Flats with Autofocuser Workflow

[kirkster501]

Hey folks, any guidance on this please?  I am getting into a pickle with it.  How do you do your flats?  The focuser is moving so even on the same filter in the same run, the focus point can be in different places.  I am confused as to the workflow and would appreciate some guidance please!   It woudl be a PITA to get flats on the same filter before every focus point move.  If your focus range is within a small window then is it OK to have an "approximate flat" that covers the focuser range for that filter for that night by reading the FITS header to see where the focuser position was?

Also want to understand flats after a meridian flip.  Should there be a different set of flats for that?

Thanks. Steve

[Jannis]

I usually take my flats the morning after where I've left my camera attached and the focus locked. I shoot straight up into the sky before the sun gets up, but after the brighter stars are no longer visible. The minimal focus change caused by a few degrees temp change is so small it haven't seems to affect any of my images at least.

As far as before/after flip i can't really say I've ever even thought about that. Dust that is stuck doesn't care about flip, and very loose dust will probably not return to the exact same position even if you flip it back for flats, but i don't know, maybe there are other reasons why people want this...?

[ollypenrice]

Small changes made to focus during the night will not have a percepible efffect on flats. Just leave the focuser where it was for tha last image and then do the flats. It's a non issue. The flip has no effect other than in risking moving dust around, as Jannis said, but I can't say I've ever known this to happen. Everything in the light path moves together as the scope tracks and flips so it has no effect. In fact, with a scope as well made as the TEC, you will almost certainly be able to re-use flats even after rotating the whole focuser with the captain's wheel, because everything creating bunnies will, again, rotate together. The ight cone will also remain centred thanks to the high build quality. With our observatory-based TEC we can often use a set of flats for months, even after rotating the back end. (Tell nobody, but I often use Lum flats for all filters, too. If they work they work. If they don't I do a separate one.)

Olly

[kirkster501]

Thanks guys for your ideas and advice!