Author Topic: Difficult vignette to remove even with flats  (Read 224 times)

Offline creznicek

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Difficult vignette to remove even with flats
« on: 2019 October 14 21:28:27 »
Hello. I'm a new pixinsight user, hoping for some direction on how to get rid of a troublesome vignetting. My equipment is a Celestron 8SE, with 6.3 focal reducer (the main source of the vignetting), and a Canon T5, no filters. I have 36 light frames at 20s each and about 20 each of flats/darks/bias frames. I combined them all in BPP. I ran through the standard workflow including DBE. And then after further processing I began to see the vignette start to show up more. Especially as i was doing noise reduction and sharpening using the luminance mask. Each step just seemed to make it worse and worse. Eventually I decided to stop and go back to the start and try to see if I could clean up the problem from the beginning. But I've not had any better luck.

I added ABE as well as DBE and I tried both division and subtraction, no better. I double checked my master flat which seemed to look good. I then tried BPP with no flats to see if they looked any better/worse in order to make sure that it was successfully applying the flat, it looked WAY worse so, yes, it was successfully applying the flat. I've searched tons of posts on pixinsight and other forums and I'm kind of at a loss. I've had this problem on another image I tried to process so I definitely need to figure this out for future imaging. I'm wondering if the flat and ABE/DBE are not fully removing the background vignetting, and then when I use luminance to make a mask for either noise reduction or sharpening if I am magnifying the problem. I'm just not sure what to do to better clear up anything in those first few steps.

I have attached a few photos: showing my initial final processed image where the vignetting was just getting worse, an image of my stacked image with flats after dbe, and an image of my master flat. I am also linking here to my data should anyone be willing to try to process things for a better result and then explain how to fix things. Includes the stacked fits image, my calibration masters and my light raws in a zip.
- Stacked Light FITS file
- Master Dark
- Master Bias
- Master Flat
- Zip of Light RAWs

Your help is greatly appreciated!
« Last Edit: 2019 October 17 16:04:29 by creznicek »

Offline ngc1535

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Re: Difficult vignette to remove even with flats
« Reply #1 on: 2019 October 15 00:13:09 »
I agree that is a difficult pattern to remove...but I suspect it is possible.
Based on the images you are showing, I suspect there is something more fundamental going on here. The signal you recorded for the object is of the same magnitude (or less) than the flat errors (flat calibration does introduce some noise). I am not certain you are ultimately going to win the battle of correcting the flat error (or other battles you will also need to fight). I suspect a longer subexposure will be necessary to make some headway.

Can you acquire images on the order of 180sec or 300sec exposures?

Basically before trying to tame the flat issue, I wonder if more battles with acquisition might be the way to go?
Do you think this might be correct?

Offline creznicek

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Re: Difficult vignette to remove even with flats
« Reply #2 on: 2019 October 17 16:15:51 »
Seems like the problem steps from either initial lights (which should be able to be cleaned up by calibration files or ABE/DBE), improperly processed calibration files (I used BPP), or better application of ABE/DBE. Couple questions:

As for the initial lights, I've seen other peoples photos with my similar setup and 20-30 second exposures without a vignette problem. I've seen others with longer exposures also trying to correct a bad vignette. I don't *think* it's the exposure time or acquisition issues. But more people can chime in if they feel that's the problem.

As for calibration files:
a) Does anyone see any problems with my processing of the calibration files?

As for ABE/DBE:
b) Does it matter if I do ABE or DBE first?
c) Should I do both subtraction and division, subtraction first?
d) What settings should I change to try to make it more effective?
e) Can I apply these over and over? I've tried 1x ABE and 3x DBE and still see a faint vignette on the left.
f) Do these help after the image has been stretched? Can I do some processing and when the vignette starts to become more visible can I try to get rid of it then?