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 Ks > 4.0 mag
- Average repeatability better than 5% for sources at Ks=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 | NJ | H | H | NH | Ks | Ks | NKs |
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
Nmag 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 Ks.
[Modified 2000 Sep 12 by S. Van Dyk.]
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