Astrometry errors in DR8

We would like to alert our users of large astrometry errors that we have identified in the SDSS Data Release 8 photometric catalog.  In particular, the regions not covered by UCAC2 (north of around 41 deg declination) the DR8 astrometry is offset 240 mas to the North and 50 mas to the West. The declination offset is nearly uniform in the affected area (at an rms level of about 60 mas).  Users trying to perform proper motion analysis, detailed astrometry, or performing spectroscopic observations should be aware of these shifts. A full description of our understanding of the errors and quality assessment plots can now be found in our  description of the DR8 astrometry.

These problems are currently being addressed and we plan to add supplemental tables with correct astrometry to the DR8 release. We will incorporate the new astrometry as the default for the DR9 data release.

An erratum has been prepared for submission to the Astrophysical Journal to correct the Data Release 8 paper’s statements on this issue.

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