[Aavso-photometry] New VSP comparison/sequence star magnitudes

[arne]

The existing comparison star database (compDB, or VSD) had
approximately 32,800 true comparison stars that were present
when VSP was released on June 30, 2007.

On October 24, 2007, we released photometry of 21,826 comparison
stars whose photometry differed by less than 0.2 magnitudes from
the original chart label. (We have released additional photometry
for new fields since then, but that new photometry is not part
of this announcement.)

Today we updated through VSP an additional 6451 stars with at
least V magnitudes, and in most cases multifilter photometry,
along with updating some of the October release stars.
(The remaining 4,523 stars are being studied for a later update.)
This latest update contains stars whose magnitude differs from
the chart label by more than 0.2 magnitudes, but less
than 3.0 magnitudes.  The vast majority of
the current release differ by less than a magnitude, and we are
confident of their reliability.  A few dozen stars have the
largest discrepancy, and for these their reliability is more
suspect. Some may be misidentifications; others may have poor
photometry in whatever catalog was used; the majority are
likely to be real. We decided to issue this release
with these less-reliable stars included.

A full description of the photometry process is given
on the homepage, and at:
http://www.aavso.org/news/compdb.shtml

WHAT THIS MEANS FOR THE OBSERVER

Panic!  No, seriously...
In the near term, what we would like people to do is to be
our quality-control experts. Print off your favorite
chart fields, and compare the new charts with your
older charts.  For stars that are significantly different, decide
for yourself (by CCD imaging, comparing with VizieR photometry,
using your eyes) whether the change looks proper.  If not, we ask
that you send us a correction using the Chart Error Tool:
http://www.aavso.org/bluegold/chet/
(you will have to be logged into Blue and Gold in order to use CHET.)
We will gather the errors and solve them as quickly as possible.
An example is W Leo, where both 000-BBR-808 and 000-BBR-809
point to the same 159 star - we'll need to delete one of those
entries.  On the other hand, Yenal's 135 star in that field now
has a more correct 139 value (as he determined independently).

Once we feel the system is stable (and also look to see if we
can add photometry on the remaining 4,523 stars), we will freeze
the system and formally request that users update their charts.
This will occur at least a month from today, and we will give
adequate time beyond the "freeze date" for everyone to update
their charts before we start complaining if you are using an
old chart.  We will also send chart-CDs to the major groups
once the database is frozen, for local distribution of charts
to those observers with poorer internet access.

Some clarification points:

- You can start using the new charts today. They really are better
than what you are currently using.  That being said, most charts
of common variables will see little or no difference.

- However, even for the fields that do change,
you do not have to update all of your charts immediately.
In fact, to lessen the load on the HQ computers, we recommend
that you just download the charts you are going to use this weekend,
and save your other chart downloads until another time.

- You do not have to use the new VSP charts after the freeze date.
We will still accept observations using *all* other charts,
whether old AAVSO charts or charts from other groups.  However, we will
strongly encourage observers to use the new photometry after
the freeze date so that the transition period is as short as possible
and so that your observations will be more scientifically useful.

Thank you for your patience on this.

Arne Henden
Director, AAVSO