USNO-A1.0 Catalog Format

The format of USNO-A is an attempt to minimize the storage requirements as well as making the catalog relatively easy to search. Apart from complaints that it wasn't in FITS format, users of UJ1.0, UJ1.3, and the preliminary catalog USNO-A0.9 seemed to find it legible.

  1. The coordinate system of the catalog is Right Ascension and South Polar Distance (SPD). This decision was based on compatibility with the ESA Hipparcos and Tycho mission and catalogs. In practice, SPD is far easier to manipulate than declination since it is positive definite.

  2. The coordinates were converted to integers in the following manner.
         (RA in decimal hours)*15*3600*100, and
         ((DEC in decimal degrees)+90)*3600*100
    Again, this choice agrees with Hipparcos/Tycho.

  3. Coordinates are given in J2000 at the epoch of the original blue plate. Somewhere on this CD-ROM set is the catalog.tar file which contains a copy of the plate database. If it really makes a difference, please consult these files since they have been corrected for every known error.

  4. The sky is partitioned into 24 zones of SPD, each of width 7.5 degrees. This is similar to the choice made by the Guide Star Catalog. Perhaps some software developed to search that catalog can be used for this catalog.

  5. In each zone, the catalog is sorted by increasing value of RA.

  6. Each of the 24 pieces of the catalog contains 3 files, and the naming convention is
           XXXX is 10 times the SPD (0, 75, 150, ... 1725)
           YYY = acc (ASCII accelerator file)
               = cat (binary catalog file)
               = lut (binary lookup table for GSC stars)

  7. Each catalog (.cat) file is a binary file containing 3 32-bit integers for each entry. The FORTRAN dimension statement looks like (3,length). In a picture it looks like:
        | RA (1) | Dec(1) | Mag(1) | RA (2) | Dec (2) | Mag (2) | ...

  8. The byte order is BIG_ENDIAN, which is the default for machines like Silicon Graphics and is opposite the default of machines like DEC.

  9. Since the catalog files can be quite long, I have found it convenient to refer to the accelerator (.acc) file, and use a combination of lseek() and read() to access the catalog file. FORTRAN direct access I/O is terribly inefficient in that it does not easily handle making a big offset then doing small reads.

  10. The accelerator file (.acc) contains the first index (1-based FORTRAN sense) for the first object every 15 minutes of RA and the number of objects in that chunk of RA. The total number of bytes in the file is given by (FIRST(96)+LONG(96)-1)*12.
  11. On this CD-ROM is a file called demo.tar which contains the source code to a program called square.f. This is a simple program that extracts all entries within something like a square chunk of sky given the user's input of RA, Dec, and size. It demonstrates how to use the .acc and .cat files. For further details, consult the README file in demo.tar.
  12. The RA takes a full 32-bit integer as does the SPD. The third 32-bit integer has been packed according to the following format.
    SQFFFBBBRRR (decimal), where

    S = sign is - if there is a correlated GSC entry, + if not.

    Q = 1 if internal PMM flags indicate that the magnitude(s) might be
    in error, or is 0 if things looked OK. As discussed in read.pht, the PMM gets confused on bright stars. If more than 40% of the pixels in the image were saturated, our experience is that the image fitting process has failed, and that the listed magnitude can be off by 3 magnitudes or more. The Q flag is set if either the blue or red image failed this test. In general, this is a problem for bright (<12th mag) stars only.

    FFF = field on which this object was detected.
    In the north, we adopted the MLP numbers for POSS-I. These start at 1 at the north pole (1 and 2 are degenerate) and end at 937 in the -30 degree zone. Note that fields 723 and 724 are degenerate, and we measured but omitted 723 in favor of 724 which corresponds to the print in the paper POSS-I atlas. In the south, the fields start at 1 at the south pole and the -35 zone ends at 408. To avoid wasting space, the field numbers were not put on a common system. Instead, you should use the following test.
    If ((zone.le.600).and.(field.le.408)) Then

    BBB = 10 times the blue magnitude.
    The range 0 through 250 contains reasonable magnitudes. 500 is reserved for a PMM flux estimator that was exactly zero, and 501 through 750 are reserved for PMM flux estimators that were negative. Only the reasonable magnitudes were calibrated: the weird ones are just as they came out of the PMM. For northern fields, magnitudes are defined by the 103a-O emulsion and filter, while southern fields are defined by the IIIa-J emulsion and filter.

    RRR = 10 times the red magnitude.
    As above except that northern plates are 103a-E emulsions and southern plates are IIIa-F emulsions.

    a) Since it is possible that a blue image correlated with the GSC but
    did not have a corresponding red image, these entries have the proper blue magnitude but have 999 for the red magnitude.

    b) For GSC entries that did not correlate with PMM entries, the Q,
    FFF, and BBB entries are zero, and the GSC magnitude is in RRR. These entries are in the range of -1 to -160 since they, by definition, have S negative.

  13. The GSC lookup tables contain many entries, each of which is two 32-bit integers long. Each pair of integers is the record number in the .cat file (==byte/12) and the GSC catalog number. They are sorted by USNO-A record number. I believe that these files are correct, but have not subjected them to extensive verification.
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