[Aavso-photometry] [AAVSO-DIS] MagType in the new extended format

[arne]

Mark Munkacsy wrote:
> Arne, thanks for clarifying that the difference between ABS and DIF is 
> not the same as the difference between "all-sky" and "differential". 
> But, in the example that's provided for SS Cyg with the new, extended 
> format (example number 2), all four measurements have different 
> magnitudes listed for the comparison star. As I understand it, the only 
> thing that would cause the measured magnitude of the comparison star to 
> be anything other than the comp star's reference value would be if an 
> ensemble was being used to set the zero point or if this was truly 
> all-sky (not differential) photometry. And my reading of that example is 
> that an ensemble is *not* being used. (And because a comp star is being 
> used, I guess I can conclude that this doesn't represent all-sky 
> photometry?) Where am I confused?
> 
The magnitudes reported in the CMAG and KMAG field are supposed to
be the raw, instrumental magnitudes returned by your software program.
They are *not* the values listed in the chart photometry table - we
already know those values, and don't need them repeated to us.  With
software here at HQ, we then subtract the CMAG and KMAG fields to
obtain (K-C), and keep track of that changing value for determining the
HQerr field.  In other cases, plotting CMAG might tell us the quality
of your night, or permit adjustment of your photometry if the standardized
magnitude for CMAG changes.  Under normal circumstances, we use the
variable-star's magnitude for almost everything, and these CMAG/KMAG
fields are for ancillary checking.

Now, I realize that some programs ask for the standard magnitude for
the reference star, and then use that to give you the magnitude for
the target star and optionally any other star in the field (like,
the check star).  They are trying to save you some of the work,
but at the same time, are hiding some of the details from you. Under
those circumstances, you don't have the instrumental magnitude for the
comparison star returned to you (shame on the software vendor!). Then
report the standard magnitude for the reference star in CMAG (yeah, I
know that it will then be a constant), the reported magnitude for the
target star in the MAGNITUDE field, and the reported magnitude for the check
star in the KMAG field.  So two out of three fields will change under
that scenario.  This is not optimal, but is your best bet based on
what your software reports.  I think, even in these cases, there are
some tricks (like including an artificial reference star in each image)
to get around the limitations, but I don't use such packages myself.

We have given these formats to all vendors, and they have all said that
they would support the format, so hopefully you won't have to worry about
these details in the future.

I hope that clarifies things!
Arne