Magnitudes used in the USNO-A catalog

USNO-A is a catalog of 488,006,860 sources whose positions can be used for astrometric references. These sources were detected by the Precision Measuring Machine (PMM) built and operated by the U. S. Naval Observatory Flagstaff Station during the scanning and processing of the Palomar Observatory Sky Survey I (POSS-I) O and E plates, the UK Science Research Council SRC-J survey plates, and the European Southern Observatory ESO-R survey plates. The PMM detects and processes at and beyond the nominal limiting magnitude of these surveys, but the large number of spurious detections requires that a filter be used to eliminate as many as possible. USNO-A's sole inclusion requirement was that there be spatially coincident detections (within a 2 arcsecond radius aperture) on the blue and red survey plate. For field centers of -30 degrees and above, data come from POSS-I plates, while data from field centers of -35 and below come from SRC-J and ESO-R plates.

USNO-A presents right ascension and south polar distance in the system of J2000 at the epoch of the survey blue plate for each object, and lists an estimate of the blue and red magnitude. For POSS-I sources, the photometric system is the photographic system defined by the O and E emulsions and filters, while southern sources are measured in the photometric system defined by the IIIa-J and IIIa-F emulsions. It is believed that the typical astrometric error is about 0.25 arcseconds and that the typical photometric error is about 0.25 magnitudes. However, these error estimates are dominated by the systematic errors incorporated in the calibration procedure, and some fields may be significantly worse. Should users be willing to locally recalibrate the astrometry and photometry, the errors arising from the PMM are believed to be in the range of 0.15 arcsecond and 0.15 magnitude.

To avoid the necessity of consulting many catalogs, objects brighter than 11th magnitude that appear in the Guide Star Catalog that were not detected by the PMM were inserted. USNO-A covers the entire sky, and goes as deep as O=21, E=20, J=22, and F=21 for objects with appropriate colors. The limiting magnitude is brighter for objects with extreme colors, and follows from the requirement for a detection on both the blue and red survey plate. Although it covers the entire sky, there are holes in the catalog in the vicinity of bright stars, regions of nebulosity, crowded fields, etc.

Magnitude Errors

There are several sources of error for the magnitudes reported in USNO-A. These magnitudes are generally flagged in the catalog. When selecting objects from the catalog, this software never disqualifies an object for failing to meet a magnitude requirement when the magnitude has been flagged. The ASCII output will display the flags beside the magnitude value. Both IRAF and Binary output will simply include the object with no comment although the Binary output will contain the flags just as the original catalog did. Below, each source of error is described.


When using the magnitude limits to select objects keep in mind that each limit acts as an independent filter. The following examples show how the filters can be used:

© Lowell Observatory 2008
Contacts: Ted Bowell and Bruce Koehn
Last modified: 2008 April 15