Affiliation
American Association of Variable Star Observers (AAVSO)
Mon, 04/28/2014 - 11:52
Hi,
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.
John
For BVI photometry,
John,
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)]
Barbara(HBB)
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,
John
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)
then
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.
Arne