VI. Analysis of the 2MASS Second Incremental Release Catalogs

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

c. PSC Photometric Precision

The Level 1 Specifications call for the precision for unconfused point sources to be better than 5% in all three bands for SNR >> 20.

This specification attempts to measure the attained photometry including all sources of error for bright sources in the "Read 2" (1.3-s exposure) photometry ("Read 1" photometry [0.51-ms exposure] of very bright sources is handled separately.) In order to meet the specification the trend of observed vs. magnitude must reach an asymptote of less than 0.05 mag for the brightest point sources.

The data set analyzed is the same as used for the Level 1 specifications sensitivity in Section VI.1a: sources observed 30 or more times in eight equatorial photometric calibration regions with |b| > 20° that were observed from both north and south. Observations of these fields were taken every hour of every night, in the same manner as the regular survey data, with the exception that the scan lengths are only 1° long. The data has passed the photometric quality check, but has not been selected on the basis of seeing or skyglow. These last criteria will act to remove some outlying data from the analysis.

Figure 1 shows the observed uncertainty, vs. magnitude for the brightest sources in these fields. Since these sources were observed at least 30 times, this analysis provides a true external estimate of the .

On the left is the bright end of the Read 2 photometry for Mt. Hopkins (North) and on the right the photometry for Cerro Tololo (South). The horizontal line is the 0.05 mag level: the trend of vs. magnitude must fall below this line at the bright end, in order for the Survey to meet the Level 1 specification.

Since this calibration data was accepted only on the basis of photometric conditions, and not selected on the basis of seeing or airglow, it is not as good as final, accepted Survey data. Still, it meets the Level 1 specification for precision. Thus, the entire Survey meets the Level 1 specification in all three bands in both hemispheres.

Figure 1

[Last Updated 2000 Feb 15 by P. Seitzer. Last Modified: 2000 Sep 5 by S. Van Dyk.]

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