Sdss astrometry7/6/2023 ![]() ![]() (Note that there are some much smaller differences between DR8 and DR9 because of astrometry, in the sizes and position angles of objects for example these are tiny and are not included in these tables).ĭifferences between the true and measured sizes and magnitudes of large galaxies, based on simulated images. These tables contain the values for positions and proper motions that are distributed in the DR9 release. However, northward of 40 deg there is a systematic offset of around 250 mas in the declination direction.įor very precise astrometry it is necessary to use the DR9 version of the catalog or, if it is necessary to use DR8, two special CAS tables included in that release: astromDR9 and properMotionsDR9, described on the DR8 astrometry algorithms page. ![]() Systematic errors introduced in DR8 southward of 40 deg declination are typically smaller than or comparable to the 45 mas systematic errors that characterize the SDSS astrometry for brighter stars. See the full details on the DR8 astrometry algorithms page. Note that the proper motions are considerably less degraded. The DR9/DR10 astrometry and proper motions are superior to both DR8 and DR7.īecause of several errors, the absolute astrometry in DR8 was degraded relative to that in DR7, and degraded substantially northward of a declination of around 40 deg. This caveat is appropriate for users of the DR8 catalog, but irrelevant for DR9 and subsequent releases. ![]()
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