Flats overcorrecting on extract OIII only (but not Ha)

That's an interesting question indeed. Could you share some lights that show this issue, together with your master calibration frames?

Go to https://upload.astropixelprocessor.com and use upload1 as username and upload1 as password.

Create a directory named “geordan-flatsproblem” and upload in there. Thank you!

For me, it happens the opposite.

Flat seems to correct well on both ha-o3 color and extract o3, but not on extract ha: it corrects well on 3 corners of the frame, except the one in the upper left where an evidence of some flat correction problem can be seen..

Ha: upper left corner is the problem while 3 other corners are ok

O3: quite well corrected 

Color: well corrected 

@vincent-mod I have uploaded the files; let me know if you need more data!

Thanks Geordan, I'll have a close look today!

So I do see a big shadow it seems on the top left, that must be the prism then. Haven't seen it like this much I have to say, but I also have no personal experience using prisms.

So I looked at the Ha and OIII and I see the problem I think. It seems that the background still has a bit of a signal for Ha, but is 0 in the OIII. My guess is that if you expose your flats for longer, this may be better. Try to shift the histogram to 70% ADU or something, it shouldn't clip of course but that might help.

Thanks -- I'll give that a shot when I can!