Except for seeing the very faintest things, astronomical twilight is more-or-less irrelevant. It is nearly as dark as it's going to get within 10 minutes of the time of nautical twilight, and for things such as picking up M30 next Sunday morning, I wouldn't be worried observing well into the interval between the end of nautical and the beginning of civil twilight---something like that not-quite-arbitrary 10-deg figure Tom P used. For ordinary deep-sky observing I'd certainly start work shortly after nautical in the evening. In a lot of CCD-related observing (professional or otherwise), the time of sunset is maybe at least as important. With our 72-inch, you start taking twilight flat-field images in the B filter about 10 minutes after sunset (it would be even earlier with a standard U filter); or start taking spectra (2" entrance aperture) with the Sun about 4 deg below the horizon. Dave Schliecher, who does comet photometry at very short ultraviolet wavelengths, starts taking his flat-fields 10 minutes _before_ sunset! \Brian -- See message header for info on list archives or unsubscribing, and please send personal replies to the author, not the list.