i'm seeing quite a range of subexposure estimates depending on my test exposure times. is there something about 3 minutes other than the short length that makes it the most trustworthy. does it have something to do with guiding? is it a problem if you're guiding during your test exposure? for example i take a 3 min test exposure, it tells me i should use 7 minutes; then i try 10 minute exposure it tells me 12 minutes. which one is correct?
thanks,
dave

Dave,

The sub-exposure calclator bases its results on background ADU.

Did you do these tests on the same night?

rb

yes. all tests done on the same night (before moon came up)i think i did a 3min, 5, 10 and 15 all yielding different estimates for subexposures.