[Aavso-photometry] What Mags to Use?

[arne]

Brian D. Warner wrote:
> I did some CCD (V filter) work on U GEM last night. The chart magnitudes are good 
> only to 0.1 precision. This seems a waste when working CCD, especially since the 
> field has some higher precision values from Arne's work.
> 
> When I asked for the photometry on the chart, there were no magnitudes other than the 
> labels, that 0.1 mag precision. I would have thought for this field that better 
> (higher precision) magnitudes would have been listed.
> 
> So, can I use the Henden photometry from his most recent U GEM file on the AAVSO 
> site, or should I stick with the lower precision values?
> 
You can use the higher precision values, and just report "Henden sequence" in
the comments field.

> I'm working V432 PER tonight. The VSP provides a nice plot but no magnitudes. There 
> are some stars from Brian Skiff's LONEOS catalog in the field, however. Is it OK to 
> use those magnitudes (given to 0.01 precision)? If so, do I use his catalog 
> designation since there are no AUID values for stars in the chart?
> 
> These are two specific examples, but maybe they can serve to provide some general 
> answers, e.g., when working a field with no magnitudes at all or with magnitudes from 
> another, reasonably calibrated source.
> 
You would use Skiff's values and report them as "LONEOS sequence" or soemthing
that makes the source clear.

Photometric calibration is a moving target.  If I calibrate a field on two
nights and publish the results, subsequent nights will move the average.
Hopefully this new average asymptotically approaches the true magnitude,
and approaches the Landolt system as well, but it doesn't always work that
way.  I work hard at getting things right, and this has been validated from
a number of other photometrists, so in general if you see a Henden sequence,
it is pretty good.  If you pick something from Brian Skiff's list, you can
be sure it also is pretty good, as Brian is a good photometrist and he has
spent years building this list of stars that he has "vetted".  For almost
every other source, you have to look at the data carefully, and not many
people have the experience nor the time to do so.  If you can, stick with
one of those two sources as your primary references.

Now, as for the AAVSO program stars and sequence/comparison stars.  Until
ensemble submission is available (coming soon with the new AAVSO Standard
Format), you must use a single comparison star for your report.  If you
do this, and report the magnitude you actually used for that comparison
star, it makes little difference what value you have used.  If you used
10.5 for the comparison star, and we later find the true magnitude is
10.53, then all a researcher has to do is add 0.03mag to your photometry
and it is on the true system.  So the final science is not impacted, but
the path to that final science is more complex than if good calibration
were available right from the beginning.

The AAVSO charts were created to provide good sequence stars for visual
observations, so the 0.1mag granularity of the magnitudes was quite
acceptable.  In addition, most of the early program stars were Miras
or CVs, where high amplitude variation was prevalent, and errors in the
sequence had little impact on the final light curve.  CCD and PEP observers
require higher precision, and that is what we've been working towards for
a long time now.  I've been doing calibration work for over 30 years,
and started with Janet Mattei in 1997 to calibrate the AAVSO program fields.
A decade ago!  Amazing how time flies.

Much of that calibration effort resulted in the .dat and .seq files you see
on the AAVSO ftp site, under
ftp://ftp.aavso.org/public/calib
ftp://ftp.aavso.org/public/calib/sumner
and in the old chart world, when you did a search for a field, you got links
to the A/B/C/D/E/F scale charts, *and* to the appropriate .dat and .seq files
for that field, if they existed.  When we moved to VSP, in order to get
something that could be released quickly, we used the Comparison Star Team's
massive effort to document all compstars used on all current AAVSO charts,
that provided GSC identification and accurate coordinates, and populated the
comparison star database (called "VSD") for VSP from that effort.  However,
this was Phase I of the effort - the next phase was to add in the best
available photometry for each of those comparison/sequence stars, and that
phase is nearly finished.  In a few days, we will release a new version of
VSP/VSD that includes accurate photometry for about 2/3 of the compstars,
and will update nearly all of the database by the end of the year.  So,
for existing fields, I'm hoping that questions like this will be no longer
asked.

So - please hold on for a few more days!
Arne