I am trying to find out, how VPhot calculates the instrumental magnitudes, but I can't.
As far as I know, the instrumental magnitude m is calculated like this: m=-2.5*log(gain*counts)
But this ist not, how VPhot does it. Maybe somebody can explain me that?
VPHOT calculate instrumental magnitudes as
m = -2.5*log10(count)
where count is the total intensity within the aperture circle pr time unit, after sky subtraction. That is, if ci is the intensity of pixel i, S is the estimated sky intensity and t the exposure time in seconds, then
count = SUM(ci - S) / t
The intensity under 'Signal Circle Statistics' on the measurement details page is SUM(ci - S), so you have to divide by exposure time before plugging it in in the first formula above to obtain the instrumental magnitude.