Flat frames for OSC images

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vichman
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Flat frames for OSC images

Post by vichman » Wed Oct 31, 2018 6:17 pm

Hello all,

I am shooting my images with my ZWO ASI071MC Pro, which is an OSC ("one shot color") camera.

I have some doubts whether I am applying my flat frames correctly.

White images coming directly from the camera have a strong greenish color, as they are not white calibrated yet, so also all my flats (taken with a pure white flat field box) are quite greenish.

I have now three options:
- take the flat frames as they are (greenish) and apply them in Pixinsight. I doubt that this will lead to good pictures.
- color calibrate the flat frames before applying them. (how?)
- shoot flat frames in black&white mode (this is possible e.g. in Sharpcap)

So far I used option 3, but I do not known if this is a good idea to apply a black&white flat frame to a color image.

Anyone any ideas?
How do you process your OSC images with flat frames?

Thanks, any help is highly appreciated!
Stefan

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Support@ZWO
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Re: Flat frames for OSC images

Post by Support@ZWO » Fri Nov 02, 2018 5:48 am

It's simple. flat frames must use same color factor as light frames.
Usually, take RAW data, which looks gary. Light, bias, dark, flat, all do the same.
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Konihlav
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Re: Flat frames for OSC images

Post by Konihlav » Fri Nov 02, 2018 9:27 am

it's not that simple.

With OSC color camera you have to:

1. attempt to have a uniform/non-gradient light for taking the flat frames (there are some "neutral" EL-panels too on the market) or shoot night-sky flats (twilight etc.)
2. check the lowest ADU value in the corners and compare to the highest ADU in the middle of the FOV so as the range of your vignetting is "reasonable"
3. you have to check it for all 3 colors R G B... if you manage to get a reasonable variation for all 3 colors in corners/center of the FOV then you may be OK with taking 1 set of flats for whole picture
4. if you have some problems (can't calibrate in some of the 3 color channels) you have to split the bayer planes into 4 - making a 4 set of raw fits referring to R G G B pixels and use this MONO :-) 4 channels to calibrate for vignetting...
:-)

BTW I do take 5 second long flats with my ASI094 but my uniform light EL-PANEL is on the absolute minimum of its brightness (am using F/3.6 BTW).

Konihlav
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Re: Flat frames for OSC images

Post by Konihlav » Fri Nov 02, 2018 9:29 am

and don't be scared if you max-STF-stretch the calibrated lights in PixOutsight and see a subtle color gradient in the data. Even the perfectly most (my images) calibrated FITs will show some color gradient in max STF in Pixi because Pixi really ultra stretches the data...

vichman
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Joined: Tue Aug 01, 2017 8:26 pm

Re: Flat frames for OSC images

Post by vichman » Sat Nov 03, 2018 12:09 pm

Hello Konihlav,

thank you very much for your advices!

May I ask what panel you are using?

Thanks
Stefan

Konihlav
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Re: Flat frames for OSC images

Post by Konihlav » Sat Nov 03, 2018 9:49 pm

I think I am using something named "16 inch Spike-A Flat Fielder" based on my own blog post http://blog.astrofotky.cz/pavelpech/?p=1160

but I also purchased some "Lacerta el-panel" some 2 years ago, but haven't used it yet :-) am too busy with other things...

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Support@ZWO
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Re: Flat frames for OSC images

Post by Support@ZWO » Mon Nov 05, 2018 2:53 pm

Looks cool :)
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