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Originally Posted by austinstkong
In relation to subs and exposure time, how would an image with 10x1min compare to an image with 20x30sec subs.
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In an ideal world *with* tracking, there would be no difference. However, in the real world, there are many, many factors that come into play, most of which are related to the abilities of your imaging and tracking gear. However, since you don't have any tracking, shorter subs are obviously better in your case, in order to minimize trailing.
Shorter subs have a bit more noise in them relative to signal (and the signal is obviously dimmer), but if you take enough and stack them, you can overcome that disadvantage. At higher magnifications, with more, but shorter subs, you also get the benefit of having a better chance at capturing fleeting moments of 'perfect' seeing conditions (that's why some people use video for some bright objects - lots of frames to choose from).
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Also, is there a way to work out how much I can leave the shutter open for before stars start to trail? (I've been using my best guess at getting an image with roundish stars and the longest exposure)
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Without tracking, the trailing starts immediately. In an ideal world, given the magnification you're using and size of the pixels on your CCD, you could calculate the time it would take for a star to move between pixels. You could then use that information to scale down your image appropriately to fit the 2 pixel trail into 1 pixel again.
In the real world, however, various factors (seeing conditions being one) 'smear' starlight over a larger area anyway, so you can get away not having tracking for a little while. How long that is exactly, amongst other things, depends on atmospheric conditions, your magnification, the quality of the optics and how much you're willing to scale down (or otherwise process - there's ways) the image afterwards.
There's definitely a lot you can do even without tracking. Trial & error and a bit of processing fairy dust will get you a long way!
