DOWNLOAD USNO A2 CATALOG

Magnitudes were within the range 11 V No effect due to proper motion, magnitude, color index, or epoch difference can explain these sections of constant offsets in and cos. Having many stars in the field of view allows "bad" star measures to be eliminated. However, in the late s a radically new possibility of calibration has been opened. The nominal astrometric precision is 0 22 to 0 25, depending on star magnitude and location on the plates, and photometric precision is 0.

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USNO-A2.0 Catalogue

The nominal astrometric precision is 0 22 to 0 25, depending on star magnitude and location usjo the plates, and photometric precision is 0. For the fainter set mag 16we used ICRF radio positions.

In other words, the one sigma error in a single USNO magnitude is 0. 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: Final astrometric solutions included iterative plate reductions after vectorial map corrections for O - C plate residuals.

USNO-A Catalogue

Landolt's stars are mostly in the celestial equator and have a rather bright limiting magnitude: This text and its associated graphic s may be freely distributed and reproduced provided that its source is clearly stated. Note that Tycho-2 and ACT represent the same system within the position errors considered.

The Astrophysical Journal, For the ACT comparison, on the contrary, such regular patterns do appear. As a result, a new full-sky astrometric catalog of over million objects was produced, the USNO-A2.

The error bars stand for the formal rms error of each bin. A total of 83 stars are included in B, 87 in V and 89 in R. A decreasing tendency of cos on Aa2 magnitudes for cataoog stars is less evident. This is exactly where the southern and northern plate surveys are split. We can get an idea of the expectation of the accuracy of a given star by looking at the following table: For the brighter sample mag 11ACT positions were used.

ESO/ST-ECF USNO-A Catalogue Server

If we measure stars in each field though we reduce this uncertainty to 0. Observed regions covered 13 in declination by an average of 35 m in right cataolg. The standard visible magnitude scale is that defined by Arno Landolt in a seminal series of papers published in the Astronomical Journal in and 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. It is very bad practice and extremely dangerous to use single USNO stars for photometry.

In addition, careful photometric calibrations were performed.

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 second reason is that the uncertaintly in the magnitude that we calculate reduces. It catallg also contribute with first epoch positions for proper motion determination in local fields.

Having many stars in the field of view allows "bad" star measures to be eliminated. Assafin1 A. However, there is a significant fraction of stars with much larger errors: Andrei2 R. The rms error in the USNO magnitude is 0. The points are the average of the position offsets, binned in cells of 0.

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

However, no significant offset is present for the northern bright stars. Dependencies with R and V magnitudes and the B-R color index were found for bright stars. This makes a standard global calibration impossible, except at the most basic level. The comparison is shown in Figure cataalogwhich displays the average offsets against V magnitudes in bins of 0. Plate measurements were referred to the ACT.