Flat frequency

[deepsouthernskies]

Hi,
I have some confusion with the taking of flats for calibration.
I have seen that some imagers take flats only every few months, whereas I have read that flats should be taken on each side of the meridian if there is a meridian flip, for instance.
Assuming that we are only considering an individual filter and binning and temperature are the same for the same optical set up, can flats be re-used  for different targets, or when the focus changes over the course of the night or from night to night.
Thanks, Al

[pfile]

the meridian flip should not matter unless you have a rotator and rotate the camera 180 degrees after a meridian flip. some people with OAGs and narrow-field instruments need to do this in order to make sure they can find the guide star they were using after the flip. but if you don't ever change the camera's orientation to the telescope, the flats will be good.

as for how often, if you have a fixed setup and never tear it down, and you don't have big problems with dust, then it is indeed possible to re-use flats for a long time. if you set up every night then you risk changing the camera's rotation and introducing dust into the system which may show up in the flats/lights.

i suppose if your filter wheel does not do a good job centering the filters each time with good accuracy then just moving the filter wheel could destroy your ability to take flats that match the lights. my camera is pretty good at this and whatever dust is on the filters is pretty well stuck on there, so the dust shadows are constant for me.

i think it's generally believed that precise focus is not super important for flats - as long as the camera is near focus it should be fine.

rob

[deepsouthernskies]

That's really helpful, Rob; thanks a lot.