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

[Steven Orlando]

Arne,

GREAT JOB!!!

Steve
----- Original Message ----- 
From: "arne" <arne at aavso.org>
To: "AAVSO-DIS" <aavso-discussion at mira.aavso.org>
Cc: "Aavso-Photometry" <aavso-photometry at mira.aavso.org>
Sent: Wednesday, October 24, 2007 6:17 PM
Subject: [Aavso-photometry] Chart and comparison-star database 
changes -Important


> 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.
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