[Aavso-photometry] Re: Lightbox usable for Ic filter flats - Summary of recent SBIG Yahoo Group thread

[Michael Koppelman]

I'm  not sure this is true. I you turn off your drive, the stars turn 
into trails. If you use some sort of min/max clipped mean or sigma 
clipping, the outliers are thrown out. I have some very, very nice 
twilight flats where you could see star trails in the individual images 
but not in the final flat. I haven't had success with sky flats that 
are not taken at twilight, but twilight flats are a fairly common 
technique as far as I know.

Twilight flats work best for me but I also use t-shirt flats made with 
an incandescent bulb. Yeah, you have to exposure B for a long time but 
that's OK.

I've also noticed lately that my t-shirt flats, while not strictly 
identical (from one night to the next) the differences are always in 
the extremity of the chip. I have one particularly flat part of my chip 
where I always put the stars of interest and that flat spot is 
virtually always the same from flat to flat (as compared by dividing 
one by the other).

Thanks for the nice summaries!

Cheers,
Michael Koppelman
http://www.lolife.com/astronomy/



On Oct 3, 2004, at 9:37 AM, Wolfgang Renz wrote:

> I think you won't find anybody trying to get robust flats for science
> work using a mean flat, especially for twilight flats. Even near the
> galactic poles there are always stars in the field, even if they're
> unseen they add signal that can really ruin photometry.