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Strange gradient in difference between master flats

auroraTDunn
in PixInsight
I just finished up a multi-night narrowband imaging run. Before I started to run everything through WBPP I decided to try a few of my new found tricks from the Fundamentals to see what I had and or changes I may want during Calibration and Post-Processing such as a NB Pedestal - that was cool a cool trick to see btw)

I also decided to look at what my differences between my master flats would be from the first night to the last (noting I never changed my imaging chain). 
So I took my first nights Flats and created a Master flat. 
Then took my last nights Flats and created a Master flat.
I opened both in PI and using PixelMath I created a new image that was the difference between the two. What I found has me wondering what happened.

First nights Master flat:
image

Final nights Master flat:
image

Nothing amazing there. But when I ran the difference between the two
image
I get a weird gradient from the bottom left to top right. Ignoring the other artifacts from the flats.
Anyone have a clue why I have this? Note that I see a similar progression from the first to last night with that gradient.

NOTE: I'm using a ASI6200. These Flats are from my Ha. But I do not see them in my Sii and Oiii. My last night was the only night I took R,B,G's and strictly just for star colors.

Comments

  • auroraTDunn
    edited July 2023
    It seems my images did not attach I have attached them below.
    Sorry about that, we just changed where/how we share data at the Univ of Colorado and have had nothing but issues with the new systems (try explaining that as a holdups on grant updates to NSF!)

    I should also note I kept everything cooled to -10 for ALL images.
    masterDark_first.png
    1915 x 1277 - 2M
    masterDark_last.png
    1915 x 1277 - 2M
    masterDark_difference.png
    1915 x 1277 - 1M
  • BlockHeadBlockHead
    Before I answer ... you took the ratio of these flats right? (not actually a difference, but divided one by the other?)

    -the Blockhead
  • auroraTDunn
    Uhmmmmm, well it seems like my name should be blockhead. I did not I actually took the difference. But I'm going to go try it the right way in just a minute!
  • auroraTDunn
    Well it 'looks' a lot better . I do still have a small gradient from the bottom left corner 0.95 to the upper right corner (0.98).
    I'll be gone all day tomorrow so I plan on running WBPP on all my data. It's still curious to me but I'm not 'as' freaked out as I was, just confused by what could be causing that.
    Before my next clear night (hopefully Saturday) I'm going to do some more testing to see what the issue may be.

    For further reference, I use a Spike-a Flat Fielder light panel. I place two layers of white TShirt, pulled tight with a rubber band, over the hood. I did an auto calc the first time with my AA+, which was 4.6 seconds. After the first set I kept all other sets hard targeted to the same value 4.6 seconds. Camera is cooled to my normal -10 before I start shooting and I've been taking 64 subs (I know that's MORE then adequate but I have 100TB for storage so I just wanted to test the difference between a higher number of subs and then try it with 32 and the probably 24) for each nightly set against my 30+ hours of lights,
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