Question about Flats - Forums

Topic starter

Frequent contributor

Dec 9, 2022

I have a CMOS OSC and use one filter at a time, either the Optolong L-Enhance narrowband filter or the Optolong L-Pro broadband filter. They are the same size (2").

My question is do I need to take flats with both filters or can I take all my flats with just one, even if I use the other filter for lights?

Thanks!
Jerry

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Ed Dixon

Dec 9, 2022 · Edited: Dec 9, 2022

My general approach is to take flats with the same setup I use for lights. Same gain, filters, and gear.

https://astrobackyard.com/how-to-take-flat-frames/

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Arun H

Frequent contributor

Dec 9, 2022

There is a good reason to use the same filter for flats as for lights.

Dust/spots will almost certainly be different, vignetting may be slightly different also due to small differences in filter position, and the response of the sensor could be different as well.

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Andy Wray

Dec 9, 2022

Take separate flats and dark flats for each filter. They take next to no time to take, so why not?

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Lorenzo Dal Molin

Dec 9, 2022

At the end of each session run with a filter you have to do the flats for that setup. Each filter will give its specific vignetting.

Jerry Gerber

Topic starter

Frequent contributor

Dec 9, 2022

Great, I'll follow everyone's advice here. It makes sense…

Thanks!
Jerry

Lynn K

Dec 9, 2022

You want think of a Flat image as a photograph of your optical system. When you change filters, you change the optical system.
If you are removing/re-installing the the filers each time you use them, more likely dust is accumulated or cleaned off. That will require a new flat field image to properly subtract the dust donuts.

Even if you use a one-shot color camera, and you are changing filters often, it may be convenient to have a closed/sealed filter wheel. It does not need to be motorized. It can be manual. That will eliminate the dust changing every time you change filters. And, flats will not not need to be taken every session. As long as you don,t disassemble the filter wheel from camera and cover the filter wheel opening.

LYNN k.

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Bray Falls

Dec 10, 2022

Flats MUST be taken through each filter

Timothy Prospero

Dec 10, 2022

Flats are made for correct imperfections on the optical train, so yes you have to take flats with every filter you use during the session.

What Insaw from my experience is the focus has a bit of tolerance so depending the situation I generally during the night there will be a shift in focus but (at least for me) it doesn't make any problems with the flats.

Cheers and CS

Timothy