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Flats and focus


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Tell me, if I'm using software to adjust focus every few degrees of temperature change, do I need to retake flats everytime I shift focus and then stack seperately for each change in focus?

Also, what sort of temperature change might I expect to cause noticeable focus change?

My setup is a 925mm f7 scope with atik 383 osc so at least I don't have to multiple the extra work by 3 or 4! 😁

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The focus shift you will experience in any given night will be something like 0.1-0.2mm at worst (for an aluminium tube, less of an issue with anything else) so you can ignore focus position with flats as long as you dont go ahead and have the focuser be at some completely out of focus position. The flat panel in front of the scope is like a kilometer off focus so the tiny little movement at the back of the focuser is of no concern to the flats.

I focus manually so cant tell what the right way to set up in software is, but i keep an eye on HFR readings during the first couple of hours of imaging. During that time i typically re-focus maybe 3 or 4 times as the scope is still cooling down from being stored indoors but after that focus typically stays more or less put and i dont need to babysit it anymore. If the night gets much colder, lets say by at least 5c, i might need to refocus well after the scope has cooled down. HFR numbers will tell you that, maybe set up a refocus call in whatever software you use when HFR has drifted by a certain amount? I know NINA can do this at least.

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Most of the stuff that shows up as dust motes on the flat is on the camera sensor window, so subtle changes in focus won’t affect this or any vignetting either. I’m fortunate to have a permanent setup so if I need to, I take the flats the following morning, using the last in focus position from the night before. When I had to take down after the imaging session I took them at the end of the imaging run.

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