Dang! Kurt, you are a rare find.   Your wrote: > B the above equation is not the same as surface brightness expressed in magnitudes per square arcsec.<   When I saw my outcome of 20 for surface brightness and Berry was saying 4*10 to the minus 6 I knew I was talking apples and oranges; but I didn’t know how to convert apples to oranges.  

From your suggestion I did down load Covington’s calculator and played with it. I am really in tune with you on this comment, > reviewing the math does help to gain some understanding of how things work.< and that is how I feel. I like to see and feel the relationships as expressed in math to see how things work. So, Covington’s calculator is nice, but you have already got me creating my own calculator, and that is just to see how things work.   I can use Covington’s calculator as a check though.  His calculator says that I need 25 minutes of exposure time for M1. I can mess with my calculator with the formulae you provided till I get in the ball park.   Speaking of ball park, Yogi once said, “theory and practice are the same in theory, but not in practice.”   Thanks Kurt   Clear sky images   Jim