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instrumental magnitudes in VPHOT

SNH01's picture
SNH01
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Joined: 2011-01-29

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

instrumental magnitudes in VPHOT
kge's picture
kge
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Joined: 2010-07-19

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

Hi Geir, Thank you, that
SNH01's picture
SNH01
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Joined: 2011-01-29

Hi Geir,

Thank you, that worked :-)

Katy

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