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
zoneXXXX.YYY
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
south(field)
Else
north(field)
Endif
- 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.