VI. Analysis of the 2MASS Second Incremental Release Catalogs

1. Comparison of Achieved Performance of Second Incremental Release Catalogs with Level 1 Science Specification

d. Point Source Astrometric Performance

The Level 1 Requirement for point source astrometric accuracy is better than 0.5´´ with respect to the reference frame. It appears that the Second Incremental Release is substantially better than this, in that the available astrometric tests indicate that the error is of the order of 0.15´´ or better.

The design of the astrometric reconstruction algorithms take advantage of two key recent advances in astrometry. First, the adoption of USNO's ACT catalog provides a deep, dense set of positional standards whose errors are known to be less than or equal to 0.030´´. Adoption of this catalog avoids intermediate astrometric catalogs and the potential for the introduction of errors. Second, each 2MASS scan is separately tied to the reference catalog. This means that should a scan have a significant error, that error does not propagate into adjacent scans.

Three separate astrometric tests were applied to the Second Incremental Release Catalog. The overall conclusions demonstrate the astrometric quality.

There is the possibility that an isolated scan may have poorer astrometry, and future studies of the stars in the scan-to-scan overlap regions can test for such errors and remove them. However, it is clear that the astrometric accuracy of the typical Second Incremental Release position is about a factor of 3 better than the Level 1 Requirement.

[Last Updated: 2000 January 24 by D. Monet. Modified 2000 Sep 5 by S. Van Dyk.]

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