- R. Cutri:
has been looking into the question of how many Tycho stars we can
expect per scan that will be faint enough for positional
reconstruction use. Since it is difficult to make accurate
a priori estimates of the JHKs magnitudes for these stars, Roc has searched
for Tycho stars which have been observed by the prototype camera. In one of
the four Tycho declination bands (0 < dec < 37.5 degrees) he found 500 Tycho
stars that were serendipitously observed in at least one band. Of these, a
surprisingly large 55% (J,H) and 67% (Ks) are fainter than J=9.0, H=8.5 and
Ks=8.0, and thus can be expected to be unsaturated in R2 and to perform well
as positional calibrators.
- S. Charnoz:
has continued his studies of the distribution of Tycho stars in
each scan. He showed histograms of the number per scan, and also
statistics of the gaps between Tycho stars. Applying Roc's
preliminary results for the fraction of these that might be valuable for R2
positional reconstruction, the median value would be about 11 stars per 6
degree scan.
- T. Evans:
showed the distribution of [R1 - (R2-R1)] delta magnitudes vs. cross
scan position, x, for ascending vs descending scans. Full details
of her results are given in yesterday's 2MASS Working Group
minutes and will not be repeated here.
- T. Jarrett:
has been looking at the effectiveness of a new galaxy magnitude
derivation method suggested by J. Huchra and S. Schneider.
Concerned more with minimizing any overall bias in the magnitudes
rather than minimizing the scatter, they proposed that only stars with peak
flux equal to or greater than that of the galaxy be removed from the
galaxy aperture, and similarly from the local background. The stellar PSF
is used to remove these stars. Fainter stars are neglected. Tom pointed
out that a small bias is inevitable with this method because some galaxy
light will be removed when the stellar PSF is fitted to these stars. Tom
used his simulator to test this method: stars are added to a real Coma field
and galaxy magnitudes re-derived in the presence of this simulated stellar
confusion and compared to the uncontaminated galaxy magnitudes. The results do
show the expected negative bias (the stellar-contaminated magnitudes are
fainter), but it is smaller than the positive bias seen with GALWORKS' nominal
method, for which an attempt is made to remove all stars and interpolate the
galaxy flux beneath removed stars (the positive bias in this method is due to
the presence of faint stars which can be more efficiently removed in the
background aperture than in the galaxy aperture). However the scatter in the
magnitudes derived using the new method is considerably higher than the
nominal method.
- D. Kirkpatrick:
has been continuing his quest for very late type stars in the
'92-'94 database, searching for objects with high proper motion.
He has serendipitously turned up two objects which are likely
to be (to have been!) asteroids. One was fortuitously observed twice on
successive scans because it fell near the edge of the first scan, and can be
seen to move between these two short epochs. The second was also observed
twice but with longer period between rescans (1 day), and was not seen at all
on the second image. Davy also reported the discovery of two more late M star
candidates.