Author Topic: Bad flats, trying to recover  (Read 2204 times)

Offline mdolenga

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Bad flats, trying to recover
« on: 2016 April 24 22:41:20 »
I have this image of Messier 106 where the dust mote seems to have moved in between my taking the photo and taking the flats (the imaging session spanned a few days).  I've tried with several different sets of flats in my library, hoping one might line up, but no matter what I do, I get this circular band to the right of the target (see attached jpg).

Any suggestions on how to get rid of it?  Fortunately (or perhaps unfortunately) this only occurs in the Luminance frames.

I tried several image integration strategies, the least gruesome seemed to be Windsorized Sigma.

Thanks in advance for any help.

Michael

Offline pfile

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Re: Bad flats, trying to recover
« Reply #1 on: 2016 April 25 09:50:57 »
david ault has a pretty good tutorial for making synthetic flats:

http://trappedphotons.com/blog/?p=756

you might try that approach to make an L flat that matches.

rob

Offline gvanhau

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Re: Bad flats, trying to recover
« Reply #2 on: 2016 April 26 12:29:55 »
Hello

The circular band (if we are talking about the same brighter band to the right of the galaxy) seems more a reflection than a dust mote (dust motes are usually darker than the background). It is very unlikely thay a normal flat will correct this. May be a synthetic flat as Rob proposes will do.

Geert
Geert Vanhauwaert

Offline pfile

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Re: Bad flats, trying to recover
« Reply #3 on: 2016 April 26 15:02:17 »
Geert, i think that's the flattened image, so the crescent is the result of the dust mote moving.

i don't know how well it would work on this image, but today i was experimenting with removing stray light gradients from otherwise good subs. what i did was to register a perfectly flattened image to the bad image, then subtract the perfect image from the one with the defect. then i smoothed the difference image and subtracted it from the bad light. i had to experiment with how much to multiply the smoothed difference image before subtracting it.

this did work, however there was a periodic pattern in the subtraction caused by the registration of the good image. when i blurred the difference image enough to eliminate the pattern, the 'flattening' did not work as well. i need to experiment with smoothing the input images to the subtraction, or maybe use a different registration algorithm.

rob

Offline mdolenga

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Re: Bad flats, trying to recover
« Reply #4 on: 2016 April 27 00:28:36 »
Geert - what I posted was a calibrated image, attached to this message is an uncalibrated one.  You see that there is a dark large spot to the upper right of M106, it's just that this spot isn't quite aligned with the flat.  My only explanation is that the dust mote shifted.  The scope used is a refractor in my backyard, equipment and conditions which have never caused me reflections in the past.

I'm working through the synthetic flat process (thank you rob), but it's taking time.  I'm tempted to just proceed with full LRGB processing of the image and then use the clone stamp tool to beat out that higher background; this might be a simpler approach.

Offline mdolenga

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Re: Bad flats, trying to recover
« Reply #5 on: 2016 April 27 00:31:12 »
i don't know how well it would work on this image, but today i was experimenting with removing stray light gradients from otherwise good subs. what i did was to register a perfectly flattened image to the bad image, then subtract the perfect image from the one with the defect. then i smoothed the difference image and subtracted it from the bad light. i had to experiment with how much to multiply the smoothed difference image before subtracting it.

I do wonder whether one could generate the ideal flat by some math applied to an uncalibrated sub and a known good image (eg. a calibrated HST image)? 

Offline gvanhau

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Re: Bad flats, trying to recover
« Reply #6 on: 2016 April 27 12:29:12 »
Geert - what I posted was a calibrated image, attached to this message is an uncalibrated one.  You see that there is a dark large spot to the upper right of M106, it's just that this spot isn't quite aligned with the flat.  My only explanation is that the dust mote shifted.  The scope used is a refractor in my backyard, equipment and conditions which have never caused me reflections in the past.
Ok, I understand now.
Not related, but I see a gradient from left to right that is not in the calibrated image, dit you apply any other process to it?

Geert
Geert Vanhauwaert

Offline mdolenga

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Re: Bad flats, trying to recover
« Reply #7 on: 2016 April 27 20:37:11 »
Not related, but I see a gradient from left to right that is not in the calibrated image, dit you apply any other process to it?


Yes, Geert, I also applied DBE to it.  I set the tolerance to 1.5, let it pick the points - I may have removed one or two that I thought were uncomfortably close to M106.