A few other things I missed, I keep the ISO to a maximum of 800, so that noise does not become an issue and restrict the exposure to between 10sec and 30sec. Less than 10sec and scintillation in the atmosphere will reduce the accuracy of the measurements and longer than 30sec field rotation & tracking become an issue on an alt/az mount without guiding (its not possible to guide in alt/az mode anyway). I also avoid a 15degree radius of the zenith also due to field rotation due to the alt/az mount.
James