# Image and Catalog Archive USNO-B1 Format

### Description of the Fields in The USNO-B1.0 Catalogue:

See Table 3 of D. Monet et al. [2003, ``The USNO-B Catalog'', AJ, 125,984] for a complete byte by byte description of the original BINARY catalogue format (most people will hopefully never have to decode this binary format). The ASCII version of the extracted subset of the USNO-B catalogue contains some (or all) of the following fields:

- ID - identification number. This number
takes on one of two possible values: it is either a zone based record
number or if the star was a Tycho2 star that was added in, this is the
Tycho2 star id. The zone based numbers are in the form ZZZZ-NNNNNNN,
where ZZZZ = the four digit USNO-B catalogue zone number (the number
is the south polar distance in integer tenths of a degree, so 1501 is
the with its southern edge at a dec of +60.1 degrees), and NNNNNNN is
the seven digit record number within the zone (the system is 1 based).
The Tycho2 ids are given in the form AAAA-BBBBB-C where AAAA = TYC1
(the GSC region number), BBBBB = TYC2 (a running number in the
region), and C = TYC3 (a component number).
- RA - the Right Ascension of the object in hours
or degrees. The epoch and equinox are those given in the file header.
The default is equinox and epoch J2000.0.
- DEC - the Declination of the object in degrees. The epoch and equinox are those given in the file header. The default is equinox and epoch J2000.0.
- sra - the error in the RA position. This is given in integer milli-arcseconds.
- sde - the error in the DEC position. This is given in integer milli-arcseconds.
- Epoch - this is the mean epoch of all the observations that went into determining the position of this object. This is given in decimal years.
- MuRa - the proper motion in RA. This is given in angular velocity units of milli-arcseconds per year. [This equals MuRA = d(RA)/dt x cos(DEC)]
- MuDec - the proper motion in DEC. This is given in angular velocity units of milli-arcseconds per year.
- MuProb - a probability estimator for the likelihood that the proper motion is correct. The range in the original catalogue is 0.0 to 0.9, with higher numbers meaning greater probability of being correct. The ascii version gives this in the range 0 to 9 (ie, 10 times the decimal version, so 1 = 0.1 etc.).
- sMuRA - the error in the proper motion in RA. Given in angular velocity units of milli-arcseconds per year.
- sMuDE - the error in the proper motion in DEC. Given in angular velocity units of milli-arcseconds per year.
- sFitRA - the overall error in the fit in the RA components. In the original catalogue, this is given as a value between 0.0 and 0.9 arcseconds, in steps of 0.1. The ascii version translates that into integer milli-arcseconds, so the range is 0 to 900 milli-arcseconds, in steps of 100 milli-arcseconds.
- sFitDE - the overall error in the fit in the DEC components. In the original catalogue, this is given as a value between 0.0 and 0.9 arcseconds, in steps of 0.1. The ascii version translates that into integer milli-arcseconds, so the range is 0 to 900 milli-arcseconds, in steps of 100 milli-arcseconds.
- NFitPt - the number of observations used in the fit for the position. The minimum will be 2, and the maximum will be 5 (since there are at most 5 separate surveys in which an object can be found). This can be a very useful discriminant as to how good an object's information is likely to be. If this has a value of 0, then that means that this object has been added in from the Tycho2 catalogue.
- Flags - this column merges four pieces of
information. They are indicated as individual bits in this integer.
The values
are:
- 1 = indicates this object has been correlated with a known proper motion catalogue star (catalogues include LHS, NLTT, Giclas etc.)
- 2 = indicates proximity to a diffraction spike or a bright star (ie a Tycho2 star)
- 4 = indicates that this star is a YS4.0 star. (YS4.0 was the intermediate catalogue used to link the Schmidt plate detections to the Tycho2 astrometric reference frame.)
- 8 = indicates that this star is a Tycho2 star that has been added into the catalogue.

- B1 - first epoch blue magnitude. This is the magnitude derived from the POSS-I 103aO plate. The valid magnitudes should span ~0 to ~22 magnitude. The formal valid range is 0.00 to 99.99. If the star is an added in Tycho2 star, then this value has been computed from the Tycho2 BT and VT magnitudes, using relations derived by A. Henden. This is a PHOTOGRAPHIC B magnitude.
- MagFlg - this gives the photometric calibration source used to derive this magnitude. On each plate, there are two photometric calibration sequences; one for the bright objects, and one for the faint objects. The bright photometric calibration sources come from the Tycho2 catalogue. The faint calibration sources come from a mixture of sources, including the Guide Stars Photometric Catalogue 2, and USNOFS CCD Parallax program fields. There is a MagFlg for each individual magnitude. 0 = the bright photometric standards on the plate. 1 = the faint photometric standards on this plate. 2 = the faint photometric standards 1 plate away (ie, on an overlapping plate). 3 = the faint photometric standards 2 plates away. And so forth.
- FldID - this field encodes both the survey, and the field id within the survey that corresponds to this observation. This is a 4 digit field, patterned SFFF, where S is the survey id, and FFF the field id. Survey id's are: 0 = POSS-I O 1 = POSS-I E 2 = POSS-II J 3 = POSS-II F 4 = SERC-J or SERC-EJ 5 = ESO-R or SERC-ER 6 = AAO-R 7 = POSS-II N 8 = SERC-I 9 = POSS-N field, taken as part of SERC-I. The field id (FFF) is the field number in the indicated survey. There is a FldID for each individual magnitude.
- S/G - this field is the star/galaxy index. This is a measure of the similarity of the detection to a stellar point spread function. The range is from 0 to 11. 0 means quite dissimilar, 11 means very similar. 19 means that there was no value computed. There is a S/G value for each individual magnitude.
- XResid - the Xi residual in the tangent plane between the measured position on this plate, and the position computed from the best fit. This is given as a value in decimal arcseconds between -50.00 and +49.99 arcseconds. There is a XResid value for each individual magnitude.
- YResid - the Xi residual in the tangent plane between the measured position on this plate, and the position computed from the best fit. This is given as a value in decimal arcseconds between -50.00 and +49.99 arcseconds. There is a YResid value for each individual magnitude.
- PltIdx - the look back index into the PMM raw scan files. There is a PltIdx value for each individual magnitude.
- R1 - first epoch red magnitude. This is the magnitude derived from the POSS-I 103aE or ESO-R plate. This is a PHOTOGRAPHIC R magnitude. See B1 Magnitude.
- B2 - second epoch blue magnitude. This is the magnitude derived from the POSS-II IIIaJ or SERC-J or SERC-EJ plate. This is a PHOTOGRAPHIC B magnitude. See B1 Magnitude.
- R2 - second epoch red magnitude. This is the magnitude derived from the POSS-II IIIaF or AAO-R or SERC-ER plate. This is a PHOTOGRAPHIC R magnitude.See B1 Magnitude.
- I2 - second epoch near-IR magnitude. This is the magnitude derived from the POSS-II IV-N or SERC-I plate. This is a PHOTOGRAPHIC I magnitude. See B1 Magnitude.
- Xi (computed quantity) - the standard coordinate Xi, in the tangent plane projection about the requested region center. This is only computed for rectangular or circular regions. The units are decimal arcseconds.
- Eta (computed quantity) - the standard coordinate Eta, in the tangent plane projection about the requested region center. This is only computed for rectangular or circular regions. The units are decimal arcseconds.
- DistCtr (computed quantity) - distance from this object to the extraction center in arcseconds.
- Gal_L (computed quantity) - computed galactic longitude, in decimal degrees.
- Gal_B (computed quantity) - computed galactic latitude, in decimal degrees.

Last updated: 2008Dec12 by sel