2MASS Bright Star Photometry comparison

The 2MASS level 1 documentation requires measurement of bright stars with the following accuracy
Photometric bias (zeropoint drift) <2% for stars with K > 4.0 magnitude
Average repeatability better than 5% for sources at K=8.0 magnitude.
Night to night zeropoint calibrations should agree to within +/-4% of the mean calibration.

Repeat scans

Several areas of the sky have been scanned on two or more occasions, by both North and South telescopes, as a test of photometric repeatability

  1. Tiles 900-907
  2. Tiles 1123-1128
  3. Tiles 1528-1534
The above comparisons are based on the psf magnitudes, which become unreliable at brighter than 8th magnitude. The plot shown below compiles default magnitude data for the brightest stars in tiles 900-907, including read 1 magnitudes for < 8th magnitude. The comparison involves observations from 13 Jan, 1998 (North) and 18 Nov, 1998 (South). The table 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.

Statistics from bright star comparison

Mag J J NJ H H NH K K NK
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

mag is the mean offset, in 0.5 magnitude bins
mag is the rms dispersion about the mean.
Nmag is the number per bin

2MASS bright stars: zeropoint

The three plots illustrated below 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 magnitude, within specification, with no indication of systematic variation with magnitude.

The same data are used to check for colour terms at this reference. There is no evidence for a significant colour term in any of J, H or KS.