Jim, I think the following links will go a long way toward helping you with what you are asking about. You should definitely look at his discussion about Optimum Magnified Visual Angle and Appendix E from his book. http://clarkvision.com/visastro/index.html <http://clarkvision.com/visastro/appendix-e.html> http://clarkvision.com/visastro/appendix-e.html Clear skies, Dale. -----Original Message----- From: Dale Hooper Sent: Monday, November 03, 2003 9:22 AM To: Utah Astronomy Subject: RE: [Utah-astronomy] Seeing Formula I could see if there is anything in the book, "Visual Astronomy of the Deep Sky" by Roger N. Clark. -----Original Message----- From: Jim Gibson [mailto:xajax99@yahoo.com] Sent: Saturday, November 01, 2003 4:35 PM To: utah-astronomy@mailman.xmission.com Subject: [Utah-astronomy] Seeing Formula To the group's knowledge, has any one run across a formula for determining whether you might be able to see an object or not? I realize that there are factors like how well one sees, what the seeing conditions are, how big and objective one is using etc. If I made a simple little formula like 1/Mag/Object Size and I applied the formula to M51 with Mag 9.6 and size 5.4' that would produce a factor of .019. If I also applied the formula to NGC 4236 with Mag 9.7 and size 18.6' I would get a factor of .0055. I have never been able to see NGC 4236. So is it valid for me (using a 12.5" objective) to say that somewhere between .019 and .0055 is a cutoff as to what I can see? I know this is going to cut some of you wrong, because since I have gotten into astronomy I have never seen soo many get soo excited over soo little. Jim _____ Do you Yahoo!? Exclusive Video Premiere - Britney <http://launch.yahoo.com/video/?1093432&fs=1&redirectURL=http://launch.yahoo.com/promos/britneyspears/> Spears