Hey all-
I have a very small patch of sky to image in, so I image 5+ objects a night over the course of weeks or months. This means I regularly have to take flats for the same object. I also use a rotator.
My method right now is to wait for a partially cloudy or hazy night, then set up a copy of my sequence, with all of the filters and rotation angles. I then disable all of the light frames, and enable all of the flat frames. After this I slew to an area of sky low on the horizon that gives me the most time available, solve/sync, and use that location as the new coordinates for all of the objects in the sequence. Thankfully the solve/sync “use these coordinates for…” dialog does not overwrite the rotation, or that’d be something I’d have to fix by hand. I then put one light frame of say 15 seconds in front of every single flat event in order to force a focus before taking the flats. Since I force autofocus on every filter change I am assuming that so long as the system is in focus the temperature doesn’t matter enough to make things too different. So I make sure the system is focused, then take that flat event, and move on.
The other thing I’ve tried is to simply take flats after every event- So take 30 60s Ha images, then do the flats right there and then (flip flat you are my hero here). The problem with this is that it significantly impacts imaging time (15-25 flat frames does actually take a fair bit of time to acquire). If I was also in an environment where other instruments were gathering data at the same time (say if I crank up another scope to do photometry), taking flats when other instruments are imaging would be unacceptable. The flip flat makes a lot of spare light around the edges.
So what I’m looking for is how other users have solved “the flats problem” when using large numbers of targets, large numbers of events, taken over a large number of nights with a rotator.
Thanks very much! Very much appreciate any input into how I could be doing this better.