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Transforming observations using VPhot

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onj's picture
Transforming observations using VPhot


I am using BSM NM and VPhot contains preloaded values for the Transformation Coefficients for this setup:

Tv = -0.010

Tbv = 0.990
Tvi = 0.994
Tvr = 1.001
Tri = 0.968

Arne also kindly supplied the value of Ti  (=0.002), which I entered using the manual input box as required.

VPhot then seems happy and can transform my (BVI) observations.

My question is how can this work (for the B observations) considering there appears to be no value for Tb? Is there another way it is being computed?

Here is my results from one night, untransformed and transformed respectively for T CrB:

B=11.486 and 11.502

V=10.177 and 10.151

I=7.525 and 7.531

I can send images if neccessary.




HBB's picture
John,  For BVI photometry,


For BVI photometry, you need the following transformation coefficients:

Tv, Tvi, Tbv

And the following equations (in order):

(Vo -Io) = (Vc -Ic) + Tvi[(vo - io) - (vc - ic)]

Vo = vo + (Vc- vc) + Tv[(Vo - Io) - (Vc - Ic)]

Io = Vo - (Vo -Io)

Bo = Vo + (Bc-Vc) = Tbv[(bo - vo) - (bc - vc)]



onj's picture
Transforming observations using VPhot

Hi Barbara,

I can now see that Tv and Tbv are sufficient to transform the B mag. I failed to notice that, as I usually think in terms of magnitude filters rather than colours.

Your equations are helpful athough I think you meant for the second "=" sign to be a "+" sign in the fourth equation.

For some reason VPhot does not use an analagous equation to transform the I-filter. It requires you to manually input Ti.

Many thanks,


HQA's picture

There are at least two ways of handling transformations:

(1) magnitude equations

B = b + zeropoint_b + k'b(X) + Tb(B-V)

V = v + zeropoint_v + k'v(X) + Tv(B-V)

Ic = ic + zeropoint_ic + k'ic(X) + Tic(V-Ic)

and the associated equations for (B-V) and (V-Ic)

(2) magnitude-color equations

solve for V, (B-V), (V-Ic)


B = V + (B-V)

Ic = V - (V-Ic)

Both methods work.  The uncertainty estimate in both cases gets a little complex, since you are solving for magnitudes and colors and you should do error propagation; most of that complexity goes away with differential photometry.  It sounds like VPHOT may be combining the two approaches, in that Ic requires the magnitude-equation, but I don't have enough experience with it to see if that is just a flag that needs to be set.


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