[Aavso-photometry] Reporting transformed data

[arne]

George Silvis wrote:
> When you report un transformed data, CMAG and KMAG are your 
> instrument magnitudes and VMAG is (at least from AIPWIN):
> VMAG= Vinstrumentmag - (Cinstrumentmag -Creferencemag). This is 
> equivalent to adjusting your Zero Point so that your 
> Cinstrumentmag==Crefrencemag.
> 
> So what should be reported if you do transform the data? VMAG is now 
> transformed, but it is unclear to me what should be reported for CMAG 
> and KMAG or if there is any value to these values. What should be reported?
> 
There are several reasons why we've asked for CMAG and KMAG to be
reported to the AAVSO.  First, (KMAG-CMAG) gives us a first estimate
as to the error in the observation for a time series.  Second, either
KMAG or CMAG by itself tells us the atmospheric transparency (were
clouds involved during a time series).  Finally, if you use a
reference magnitude that is incorrect, then the researcher can
adjust your observation at a later date by inverting the equation you
show above, obtaining the Vinstrumentmag, and then reusing the
equation with the correct Creferencemag.

It is the latter "use" of these values that will be incorrect at
second-order if the Vmag is transformed and Cmag/Kmag are instrumental.
We don't tell you whether Cmag should be instrumental or
instrumental-transformed, and to be honest, it depends on how you
do your differential photometry.  I think that level of confusion is
ok for now; otherwise, HQ would have to ask for *everything* involved
in your processing and the form would get even more complex.  So use
Cinstrumentmag or Ctransformed as you see fit, and if you want to be
a bit more descriptive, you could put a comment in the notes field
indicating how CMAG was obtained.

> Also, Cohen's article spoke of submitting your transform coefficients 
> to the AAVSO. Is that still done and to whom?
> 
At the time of the article, the transformation coefficients were to
be sent to the chair of the CCD committee (Gary Walker).  We are
in the process of developing a database at HQ to handle observer
transformation coefficients, so I'd send the coefficients to Gary
for now and then we will gather them from him when the database
is ready.

> Suggestions/Instructions?
> 
> 
> I've finally computed my transform coefficients with help from the 
> forum here and Lou Cohen's article. I've got a spread sheet (don't we 
> all!) that takes 4 AIPWin reports and kicks out the numbers.
> 
> Working with 3 image sets of M67 I derived for my CB245 camera the 
> following coefficients:
>      Tbv    Tvr    Tri     Tv     Tr
>     1.071  0.995  1.013 -0.067  0.098
> +/- .014   .014   .036   .007   .016
> 
> Does anyone else in the group have transform coefficients for a 
> CB245? Do your values look anything like these?
> 
The coefficients look good.  You might try Tvi as well.  My
assumption is that you were using (V-Rc) for your Tr coefficient;
you might try the equation using (V-Ic) as the color index
instead, since (V-Ic) is nearly double the size of (V-Rc) and
the fit will be easier.
Arne