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Hi Jeff,
There is a free ccd exposure length calculator on the net. Try googling it.
It may be by Ron Wodaski. As I recall 10 minutes was ideal for the 2000XM with the KAI2020 chip.
I think practical considerations come into it.
A lot go 1x1 binning for luminance and 2x2 binning for colour. One set is to do 15 mins at 1x1 for luminance and 10 minutes at 2x2 for colour.
This way you only need 2 darks - a 10 minute one at 2x2 and a 15 minute one at 1x1.
Planes, meteorites (more at a dark site), clouds, etc make long exposures more likely to be unusable. But more likely unless you have a very expensive and high quality mount 30 minutes is not going to be easy.
So 10 minutes 1x1 for luminance and 10 minutes 2x2 RGB or even 15 mins for luminance would be my suggestion.
It can be quite frustrating getting LRG and then clouds come in and no blue yet.
So another tip is to do one LRGB at a time rather than 6 L then 3 R, 3 G, 3B as you can't get that colour image at all until you have all of them.
30 min subs really requires excellent polar alignment (highly accurate) and a highly accurate mount and autoguider and stable clear skies (nothing worse than up to 25 minutes and you can see some clouds sailing over!).
Greg.
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