[Aavso-photometry] Questions about Z And Photometry

[arne]

Steven Orlando wrote:
> What is the difference between a reference and a check star?
> 
> AIP4WIN doesn't use the term reference star. Maxim DL does, and when you do 
> photometry in MaximDL, you get in the csv file that the REF all zeros.
> 
> When I submit my exoplanet data to Bruce Gary, he likes to use an artifical 
> star as a reference.
> 
> Can someone please sort this out for me? I am confused!
> 
All of these terms (comparison, reference, check) refer to stars whose
standard magnitudes are known (or, at least, they are known constant
stars in the field if you are doing differential work).

The nomenclature of comparison and check comes from the photoelectric
photometry days, when you measured each star separately.  You would
usually measure the target, then the comparison, then the target,
then the comparison, and finally the check star in a specific sequence
of movements.  The target and comparison were the primary stars for
the differential photometry.  The check star was just there to "check"
for variability of the comparison star.  If it was found to be also
variable, then you could at some level recover your original differential
photometry and then go select a new comparison star for future measurements.

With CCD observations, you measure all stars simultaneously, so the
naming of comp and check is not as clean-cut.  In fact, many observers
measure the magnitudes of many comparison stars in the field to form
a "master comparison star" - the process of ensemble photometry.  However,
the terms have remained for those who wish to submit simple photometry,
where you measure the magnitude of three stars in each image (the target,
a star you've selected as the comparison, and a star that you've selected
as the check).  That still guards against comparison-star variability
and also gives a reasonable first approximation to the photometric
error (from the K-C difference).

As for what each software package names things, and what they output
in their files, you will have to read the documentation.  Hopefully
most of those differences will go away when the AAVSO standard formats
are announced in a couple of weeks.
Arne