You might find the table at the bottom of the webpage handy if you don't have any way of tracking the stars , just a fixed tripod and a camera and a lens.
http://www.eskimo.com/~rachford/widefield/calc.html
Interesting that a 200mm f2.8 (71mm diam) telephoto can record stars as faint as +11 mag in just a couple of seconds , w/out tracking and you can still get round star images.
(ON film) , I'm betting a DSLR can do a lot better than that .... would be nice if someone made the appropriate measurements to get the correct constants for DSLRs in the equation
Mag = 8.0 + 2.512 * log10(A2/A1)
Tracking accuracy wiould also need adjustment for pixcel sizes on the chip I think so :
T = P / 8.33 would change.
So would maximum image size for sharp stars :
M = P / 33.33
But handy anyway I think. I use it when I am on holidays and have left the handy GEMs at home , but still have my camera and a couple of lenses handy.
Got some very nice photos of the comet last year that way.
You can easily plug in the info for your 10" dob and get a handle on max exposures you can do and still have nice round star or "sharp" moon or planet images recorded without tracking.
A good way to do some poor man's astrophotography.