[Aavso-photometry] [AAVSO-DIS] True Chart Confessions

[arne]

Bob Stine wrote:
> Here's a mistake I've frequently made: I make a visual observation under the
> dark sky and dim red light, using what I think are the two closest comp
> stars.  Only later do I find out that somewhere on the chart was a comp star
> even closer in brightness that I failed to notice in a crowded field in the
> dark.
> 
> Has this happened to anyone else?  If it's only me, then stop reading here.
> But if this is a common issue, then I have an idea to fix it.  When one
> prints a chart using the AAVSO Variable Star Plotter (VSP) and one selects
> the "Table of Photometry" option, lo and behold, there is a column
> containing the magnitudes of the comparison stars in descending order.  What
> if we could have that column printed along, say, the right margin of the
> chart?  That would give us a "Table of Contents" of all the comps in the
> sequence for that chart.
> 
As mentioned by others, we used to have such a feature on CCD charts,
but the table took up valuable space on the chart and did not include
sufficient information for use by many software packages (such as
coordinates, star name, sufficient color and error information etc.).
So we removed the table to a separate page and made it available for
all charts, not just F-scale CCD ones.  There is never a perfect solution,
but the case mentioned above usually only happens for fields that you
don't observe often, or that have a very dense set of comp stars.
Usually your errors will go down with a wise choice of bracketing
comp stars for visual observations, so not finding that one better
comp might make a small difference.  My guess is that your concern
would cause you to inspect the chart the next day and see if a
better pair was available, and you'd probably use that better pair
the next time you observed the variable.

Wolfgang makes the valid point that the chosen sequences are primarily
geared towards the visual observer, spanning more than the full range
of the variable.  For ensemble work, you'd rather have lots of
comps with a smaller magnitude range to improve their signal/noise
and weight in the solution.  We are headed in that direction with
our calibration work, providing good magnitudes and colors for all
stars surrounding a variable, thereby giving observers the option
of selecting their own comps based on their needs.  Seqplot (available
if you are a sequence team member) is the pilot "tool" for such
selection, giving you access to all photometry that I've done at
NOFS or SRO (and soon APASS).  It does use star color on the display
screen to identify which field stars have appropriate color for
typical sequences.  We thought about making VSP plot in color, but
then decided that color would not be helpful at the telescope.
Besides, the existing catalogs used for obtaining the position and
magnitudes of the field stars don't include reliable color information.

Keep giving suggestions, and as we get nearer to the final calibration
solution, we will start implementing more of these observer-oriented
features.
Arne