VI. Analysis of the 2MASS Second Incremental Release Catalogs

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

h. Extended Source Photometric Precision

The Level 1 specification for photometric precision for extended sources is better than 10% (0.109 mag) for sources at H < 13.8 mag. The 2MASS data appear to meet this requirement as stated and mostly also as extended to other bands.

This specification sets out standards for realized photometry including all sources of error, and is meant to indicate the accuracy of photometry for moderately bright sources that might be used for Tully-Fisher studies, for example. The specification is interpreted to mean the error of sources in the half magnitude bin from 13.3 < H < 13.8 mag. The specification was based on assumption before the Survey began that H-band was to be preferred for Tully-Fisher work, but since subsequent indications are that J and K_s are probably at least as good, the specification could be extended to the precision of J and K_s magnitudes in a similar magnitude interval relative to the sensitivity limits.

We can test this longer-term accuracy fairly well by comparing data in the overlaps that was taken at well-separated times. We show below the uncertainty for sources in the appropriate magnitude intervals where the duplicate measurements were made at least one month apart:

	Ks-band 20 mag arcsec-2	7´´ circular
	isophotal magnitude	aperture magnitude
Mag Interval	uncertainty	uncertainty
14.1 < J < 14.6	0.086	0.067
13.3 < H < 13.8	0.092	0.074
12.5 < K < 13.0	0.116	0.071

Note that all but the K_s-band isophotal elliptical magnitudes have better than 10% errors. Even if we consider the contribution of a potential 0.044 mag uncertainty in the uniformity, this source of error (added quadratically) would not push any other of the uncertainties over the 10% level.

[Modified 2000 Sep 11 by S. Van Dyk.]

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