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Hi all!As summer is getting closer, I think it's time to look at extinction/airmass.While some reduction packages have an airmass claculator, others don't, so I think it would be best to integrate airmass into the Excel spreadsheet.I haven't looked at the math in dedai, but I understand that for a first order compensation of extinction effects, it boils down to computing a singe number, the airmass coefficient, for each of the involved stars (variable and comparison stars). Air mass will depend on:-coordinates of the stars (we can stick to the set of comparison stars we use in the spread-sheet, no need for user-input)- Earth-coordinates of the observation point (user has to input this once, we could include a HOWTO using google-maps to find coordinates accurately ) - Date/time of observation. What do you think?CSHB