The plot shows the apparent magnitude of the moon in V and B as a function of lunar phase (phase=0 is new moon).
We measured the flux in images in which the filter was reliably V or B and used the transformations determined from NGC6633 (i.e. http://earthshine.thejll.com/#post229) to get the apparent magnitude.
These data have been obtained for a large range of airmass (z) -- from z = 1 to 10, with most of the data in the range z = 1 to 3. We derived extinctions of 0.10 mag/airmass for V and 0.17 mag/airmass for B, by comparing to the apparent V magnitude from the JPL ephemeris for the Moon (http://ssd.jpl.nasa.gov/horizons.cgi) (more about this below). The solid black line shows the V apparent magnitude as a function of phase after extinction correction, and adjusting the zeropoint by 0.2 mag in V to fit.
Note that B is ~ 1.0 mag fainter (i.e. B-V ~ 1.0, as we'd expect).
The airmass fits are shown above : the plot is the difference between the apparent magnitude from the JPF ephermeris and our transformed instrumental V band (or B band) magnitudes, shown as a function of airmass. The two lines show 0.10 mag/airmass (V band) and 0.17 mag/airmass (B band) -- they are not fits. There are some bad outliers, especially in the V band, which are probably due to the incorrect filter being in the beam.