[Aavso-photometry] Transforming

[arne]

Barbara G. Harris wrote:
> Here are the TCs that I obtained about 1 month ago.  I re-imaged M67 a few
> nights ago to see how repeatable my numbers were but I have not performed
> the calculations yet for those images:
> Tbv:  1.268
> Tvr:  0.981
> Tri:  0.868
> Tv:  -0.189
> Tr:  -0.173
> 
> What do the negative numbers for Tv and Tr signify?  How do I know if a
> particular star that I am observing has a constant color?
> 
Barbara was using V-R for her coefficient.  So the equation is

(V-v) = zeropoint + Tv*(V-R)

to first order.

This has the general form

V_true = v_instrumental + Tv*(V-R)

If the coefficient is negative, that means for stars with large
(V-R) (i.e, "red" stars), you must decrease v_ins to obtain the
true magnitude.  A decrease means v_ins was measured too faint.
So Barbara's system is giving fainter measures for red stars than
a fully standard system would give.  This means her V filter has
a central wavelength slightly blueward of the standard system.

Normally, Tv is pretty close to 0.0, as the typical glass V filter
plus a CCD gives a good representation of the Johnson bandpass.  Typical
values are +/- 0.10 or so.  I am surprised that Barbara's coefficient
is so large.  The color coefficients (Tbv, Tvr, Tri) all seem to be
reasonable, though Tbv seems a bit large as well for a thinned,
backside illuminated chip.  I'll talk to Barbara off-list to see
if I can understand what is going on.
Arne