Reposted from the AAVSO mailing list - Kurt ========================== Date: Wed, 24 Oct 2007 18:17:10 -0400 From: arne <arne@aavso.org> Subject: [Aavso-photometry] Chart and comparison-star database changes - Important To: AAVSO-DIS <aavso-discussion@mira.aavso.org> Cc: Aavso-Photometry <aavso-photometry@mira.aavso.org> Message-ID: <471FC466.6040409@aavso.org> Content-Type: text/plain; charset=ISO-8859-1; format=flowed 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. _______________________________________________ Sent via CSolutions - http://www.csolutions.net