|
Sky flats can be a problem if you have lots of filters/binnings you need to take while the sky brightness is dropping fast.
You have to work in strict order of sensitivity (least sensitive filter/bin combination first, upto most sensitive), but even then, you are forced to compromise on how many images you can take.
For a median combine to reduce noise, you really need a fair number of images. 5 is not very effective.
I now use a homemade EL panel, with several layers of white paper in front to reduce the level to something that lets me use a reasonable exposure time (typically between 2s and 8s).
I will take at least 25 images (along with matching darks). Since they are all at the same intensity, the differences are essentially the gaussian noise, so median combining really cleans them up nicely.
I've analysed the flats images for artifacts produced by the panel/paper, and can't find any. It's working well for me for precision photometry.
Agreed, always take flats after the session, otherwise you cannot change ANYTHING in the optical train.
Ivan
|