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R. L. Williamson II
Space Telescope Science Institute, 3700 San Martin Drive, Baltimore,
MD 21218
D. J. MacConnell
Computer Sciences Corporation, Space Telescope Science Institute,
3700 San Martin Drive, Baltimore, MD 21218
W. J. Roberts
Baltimore, MD 21210
Keywords: Proper Motions, Schmidt Plates, Astrometry, Guide Star Catalog, Palomar Sky Survey
Using two PDS 2020G microdensitometers, scanning the early 1950's-epoch
POSS E plates north of 
= 
was completed at the Space
Telescope Science Institute about three years ago. These scans, together with
scans of the Palomar Oschin Schmidt ``Quick V'' plates taken for the
HST Guide Star Catalogue provided a baseline of about 30 years from
which proper motions may be determined.
Extractions from the scan database are typically done over small regions, no
larger than 
on a side, so that distortions are minimized. An in-house
software program generates an inventory of all of the objects in an
extracted field for each epoch, and outputs a file that contains positions
(in both linear units and equatorial coordinates), rough magnitudes, and a
``star'' or ``non-star'' classification for each object. Objects that are
classified as stars in both frames are matched using a nearest-neighbor
algorithm in both spatial directions. The program eliminates those with
residuals 
as potential proper motion stars, but retains
in the solution those with proper motions, 
, not exceeding
yr
.
The number of reference stars remaining is typically 25 to 30. After
matches are made and accepted,
we then determine a solution for an affine transformation from one frame to
the other using the least-squares, singular value decomposition routines from
Press et al. (1992). With our material, we have the most confidence in
results for stars with visual magnitudes in the range 
, and
with proper motions in the range
yr
yr
. We work
in either of two ways which we call ``Selected-Target'' and ``Survey'' modes.
In this mode, the position of a specific star is centered in an extraction
window for each epoch, and the motion is determined for that star. We have
been obtaining proper motions of: candidate nearby stars for H. Jahreiß ,
verifying that some having motions up to 
yr
which are not listed
in the Luyten or Giclas catalogues; hot DAs from the Palomar-Green (PG) Survey
for J. Liebert; sdO stars in the PG survey for R. Saffer; faint, high-latitude
carbon stars; and low-mass, post-AGB stars for M. Parthasarathy. Other
collaborations are pending.
Figure: POSS Image of region around PG0923+533 and resulting plot from program.
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Figure 1 shows a print ( left) of the area around one of our single
targets, PG0923+533, and the resulting plot ( right) from the analysis
of the field for proper motions. Arrows indicate the magnitude and
direction of the proper motion for the stars. We determined a proper motion
for this target of 
yr
and 
.
We also carried out a search over the full 6^o field of POSS Area 321 near the North Galactic Pole using a 50% overlapping sub-plate technique. Our epoch difference is 31.92yr compared with 17.04yr for Luyten's neighboring area 322 and 12.98yr for area 478 (Luyten 1973). This method is vulnerable to cosmetic defects, especially for faint objects, and the accuracy of the star/non-star classifier.
Of 123 Luyten proper motion stars in the common area, we find 44 certain and
two possible correspondences; the faintest Luyten R mag of these is 19.8.
Of 77 Luyten stars not found, 24 are too faint [R(Luy) 
] to
appear on the ``Quick V'' plate and two are too bright (8
mag).
Of the 51 remaining, 34 have 
yr
---i.e., perhaps
too small for us
to detect. Of the remaining 17, six are near our faint or bright limit. The
largest undetected Luyten proper motion is 
yr
for a R=18.9 star.
This and other cases of missing known proper motion stars, as well as
spurious proper motions, may be attributable to the star/non-star classifier
being too conservative and hence eliminating real stars from one or
both frames.
Figure: Plot from the program of the area around target PG2129+150 and the
POSS Image of the region.
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Such a case is illustrated in Figure 2 which is a plot showing the stars
matched in the field of PG2129+150; the target star, marked with a ``t" in the
figure, show no motion, but two others, separated by about 9', seem to be a
common proper motion pair with 
yr
and 
.
However, upon inspecting the actual photograph, also in Figure 2, we see that
two stars of the right separation and position angle are present in each
position. For these spurious motions to be computed, it must be that
the star/non-star classifier classified the northern member of each pair as a
``star'' and the southern members as ``non-stars'' in the early-epoch plate
output file, and then reversed the classifications in the late-epoch plate.
We are pleased to acknowledge the support of the Space Telescope Science Institute and its Catalogues and Surveys Branch, and in particular, B. Lasker and B. McLean.
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