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Hi,
I do the following,
On a bright cloudless day i point my scope at the blue sky away from the sun. I stretch a grey or white bonds t-shirt across the end of the scope and take an exposure that gives a histogram spike about one third of the way across the range. Usually much less than 1/3 second is required, depending on the brightness of the sky and the thickness of the T-shirt.
Note, This will only work if you have left your scope focussed from the previous night with the camera and anything else (reducer etc) attached and in the same orientation. The orientation is easy. I just have a few small pieces of duct tape on the nosepiece and the focusser marked with a pen.
If you wish to take flats at any time though you will need a light box. Then you can take them right after your imaging session before you tear down.
The T-shirt method lets you get some good flats and allows you to practice your technique before going to the trouble of building a lightbox.
I find my flats last a few months, until I clean all my equipment then i take some more. I dont bother with a lightbox, though one day i may build one.
Usually I take 3 or 4 flats pointing at different parts of the sky. I just give them to Maxim or DSS and let it worry about creating the master flat when it calibrates my subs.
Cheers
Paul
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