Beginner reduction spreadsheet valid at all air masses?
I've had an email exchange with Brian off the forum on this topic. I thought it might be of interest for others to see what I've been doing.
In the documentation on DSLR photometry, it is stated that the beginner reduction spreadsheet should be used only at relatively low air mass.
I've done some tests on a star field in Grus, with only slightly more than 1 degree separation between the comp, check and target stars (DSLR images taken through a 600mm focal length refractor).
The results suggest that the spreadsheet can be used at any air mass with no or minimum loss of precision. Remember, this spreadsheet does not specifically include calculations to correct for air mass.
I'd be curious to hear if anyone else has tried this. If you haven't, you might find it interesting to give it a go.