British Astronomical Association, Variable Star Section (BAA-VSS)
Wed, 11/15/2023 - 20:46

Thinking about taking flats.

If you change filters does that mean your flats are obsolete because I can't see how

the filter wheel can possibly return the filter back to its original position.

Given that in a photometry session you are likely to be changing filters a lot.

American Association of Variable Star Observers (AAVSO)
Variation in flats

     If the mechanism for changing filters involves a wheel or a slide, there ought to be some sort of mechanical detent or perhaps fixed number of steps in an electronic set-up so that the filter positions are repeatable to within small tolerances.  If this is not the case, what you'll see is that the various dust-doughnuts or other things normally taken out by the flats produce little ghost images immediately next to where they were when the flats were taken.  This is often not utterly detrimental to the photometry, but obviously one wants to look for such things.  It can also be the case that dust flecks etc can accumulate during a single night, and some suggest taking flats both at dusk and dawn (for twilight flats). 

     The thing to do of course is to examine the images carefully to see what's happening.