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Flats with bright corners

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I have bright corners on  my O3 and S2 flats, whereas my Ha flats a more normal with 4 darker corners.

Could this be due to deterioration of the filters at the edges? My O3 and S2 filters are of a older generation than my Ha filter.

S2 flat screenshot below.





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Are the filters the same optical diameter or are your older OIII & SII filters in heavier mounts? Some older filter mounts/holders have a smaller internal diameter than newer filter mounts - that may cause vignetting at the corners.

Just a thought

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It's not the vignetting overall that I'm looking at, it's the brightening of the lower left corner that has me perplexed. 

It seems to calibrate out ok so I'm not overly bothered, more curious really

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Are there any other lenses in the image train - a reducer of something like that? My collimation is marginally out on my RC8 and this caused uneven flats similar to this - bright on one side. Calibrates out on the image OK so for now I put up with it. To be honest I am just a coward and putting of the collimation jobs on the RC. I'll do it after galaxy season.

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Thanks for chipping in 👍.

No other lenses, not using a coma corrector. I did think it might be collimation but the Ha filter (which is the newest generation) doesn't have this effect, just the older o3 and s2 filters. 

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