[Aavso-photometry] BVRI Program

[Brad Walter]

 Arne can you share the standard deviations for your Sonoita measurements of
VX GEM?


Message: 2
Date: Thu, 08 Nov 2007 16:35:12 -0500
From: arne <arne at aavso.org>
Subject: Re: [Aavso-photometry] BVRI Observing Program
To: aavso Photometry <aavso-photometry at aavso.org>
Cc: Keith Graham <kag at core.com>
Message-ID: <47338110.5040501 at aavso.org>
Content-Type: text/plain; charset=ISO-8859-1; format=flowed

Good question.
There are some holes in the current VSP/VSD update.  First, all stars with
magnitudes more than 0.2mag different than their old chart values were not
uploaded.  Second, all CCD charts, BVRI photometry charts, etc.
had photometry listed on them that did not get copied into VSD.  This will
happen during the January load.

The CCD BVRI program picked 8 stars to follow: S Per, U Ori, VX Gem, DH Dra,
VX UMa, W Leo, RU Vir, and RR Boo.  Many of the comp stars were bright (some
of these are bright Miras, after all), and those were filled in using Tycho2
values during this initial load of the comp star database.  I'd recommend
using the original BVRI charts and not VSP for these 8 stars until we
properly populate the database.

On the other hand, a couple of those fields were observed by me for other
reasons.  An example is VX Gem.  It is interesting to compare my 3-night
BVRI calibration at Sonoita with the calibration on the BVRI chart:

91, 94 101 and 104 are saturated at SRO, as our goal was accurate photometry
from 11-15mag

ID     B       V      Rc      Ic
110  12.035  10.990  10.440   9.940 SRO
      12.043  11.017  10.466   9.956 CCD

115  12.172  11.509  11.134  10.780 SRO
      12.155  11.509  11.138  10.776 CCD

122  12.899  12.219  11.848  11.516 SRO
      12.893  12.235  11.867  11.503 CCD

127  13.266  12.715  12.384  12.079 SRO
      13.284  12.746  12.412  12.095 CCD

128  13.898  12.826  12.257  11.735 SRO
      13.928  12.859  12.284  11.669 CCD

139  14.428  13.940  13.624  13.324 SRO
      14.478  13.989  13.673  13.196 CCD

You can see that, in general, the agreement is quite good.  As the stars get
fainter, there appears to be a systematic offset for BVR, and a very obvious
difference at Ic.  Who is right?  You might observe yourself and see which
set of magnitudes comes closest to your own transformed values, using
perhaps the 115 star as the zeropoint.
I'd like to hear your answers!  The Howell, Mattei and Benson paper (1993,
JAAVSO 22, 2) indicated that they had two clear nights and two partly cloudy
nights, and that they covered 19 fields (of which
8 were used to create the BVRI program).  I don't know if the other
11 were finally processed.  They indicate that they did 3 measures on one
night for each of the fields, and did not give the standard deviation of the
measures in the paper.
Arne


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End of Aavso-photometry Digest, Vol 48, Issue 5
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