USNO A1.0 November 1996
USNO SA1.0 December 1996
This is READ.AST, the file with the discussion of the astrometric calibration
of USNO-A. Please refer to READ.ME for an introduction to the catalog.
Summary:
The astrometric calibration of USNO-A is based on the Space Telescope
Science Institute's Guide Star Catalog version 1.1, hereinafter GSC.
This is a temporary calibration, and it will be replaced with a
calibration to the European Space Agency's Hipparcos and Tycho catalogs
as soon as they become available (current estimate is June 1997).
We believe that a typical astrometric error is about 0.25 arcseconds,
but for stars a few magnitudes brighter than the plate limit and away
from the corners, the error may be as small as 0.15 arcseconds.
Coordinates are computed in the system of J2000 at the epoch of the
survey blue plate. Proper motions were neither computed for nor
applied to the coordinates in this catalog.
Whenever possible, we have adopted Pat Wallace's SLALIB for computing
quantities associated with position and angle. Details about these
routines and permission to use them should be obtained from the
author at ptw@star.rl.ac.uk.
Source Code:
binary/acrs - projection of ACRS to survey plate coordinates
binary/ppm - " " PPM " " " " "
binary/gscgen - " " GSC " " " " "
newbin/tychogen " " Tycho Input Catalog " " " " "
binary/gsctaff - Taff-o-grams for various surveys
binary/autogo - fit POSS-I O to projected GSC
binary/autoge - " POSS-I E " " " "
binary/autogb - " SRC-J " " " "
binary/autogr - " ESO-R " " " "
catalog.tar - electronic version of the various plate logs
binary/ugapX - the various routines that make the catalog
Strategy:
Using the reference catalog (GSC1.1) and the information contained
in the plate log (possi.cat and south.cat in catalog.tar), SLALIB
is used to compute the observed place for each catalog star.
The PMM coordinates are corrected for the nominal cubic distortion
of the Schmidt telescope (using SLALIB's SLA_PCD, etc.) and
compared to the projected catalog. A best fit using up to cubic
terms is computed and the residuals are saved. After doing this for
a significant number of plates, the residuals are binned according
to their location on the plate, and an approximation for the
systematic field distortion of the Schmidt telescope is determined.
(These are called Taff-o-grams in the code in recognition of Larry
Taff's demonstration of their significance.) The fitting procedure
is repeated, this time including the systematic field distortion
map, and this fit is adopted for the generation of the catalog.
The Individual Plate Solutions:
For a particular field, the plate log was consulted to get the
various parameters (date, time, emulsion, etc.) for the plate.
Unfortunately, there were a substantial number of typographical
errors in the original versions of these logs, and every effort
has been made to track down these errors and correct them. We
believe that the versions contained in this CD-ROM set are more
accurate than the ones we started with, and all of the errors
that we could fix have been fixed. With the exposure data,
SLALIB is used to compute the best estimator of where the stars
should be found. In order, we used SLA_MAPQK, SLA_AOPQK, and
SLA_DS2TP to go from catalog to apparent to observed to tangent
plane coordinates.
The PMM produces coordinates for each detection in integer hundredths
of a micron on its focal plane. Actually, there is a systematic problem
in the introduction of temperature and pressure into the PMM logic,
and its version of a micron can be off by as much as one part in
10^5, but they are sufficiently close to microns for this discussion.
The coordinates have had the individual platen zero points subtracted,
and the nominal center of each plate appears at approximately (170,175)
millimeters. SLALIB provides a utility for removing the nominal
pin cushion distortion of a Schmidt telescope, and this correction
is applied to the raw PMM coordinates.
With the exception of systematic astrometric errors in the Schmidt
telescope, the projected catalog and undistorted PMM coordinates
ought to agree with each other. The mapping is done using cubic
polynomials in X and Y, although linear terms are sufficient except
when doing the full-plate solution. No sub-plate solutions are used:
a single fit in X and Y is used to describe the whole plate. These
solutions are saved as are the residuals computed for each match between
the PMM and the reference catalog, and this process is repeated for
every survey plate.
When many solutions are available, the residuals are combined
according to the position of the object on the plate by the
code in binary/gsctaff. For USNO-A, a mean distortion pattern
was computed for each of the three Schmidt telescopes involved.
However, it is clear from examination of subsets of the data that
there are significant differences in the shapes of the distortion pattern
as a function of zenith distance (actually declination but most survey
plates were taken near enough to the meridian). In future releases,
we intend to use zonal versions of this correction. The residuals
are binned in a 32x32 grid, and a 2-dimensional smoothing spline is
used to expand this to a 65x65 grid. This corresponds to boxes
about 5 millimeters in size on the plate.
With the systematic correction determined, the astrometric solution
is repeated using the same catalog projection but adding the systematic
correction removal to the pin cushion distortion removal in the
pre-processing of PMM coordinates before fitting. Again, a single
cubic fit in each coordinate is used to describe the entire plate.
Assembling the Catalog:
Two separate astrometric fits go into each field. First, the red
plate is mapped on to the blue plate, and then the blue plate is
mapped on to the reference catalog. The code is complicated only
because of the large number of detections in each field, and the
importance of applying each fit in the proper order. This process
is done in binary/ugap012, and extra software is inserted to verify
that each step worked properly. The output of ugap012 is a set of
rings on the sky that follow from the surveys being taken in rings
of declination. Because of the relatively slow response of our
CD-ROM jukebox that stores the raw catalogs, it takes about a week
to do this phase of the preparation of USNO-A.
The rings of various declinations are merged into zones of constant
width by the code in binary/ugap3. The zones are examined for
duplicate detections by the code in binary/ugap4. This program
makes a list of all entries to be removed (the TAGs) and saves
multiple observations of the same object in the sameXXXX.dat file
for the photometric calibration. The important routine in ugap4
is nodup.f which finds the multiple detections. For USNO-A, the
radius was taken to be 1 arcsecond. In the polar regions, the
xynodup.f routine is used and the double detections are removed in
coordinates on the tangent plane, and a radius of 15 microns was used.
Finally, the code in binary/ugap5 removes the TAGged entries and
produces the final catalog. This catalog incorporates the astrometric
calibration, but not the photometric calibration. Routines to
check each step appear in binary/ugap3x, binary/ugap4x, and
binary/ugap5x. A powerful debugging tool is plotting the entire
sky because the eye is very sensitive to systematic errors at plate
boundaries, etc.
Finally, the code in binary/ugap7 applies the photometric calibration,
and the code in binary/ugap8 projects the catalog in Galactic
coordinates. The partition of the catalog files on the various
CD-ROMs is done in binary/ugap6.