USNO-A2.0 Catalog Format

Things that are the SAME or DIFFERENT with respect to A1.0 are highlighted.
  • The coordinate system of the catalog is Right Ascension and South Polar Distance (SPD). {SAME}

  • The coordinates were converted to integers in the following manner. {SAME}
         (RA in decimal hours)*15*3600*100, and
         ((DEC in decimal degrees)+90)*3600*100

  • 3) Coordinates are given in J2000 at the epoch of the mean of the original blue and red plates. This is DIFFERENT from A1.0. In A1.0, we didn't have enough confidence in the removal of systematic errors, so we quoted the blue position. In A2.0, we hope that the mean of the red and blue positions is more accurate than either alone, so we are quoting the mean position. For POSS-I, this is a trivial change since the plates were taken as pairs. For SRC-J and ESO-R, there could be a small error introduced for stars with relatively small proper motions. Large motion stars would not be selected because their blue and red images would not lie within the 2 arcsec selection aperture. As before, there is a file CATALOG.TAR that lists the epoch of each plate.

  • 4) The sky is partitioned into 24 zones of SPD, each of width 7.5 degrees. {SAME}

  • 5) In each zone, the catalog is sorted by increasing value of RA. {SAME}

  • 6) Each of the 24 pieces of the catalog contains 2 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)
    This is DIFFERENT than A1.0 in that the LUT files no longer appear. USNO-A2.0 has no dependence on any Guide Star catalog, and the cross correlation has not been done by USNO.

  • 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 the following. {SAME}
        | 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. {SAME}

  • 9) The third 32-bit integer has been packed according to the following format.
    0QFFFBBBRRR (decimal), where

    0 Unlike the USNO-A1.0, no correlation with GSC is attempted.

    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.
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