[Aavso-photometry] Calibration Files and Photometry Tables

[arne]

bailyhill at aol.com wrote:
>  
> I am sorry to see the CCD box go--but I understand the confusion.  For 
> me, the Photometry Table with RA and Dec just does not work.  Its too 
> much work to get the values.  I have only done it once, and that was for 
> a star that had 10 Years of history that we wished to continue.  I had 
> to use a scale/ruler, get out my calculator, covert units, draw lines on 
> the chart, etc.  Big PITA.
>  
After talking off-line with Gary, I realized that his problem was his
confusion between my calibration files and the VSP/VSD photometry table.
If Gary can get confused, perhaps it is time to describe the process a bit.

The .dat files found at
ftp://ftp.aavso.org/public/calib
are the output from my photometric pipeline, and list all stars in each
observed field for which I have calibrated photometry.  The file:
ftp://ftp.aavso.org/public/calib/README.calib.txt
describes how the data are collected and the format of each file.
Note that the files are named roughly according to its common name
(like v838mon.dat).  There are also two master files: master_nofs.dat
and master_sro.dat that include all stars calibrated from each
respective site in single ascii files.  These are quite large
(about 500K and 100K stars respectively), so don't try
to load them into Excel!  Even for a single field, the number of
stars might range from a few dozen to thousands, depending on magnitude
depth and size of the CCD field of view.  I calibrate *everything*.

The README file also gives you two options for plotting the files, either
from GUIDE or from an excel spreadsheet.  I am not sure these two options
will work with the file versions since about 2005, when I added the (V-Ic)
color index to the files, but try them and see.  If they don't work, we
can get Bill and Lew to update their programs.  I also have a Mac OS X
program that will plot the files from an X-window (my favorite option).
Note that the .dat files are not really designed to be used manually.
The coordinates are given in decimal degrees, and each line contains lots
of information about magnitudes, errors, number of nights observed, etc.
that make it difficult to parse and understand.  There are lots of
really faint stars with large error in each file.  The sorting won't
make sense to you.  Use computer aides!

When a new object is announced, I often put it on the SRO program to be
observed if it is visible from southern Arizona.  The weather has been pretty
poor this year, so it might take a week or two before I get enough photometry
to release a .dat file.  Once that is done, one of the sequence team members
looks at the calibration file and chooses a set of comparison stars for the
field.  The criteria are essentially those given in John Toone's JAAVSO
article:
http://www.aavso.org/publications/ejaavso/v34n1/76.pdf
choosing stars with a small color range, good range in magnitude and
non-variable if possible.  With only two or three nights of calibration,
variability is not always obvious, but there are often other sources like
ASAS or NSVS to check for the brighter stars to guard against including
a variable in the sequence.  These sequences are typically 10-20 stars,
depending on the expected variability range.

Once a sequence is prepared, it is uploaded to the comparison star database
(VSD) that is used for chart generation.  After the sequence is uploaded, then
any chart plotted will show the new sequence.  VSP has two options for
displaying the stored sequence: a check-box on the fill-in form that just
displays the sequence and not the chart; and from the displayed chart, where
there is a clickable link at the top that will display the sequence.
This is the "photometry table" that I refer to, and is similar to the
"CCD Box" that used to be displayed directly on the chart, but with much
more information (such as coordinates).

I hope this makes things a bit clearer.
Arne