Flat frames with autofocus

[Gumminess8083]

Hello everyone, how should I deal with flat frames when using an autofocus motor?

I usually take my flat frames using NINA's flat wizard, but, since I frequently run autofocus sessions during my imaging sessions, I don't exactly know what my focus was during each specific image.

Should I keep track of all focus values throughout the night and take separate flat frames for each of them, or can I just eyeball it and take some flat frames at whatever focus value the scope is at the end of the session?

[cbrunfranc]

I'm not an expert but from what I have read you do not need to have a perfect focus for flat frame.
I do have also an autofocus and I only do flat frame at the beginning with a "rought" focus and the calibration is great

Christophe -BF-

[teoria_del_big_bang]

I agree with @cbrunfranc, in general the focus position should not change much throughout the night as it is really only changing due to thermals (as far as I am aware). 
If you look at the actual focus position then it may look to have changed a lot because the figures are so much different but these are so fine adjustments in reality the focus tube should have moved very little so one set of flats will suffice for all frames.
For what it is worth I would normally take flats at the very end of a session (automatically programmed into NINA) so the last focus position used for images should be as good a focus as you can get for Flats but again whether that matter I really doubt it, most of the things you are calibrating out of the final frames are so fae out of focus anyway.

Yes, ideally  you need different sets of flats for each filter used in a session (although many argue not and indeed seem to produce great images using same flat for all filter so I cannot argue although separate flats for all filters is my preference as they do not take long to take) but I think the only time you then need to take a further set (for each filter) is if you have rotated the camera between two different targets, for instance, so often as I have an automatic rotator I may have to do this.

Steve

Edited November 1, 2022 by teoria_del_big_bang

[AstroMuni]

1 hour ago, Gumminess8083 said:

I usually take my flat frames using NINA's flat wizard, but, since I frequently run autofocus sessions during my imaging sessions, I don't exactly know what my focus was during each specific image.

The way I understand it, flats are mainly taken to help correct issues as a result of artifacts in the optical train (eg. dust etc.). I find that the focus doesnt alter too much, if you havent altered the optical train, hence I literally take flats once every few months. If I do find residual artifacts after an imaging session then I know I should get a new set done. Perhaps its just me not being too particular and pixel peeping,  but I dont find any great advantage by taking flats after every session as I cant visibly see any appreciable improvements.

[Gumminess8083]

Thanks for your replies, everyone!

I'll keep shooting them at the end of the session without fussing too much about th edetails then