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LRGB Flats After Each Filter or at End of Session?


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Do you guys do your flats after you complete your lights through each filter, or are you waiting to the end of the session to do all your flats at once?

My only concern with waiting to the end of the session is that the focus may have shifted slightly somehow or that a little dust bunny may have been present in your light frames but by the time your filter wheel spun a few times, it wouldn't be present at the end of your session if you decided to take your flat frames at that point rendering an error in your flat frame.

Thoughts and comments?

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I take my flats at the end of the imaging session each night - the focus is adjusted if necessary during the actual imaging run, so should be spot-on at the end just like it is at the beginning. The flats only take a few minutes to get, I usually take 20 for each filter I've used that night. It's important that the rotation of your CCD/DSLR is identical to your imaging run otherwise the dust bunnies etc will not line up.

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Even with parfocal filters, the focus for each filter can be slightly different which is why I run a focus routine for each filter. Is it reasonable to ignore this fact and just shoot flats at the end without considering this slight focus difference?

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Yes, ignore it. Flats are so far out of focus anyway that the very slight tweaks between filters, or even the same filter if the temperature drops during a session, make no practical difference to the flats. It is big focus changes but mainly camera rotation that you need to avoid if you want your flats to match up well.

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The only concern would be the apparent size of the dust bunnies being different due to different focus. But this is something I could probably test/quantify in a matter of minutes indoors with some test flats. Thank you.

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