The global 2 minimization procedure by M. Weinberg and S. Nikolaev, described here has produced photometry of stars in the 2MASS Calibration Fields with exquisite internal uniformity and self-consistency. The internally consistent photometry of the secondary standard stars developed from this procedure are used for nightly 2MASS photometric calibration.
The primary and secondary standard stars that are observed during normal survey scans are calibrated using the nightly photometric solutions, and these calibrated magnitudes are reported in the Point Source Catalog. We can examine the difference between the photometry reported in the PSC and the "best" magnitudes for those stars from the global 2 minimization procedure to measure the internal consistency of photometry reported in the PSC and the 2MASS natural photometric system.
Of the 1036 primary and secondary 2MASS calibration stars, 470 are included in the PSC for the Secondary Incremental Data Release. Figure 1 shows the sky coverage map for the upcoming release, and marked in green the positions of the calibration stars found in the PSC. Shown in red are the positions of calibrators not found in the PSC. All calibrators that are in the coverage area of the release are in the PSC.
In Figures 2, 3, and 4 the difference between the "truth" photometry and the PSC photometry for each calibration star is plotted against the true magnitude. The right side of each figure provides a histogram of the photometric residuals.
The mean and RMS photometric residuals in each band are shown in the three plots. The RMS residual in all bands is ~0.03 mag. No significant bias between the the PSC and "truth" photometry is seen.
There are a couple of stars that show relatively large (>0.1 mag) discrepencies between the PSC and truth photometry. J. Gizis has begun an investigation into these measurements; the results can be seen here.
|Figure 2||Figure 3||Figure 4|
[Last Updated: 2000 February 9 by R. Cutri. Modified 2000 Aug 29 by S. Van Dyk.]