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