NOT between my Light and Flat frames - but it was from the Masterdark I used to calibrate the Light Frames. 

I found the difference by looking at the Master Flat Dark vs. the Master Dark for the Lightframes.

The Flat Dark - with an offset of 50 - had a K value of around 500, and the Master Dark without the Offset had a value of around 15.

I must redo my Dark Frames for Light calibration to match my new Offset setting. 

Thank you, @Dan Watt, for putting me on the right path to find the problem.
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