David; You might consider doing an experiment. Try shooting a couple of fields of view that have a variety of objects that have differing magnitudes. Consider NGC 7331 in Pegasus or the Pelican Nebula in Cygnus, the outer layers are much fainter than the inner bright sections. Open up you planetarium program and see what the faintest galaxy is around 7331. Like that. Take on those areas with several different exposure times and see what the results are versus your visual observation from, say, the All-Arizona site. Hope that helps; Steve Coe -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.