[Aavso-photometry] Reality Check - Basic Concepts (Flats)

[arne]

David Trowbridge wrote:
> I generally take my flats right after sunset, aiming the telescope low in
> the eastern sky at an azimuth roughly opposite to where the sun went down. I
> put a T-shirt over the end of the telescope and take 4-5 images with each
> filter, then stack them. Would aiming higher in the sky, close to the zenith
> give better flats?
> 
> Occasionally I use a light box with incandescent bulbs, but it requires a
> long exposure, especially with the B filter. I probably need to replace the
> incandescent with halogen bulbs.
> 
Lots of hidden details in a question like this,
The T-shirt effectively randomizes the incoming light, so it will
make only a small difference where in the sky you are pointed.
The preferred sweet spot is straight overhead.  Note that many
amateur systems have some mirror flop, so pointing low on the
horizon, a position you rarely put the telescope into when
observing, may give a different optical path than pointing
in a more reasonable position.

4-5 flats per filter is marginally ok, depending on how deep you
expose and what your chip full well is.  "stacking" means different
things to different folks.  Average or median or what?  normalizing? etc.
Berry/Burnell have a good description of how to create good flats.
In fact, lots of books out there that would be helpful at this
level of question, and you should probably read one or two (or
look at the AAVSO CCD manual if you want something free).
Arne