VI. Analysis of the 2MASS Second Incremental Release Catalogs

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

f. 2MASS Bright Star Photometry Comparison

The 2MASS Level 1 specifications require measurement of bright stars with the following accuracy:

Photometric bias (zeropoint drift) <2% for stars with K_s > 4.0 mag
Average repeatability better than 5% for sources at K_s=8.0 mag
Night-to-night zeropoint calibrations should agree to within ±4% of the mean calibration.

i. Repeat Scans

Several areas of the sky have been scanned on two or more occasions, by both the north and south telescopes, as a test of photometric repeatability. The comparisons are based on the PSF magnitudes, which become unreliable at brighter than magnitude 8. Figure 1 compiles default magnitude data for the brightest stars, including "Read 1" (R1) magnitudes for < magnitude 8. The comparison involves observations from 1998 Jan 13 (north) and 1998 Nov 18 (south).

Figure 1

ii. Statistics from Bright Star Comparison

Table 1 gives the mean offset and rms as a function of magnitude. Given that these data originate from a single night (each), the values are within the formal specification.

Mag J _J N_J H _H N_H K_s _{K_s} N_{K_s}
6.25 0 0 0 0.036 0.039 3 0.019 0.018 8

6.75 0 0 0 0.040 0.031 18 0.015 0.017 20

7.25 0.048 0.026 14 0.028 0.028 39 0.021 0.030 44

7.75 0.052 0.025 34 0.027 0.035 54 0.016 0.045 69

8.25 0.044 0.043 55 0.017 0.051 93 0.017 0.034 106

8.75 0.046 0.032 83 0.021 0.039 148 0.013 0.035 163

where
mag is the mean offset, in 0.5 magnitude bins,
_mag is the rms dispersion about the mean, and
N_mag is the number per bin.

iii. 2MASS Bright Stars: Zeropoint

Figures 2, 3, and 4 show the residuals between 2MASS observations and literature photometry (from Leggett, 1992, ApJS, 82, 351) for a subset of nearby stars from the Gliese/Jahreiss CNS3. The rms uncertainties are <0.03 mag, within specification, with no indication of systematic variation with magnitude.

Figure 2 Figure 3 Figure 4

The same data are used to check for color terms in Section VI.6b. There is no evidence for a significant color term in any of J, H or K_s.

[Modified 2000 Sep 12 by S. Van Dyk.]

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Mag	J	_J	N_J	H	_H	N_H	K_s	_{K_s}	N_{K_s}
6.25	0	0	0	0.036	0.039	3	0.019	0.018	8
6.75	0	0	0	0.040	0.031	18	0.015	0.017	20
7.25	0.048	0.026	14	0.028	0.028	39	0.021	0.030	44
7.75	0.052	0.025	34	0.027	0.035	54	0.016	0.045	69
8.25	0.044	0.043	55	0.017	0.051	93	0.017	0.034	106
8.75	0.046	0.032	83	0.021	0.039	148	0.013	0.035	163