[Aavso-photometry] Flat frame quality and its effect on photometric accuracy

[arne]

George Sjoberg wrote:
> Hi Lionel,
> 
> Couple of questions about your Flat frame acquisition with the GEM. When you say you take 10 to 25 flats for each filter is that 10 to 25 flats for each side of the meridian? (A total  of 20 to 50 for each filter?) For your masters do you average or median combine (or maybe some other)? What length exposures do you limit yourself to? I've been doing 24 (12 in each orientation) and i'm lucky if I can get two filters done at dusk and two more at dawn. I've become very frustrated at the lack of repeatable positioning of the sbig filter wheel (I'm using the same exact camera as you) Don't have any idea about what to do about the filter wheel.
> 
A couple of hints, for filter wheel positioning.
- always approach from the same direction.  For example, give a filter
command to go to the first filter position, then back off to the
desired filter position.

- keep the filters as clean as possible.  Repeatability is not
a great issue except for dust.

- if you only have a couple of dust specs, avoid placing important
stars near where they occur.

Very few vendors do filter wheels very well; it is hard to make
a precise positioning system for a reasonable cost.  You should
look at the specs for your wheel to see what you should expect.

George, you have 4 sets of flats: east/west plus evening/morning.
What kind of differences are you seeing on your flats?
Arne