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transformation confusions

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transformation confusions

I wonder if someone would have time to clear up some confusions for me please?

1. The Vphot admin page allows input of several TC's - do the 'Filter band coefficients' equate to Tv Tb and Tr etc?
2. I assume the 'colour index coefficients' are self explanatory (e.g. Tbv for b-v)
3. I am most confused however with Tv, Tb etc - surely these will be airmass dependant if they are simply slope of for example Tv (catalogue Vmag - measured vmag) versus CB-CV.
4. I don't get airmass in my fits but i see vphot calculates it - however, this calculated airmass is not translated to reports for time series I do - is it possible to have the calculated airmass tranferred to time series reports?

Thanks in advance if anyone has the time,



Transformation Coefficients


Hi Martyn

Photometric data reduction consists of 4 parts.

raw magnitude,
color transformation (Tv etc)
extinction (k;v etc.)
system zero point.

The air mass and thus extinction has no connection to the color transformation coefficients.  Extinction is affected by color, pwever with the shorter wavelenghts affected the most. Each item is handled by itself. For differential photometry the zero point and extinction usually cancel out unless the progrm and comparison stasr are separted by a significant air mass and thus have significant different extinctions. There is confusion about the Tb and Tu versus Tbv and Tub. Someone should explain how VPhot works in that regard. Normally the V transformation is called epsilon, the B-V mu and U-B psi so I am not suret what Tb and Tu are.


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