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experiment



Now that the Fall good photometry weather is approaching for midwestern sites,
I wonder if one of you would be willing to do the following experiment for me.
  Pick a good photometric night (no clouds present), and scan all night long
(approximately 30-40 frames?).  For the V filter data, find all Tycho stars
in each frame, and set the zero point of the instrumental magnitudes to be
equal to the mean value of the Tycho stars. Note: just a constant offset,
please; do not perform a linear fit to the data.  Output a file containing,
for each star identified with a Tycho star, the following information:

     frame  x  y  mag'  V(tycho) R(tycho)

where frame = filename for each frame, x and y are the CCD-based coordinates
of the star, mag' is the adjusted instrumental magnitude for the star,
V(tycho) and R(tycho) are the V and R magnitudes for the star from MG's
transformed equatorial Tycho catalog.  (I'd prefer the Bt and Vt along with
Bterr and Vterr instead of Rtycho, but those do not appear in MG's catalog).
I would prefer an aperture magnitude, or the Star psf magnitude as long as
that psf size does not change through the night.
  What I want to do is plot (mag'-V) vs. x to see if there is a strong
correlation between declination and magnitude error, and vs. y to see if
there is a time-dependent correlation.  You can also get other plots, such
as magnitude vs. color vs. position.  It works best if you
have lots of stars in the plot, which is why you need to combine the data
for many frames.  MG has done an equivalent calculation for his site, but
I would like to see the data in more detail.
  There will always be some scatter based on the color of the star, since
the TASS instrumental V is not exactly on the Johnson system and the
transformations I gave in an earlier email for the Tycho catalog are not
perfect.
Arne