Steve, The individual exposure times tend to be driven by the brightest objects in the field of view. The goal is to prevent saturation of anything except perhaps the brightest star or two. If you allow any object to saturate completely, you can never bring back any detail. It will just be white. A normal histogram for a single exposure has a few bright stars that are near saturation, and a whole bunch of stuff that is almost black. The processing phase targets enhancing the almost black parts of the image. With my equipment and filters, I most often use 10 minute subs, but often reduce that to 5 minutes for bright objects, and go to 20 or 30 minutes for really faint stuff. I don't use my DSLRs for imaging, so I really can't speak to exposure times with those cameras. David Rankin could probably help you with that, since he uses a DSLR. I like using a short focal length for my guide scope. Mine guide setup is a Mini Borg 45ED refractor with a focal length of 325mm, fitted with a Starfish autoguider. It gives me plenty of guide stars to choose from, no matter where I point it. I typically use 3 second exposure for the autoguider. I hope that helps a bit. Cheers, Tyler -----Original Message----- From: utah-astronomy-bounces@mailman.xmission.com [mailto:utah-astronomy-bounces@mailman.xmission.com] On Behalf Of Stephen Peterson Sent: Wednesday, November 03, 2010 7:12 AM To: utah-astronomy@mailman.xmission.com Subject: [Utah-astronomy] Astrophotography questions Starting to reeducate myself RE astrophotography and have a couple of questions: What should my individual images for stacking look like? I know stacking increases S/N. Assuming typical suburban light pollution, at say ISO 800 what is a typical exposure for eg M27, or M8 with f4 optics? How light/dark should the background be, i.e. how much skylight can I tolerate and subsequently "remove" by stacking? Confirm that for auto-tracking a short FL refractor is better than a longer FL one. The Solitaire tracker has trouble finding a suitable star, say near M27, with my f9 85mm refractor. Thanks for any help. Steve _______________________________________________ Utah-Astronomy mailing list Utah-Astronomy@mailman.xmission.com http://mailman.xmission.com/cgi-bin/mailman/listinfo/utah-astronomy Visit the Photo Gallery: http://www.slas.us/gallery2/main.php Visit the Wiki: http://www.utahastronomy.com