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 firstname.lastname@example.org. 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.