Flats overcorrectin...
 
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Flats overcorrecting on extract OIII only (but not Ha)  

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(@geordan)
White Dwarf Customer
Joined: 3 years ago
Posts: 18
June 16, 2021 02:15  

Hi,

I am using a OSC camera (ASI294MC Pro) with the STC Duo-Narrowband filter.  My OAG prism intrudes on the light path, covering the top left corner.  Normally, flats help mitigate this.  When I use Ha-OIII Extract Ha, the calibrated image looks fine:

image

But if I change the debayer algorithm to Ha-OIII Extract OIII, the corner that is shadowed by the OAG prism brightens up (and you can see more of the ASI294MC's characteristic amp glow in the top right):

image

This happens whether or not I take flats with the Duo-Narrowband.  Why are the calibrated OIII images not appearing to take flats into account correctly?


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(@vincent-mod)
Universe Admin
Joined: 4 years ago
Posts: 3550
June 17, 2021 09:30  

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!


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(@ippiu)
Red Giant Customer
Joined: 2 years ago
Posts: 79
June 17, 2021 12:49  

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

ha

O3: quite well corrected 

o3

Color: well corrected 

color

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(@geordan)
White Dwarf Customer
Joined: 3 years ago
Posts: 18
June 17, 2021 19:57  

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


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(@vincent-mod)
Universe Admin
Joined: 4 years ago
Posts: 3550
June 18, 2021 10:31  

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


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(@vincent-mod)
Universe Admin
Joined: 4 years ago
Posts: 3550
June 18, 2021 15:27  

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.


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(@geordan)
White Dwarf Customer
Joined: 3 years ago
Posts: 18
June 19, 2021 09:22  

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


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