Although smaller, a clear cos signature is also present for these southern bright stars. For the GSC comparison, no proper motion was available. Each point is the average over 1 h bins in right ascension. The more stars that we use to calculate this average, the better and more exactly we define the average. However, there is a significant fraction of stars with much larger errors:

usno a2 catalog

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A by-product of the Hipparcos observations was extremely accurate photometry of millions of stars. Camargo3 R. Note that both these values include the built-in error that comes from the transformation of the USNO magnitude to the Landolt magnitude scale. The statistics in Table 1 refer to the average offsets of the fields according to the northern and southern parts of USNO-A2. It is very bad practice and extremely dangerous to use single USNO stars for photometry.

Observed regions covered 13 in declination by an average of 35 m in right ascension. In addition, careful photometric calibrations were performed.

Each point is the average over 1 h bins in right ascension. The reason for using all stars is two-fold apart from the one given above. In this way, a striking, systematic offset of -0 17 is found for the bright mag 11 stars in the southern part of the catalog. No dependence on the star distance catlaog the plate center was found. We are grateful to the anonymous referee for his valuable notes and suggestions.


ESO/ST-ECF USNO-A Catalogue Server

Final astrometric solutions included iterative plate reductions after vectorial map corrections for O – C plate residuals. The standard visible magnitude scale is that defined by Arno Landolt in a seminal series of papers published in the Astronomical Journal in and However, in the late s a radically new catalov of calibration has been opened. A total of 83 stars are included in B, 87 in V and 89 in R.

Many people who use the USNO catalogue habitually are unaware of how it was prepared and its limitations. The points are the average of the position offsets, binned in cells of 0. The comparison is shown in Figure 4which displays the average offsets against V magnitudes in bins of 0.

ESO/ST-ECF USNO-A2.0 Catalogue Server

The dispersion seen in the northern part of the plot for the GSC comparison is unusually larger than in the others.

Although smaller, a clear cos signature is also present for these southern bright stars. Plate measurements were referred to the ACT.

usno a2 catalog

Magnitudes were within the range 11 V The error bars stand for the formal rms error of each bin. A decreasing tendency of cos on R magnitudes for south stars is less evident.

USNO-A2.0 Catalogue

This text and its associated graphic s may be freely distributed and reproduced provided that uson source is clearly stated. The photometric transformations that are calculated for the three colours B, pseudo-V, and R to convert USNO photometry to the standard Landolt scale are: For the brighter sample mag ysnoACT positions were used.


In this case, however, the linear dependencies of the southern offsets with regard to R magnitudes and to B-R color index appear to be much more relevant. The nominal astrometric precision is 0 22 to 0 25, depending on star magnitude and location on the plates, and photometric precision is 0.

As we can see, the USNO and Landolt magnitude scales coincide very exactly at magnitude 12, at the magnitude limit of the Tycho stars. The second reason is that the cztalog in the magnitude that we calculate reduces.

usno a2 catalog

For the fainter set mag 16we used ICRF radio positions. This result is expected due to the way that the USNO magnitudes were calibrated see below.

USNO-A A Catalog of Astrometric Standards

This is exactly where the southern and northern plate surveys are split. Although not as striking as Figure 1Figure 2 also displays some unexpected sinuous features and nonzero constant offsets in the southern and northern parts.

A detailed description of these surveys is given in Lasker et al.