Flats for full/partial disk

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FlankerOneTwo
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Flats for full/partial disk

Post by FlankerOneTwo » Sun Feb 04, 2018 11:14 pm

Hi all,

I've been experimenting with masking off the sky background in Photoshop in order to get partial flat frames to work for full or partial disk images. Basically I create a layer mask on the partial flat, using a histogram stretched copy of the flat itself. This allows me to reveal an underlying layer which I fill with a gray level selected based on the desired correction. It's pretty easy to use the healing brush to fade out details I wish to preserve if they show up on the pseudoflat. Since I'm only generally needing to lighten dark areas such as limb darkening or Newton rings, I try to make the final mean match the max of the flat; I would think that matching the mean of the flat should yield roughly equal lightening and darkening. I then apply the flat to the source image in AutoStakkert (not Photoshop, since it doesn't do true flat field correction).

This seems to work reasonably well on the few test images I've run so far using pseudoflats created by blurring the source image, I would think it should work for "real" flats as well even if they contain the limb of the solar disk. I'm wondering if anyone else out there has an alternative technique, or any thoughts or suggested refinements on this approach. Obviously better not to need to use a flat, but might be a useful means to salvage /tweak some data. Thanks!

Example on full disk for limb darkening - raw image
full disk.jpg
full disk.jpg (25.59 KiB) Viewed 178 times
pseudoflat with fill layer
full disk median.jpg
full disk median.jpg (20.44 KiB) Viewed 178 times
Calibrated image
full disk calibrated.jpg
full disk calibrated.jpg (28.17 KiB) Viewed 178 times
Last edited by FlankerOneTwo on Sun Feb 04, 2018 11:20 pm, edited 1 time in total.

FlankerOneTwo
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Oh, I get it now!
Posts: 30
Joined: Wed Oct 11, 2017 8:11 pm

Re: Flats for full/partial disk

Post by FlankerOneTwo » Sun Feb 04, 2018 11:16 pm

Another example for Newton rings (visible running top left to bottom right) - source image
disk barlow raw.jpg
disk barlow raw.jpg (25.01 KiB) Viewed 176 times
pseudoflat with fill
disk barlow pseudoflat.jpg
disk barlow pseudoflat.jpg (14.81 KiB) Viewed 176 times
calibrated result - still a bit of the rings in there, might tweak the pseudoflat a bit
disk barlow calibrated.jpg
disk barlow calibrated.jpg (25.1 KiB) Viewed 176 times

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Montana
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Re: Flats for full/partial disk

Post by Montana » Mon Feb 05, 2018 12:38 pm

Very nice results, that seems to work very well :)

Alexandra

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