[Aavso-photometry] Chart and comparison-star database changes - Important

[arne]

As of Wednesday, October 24, 2007, we have updated about 2/3 of the stars
in the Variable Star Database (VSD or the comparison-star database) of the
Variable Star Plotter (VSP).  These stars now have accurate V-band
photometry, and in most cases, multiband photometry.

This is Phase IIa of the comparison star database project.  Phase I
was the documentation of every comparison star found on every
chart at the AAVSO, as of about 2004.  This resulted in a database of
around 31,700 stars, where the position and chart-magnitude of each
star was recorded.  It was a massive effort, lead by Vance Petriew,
but involving dozens of volunteers.  However, no precision photometry
was recorded, even if these were CCD-f charts with a table of
BVRI photometry included, or where Sumner sequences could be found
using the online chart search engine.

These sequences are now included in VSD/VSP.  If you pull up a chart for
a field like TT Ari, you can now obtain the BVRI information for
the majority of the comparison stars.  This Phase IIa update was
a massive effort in itself, and would not have been possible without
the assistance of Grzegorz Pojmanski for ASAS, Jean-Claude
Mermilliod for GCPD, Michael Koppelman, and Aaron Price.  We've also
used the hard work and labor of many surveys, such as Tycho, TASS, SDSS,
2MASS, and Brian Skiff's LONEOS photometric calibration database.

For those of you who have never looked for the photometry available
for the chart sequence stars, there are two methods through VSP.
First, near the bottom of the form is the option "CCD Box Limit".
This is the number of sequence stars for which you want photometry
displayed on the chart itself.  Set that number to a value greater
than zero, and you will find a box printed in the lower right of
your chart.  Second, you can also click the box at the form
bottom labelled "Field Photometry" and "Do not plot a chart, just
give me a table of photometry."  This gives you an html table
of the photometry, along with coordinates for every star.

Why have only 2/3 of the stars been updated?  The main reason is
that the remainder are significantly discrepant from their chart labels.
A star labelled as 108, for example, might have accurate photometry
that indicates a label of 104 is more appropriate.  We want to check
these stars thoroughly and ensure that the photometry and labels
are correct, and that will take a couple of months.  In the meantime,
we wanted to give you the benefit of the majority of stars where the
differences are minor (less than 0.2mag).  What is encouraging
to me is that 21,826 of our comparison stars *were* accurate,
meaning our observers were providing nearly correct estimates.

However, it *does* mean that many charts are slightly different
than the online version, or even the chart you created with
VSP a week ago.  For visual observers, you will be hard-pressed
to measure the difference.  However, for all observers,
we request that you be extremely careful in your reports.
It is *essential* that you indicate the chart_ID/date that
you use.  For now, you can continue to use your existing charts,
but we urge downloading new versions over the next few months
and using these new, more precise, labels and magnitudes.

Our intent is to make the AAVSO charts as accurate as possible.
There are several charts where the sequences are well-known to
be highly wrong; othere with scale errors; still others where
accurate photometry would help the CCD observers.  We have
long debated whether we should continue using charts that
we know are wrong, as maintaining the "wrongness" at least
means consistent light curves.  At the same time, we know that
those fields with bad charts are underobserved because people
know they can't estimate properly.  We are taking the baby
step now of minor tweaks to existing sequences.  I hope it
does not inconvenience too many observers - I know it will
take time to get used to even small changes to your favorite
fields.  Please bear with us, and realize that the end product
will be better science for the astronomical community.