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You're off to a flying start there Duncan.
Here's what I do with flats (and sometimes I don't use them as occassionally I get a flat that worsens an image for some reason).
1. White T-shirt over the end of the scope.
2. Point towards the west about 15-20 degrees up.
3. At dusk I take a flat for each filter. Usually I go for at least 3 seconds
(not so long if using an SBig but FLI and Apogee with a shutter requires that long otherwise you see the blades of the shutter in the flats).
I go for an exposure around 20-23,000 ADU.
You have to be fast and efficient as it is amazing how fast dark encroaches on you. You'll see the ADU number drop with each flat.
Make sure the camera is at the same temp you will be imaging at.
I take:
Luminance 1x1
RGB each 2x2 and one for Ha 1x1 if I am shooting Ha.
Don't change the position of the camera when you go to image it has to line up with the flats or they are useless.
Telescope should also be in focus even though its a flat as that also would be a difference between the flat and the light.
Use median combine, I usually take 3 flats.
I also take a bias at the same temp. I take 3 and median combine them.
These are subtracted when making the master flat.
I use CCDstack where its all very easy and I use CCDsoft to control the camera.
You don't want a flat that is too bright like 50,000 ADU. That'll degrade the image. Go for about a 1/3rd of total exposure ability of the camera - ie 22,000 ADU (fully exposed is 65,535).
Hope this helps.
Greg.
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