1. new column I =  (starC1-sky) / (starC2-sky)
     these are ratios of intensities

2. convert to differences of instrumental magnitudes

        new col delta_m = 25 - 2.5*log10(I)

3. calibrate to create real magnitudes

        pick one reference star
        look up its magnitude in a catalog
        find the offset

                offset =  (real mag) - (instr mag)

        subtract that offset from ALL instrument mags


   suppose in image 48, star C2 has instr. mag  14.50
           in catalog, star C2 has mag V = 13.45

         -> offset =  13.45 - 14.50 =  -1.05
    then
      add -1.05 from all instr. mag in image 48