Astrometry of Landolt Catalog

The Landolt photometric catalog has a well-known poor astrometry (there are errors of tens of arcsec for some stars).

How can we get a better astrometry?

This is the method I used to improve the astrometric accuracy of the Landolt catalog. The first step was to get a 5" accuracy astrometry, the next one was to match with the USNO catalog.

I used theAladin tool provided by the Centre de Données astronomique de Strasbourg, in order to get a digitized image of (almost) any part of the sky, with RA and DEC coordinates. For the Landolt fields, I used the Digitized Sky Survey (DSS) of the STScI. Two generations are available, DSS1 and DSS2 with a pixel size of about 1.7" for DSS1 and about 1" for DSS2. These images gave me the coordinates of each Landolt star, based on the finding charts provided by Landolt.

A first step was to localize roughtly each star after its finding chart. The second one was to try to get its coordinates as accurately as possible. I used DSS2 (smallest pixel size) to do that. Afterward I noticed that astrometry of DSS2 is systematically off for a few fields (as for SA110 for example).

At this level the accuracy in Landolt coordinates is less than 5".

Finally I used the USNO-A2.0 astrometric catalog to refine the position of each star.

Both my "new" Landolt catalog and the USNO have been matched:

In total 498 (on 526) Landolt stars have now USNO coordinates, which means an accuracy of 0.25". For the remaining 28, I put in the "new" catalog the coordinates I got by eye (with an accuracy typically < 5").

The astrometrically improved Landolt catalog

To download this "new" Landolt catalog which has exactly the same shape of the original one. Coordinates are for J2000.0:

First use: quick study of the USNO catalog photometry

We can examine the photometry of the USNO catalog.


Landolt Faint Photometric Standards: ING La Palma Technical Note no. 95

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Guillaume Blanc - august 2001