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instrumental magnitudes in VPHOT

SNH01
Affiliation
American Association of Variable Star Observers (AAVSO)
Wed, 08/17/2011 - 09:54

Hello,

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?

Thanks
Katy

kge
Affiliation
Norwegian Astronomical Society, Variable Star Section (NAS)
Wed, 08/17/2011 - 14:10
instrumental magnitudes in VPHOT

Hi Katy,

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.

Geir