The hourly observations of standard fields allow the long term site characterstics to be monitored. The figures in this section are based upon the nightly average of all calibrator scans during photometric periods. Night 1 is defined as 1997 March 1 UT. The J, H, and Ks data are shown as blue, green, and red respectively.
Figure 1 shows the strong seasonal dependence of the Ks backgrounds at the Mt. Hopkins facility. The Ks background is dominated by thermal emission. The gaps of Night 154-187 and 507-565 are due shutdowns during the Arizona monsoon season. The H backgrounds are dominated by airglow (OH) emission. They show considerably more night to night variation. Variations within a night can be more than a factor of two at H band.
The best images for the Mt. Hopkins facility are ~2.7´´-2.8´´ FWHM, as shown in Figure 2. The camera pixels are 2´´. Image sharpness, particularly at Ks, improved after telescope adjustments were made at the end of the 1998 monsoon season (Night 565). The FWHM increases during the summer, which may be a combination of the atmospheric conditions and the poorer focus behavior for the Mt. Hopkins telescope at high temperatures.
The achieved signal-to-noise (S/N) for faint stars depends primarily upon the background and image sharpness. The Level-1 specifications for 2MASS are S/N=10 at 14.3 at Ks, 15.1 at H, and 15.8 at J. Analysis of repeat observations of the calibrator fields has been used to determine empirical correlations describing the magnitude at which S/N=10 is achieved as a function of background and the image sharpness. These correlations are used during the Quality Assesment grading. Figure 3 shows the estimated limiting magnitudes. The combination of high backgrounds and poor seeing compromises the July data at Ks band; the Mt. Hopkins data taken during the winter is typically 0.6 mag deeper. On average, the H data is better than the specification, but portions of nights with high airglow can be compromised. The J sensitivity nearly always achieves S/N=10 at J>16.
The zero-point measures the relationship between flux and camera counts(DN). More positive zero-points are more sensitive, in the sense that there are more counts for a given flux. Figure 4 shows that the J zero-point varies more strongly on a night-to-night basis than H or Ks. This is consistent with the observation that within a photometric night, a constant zero-point is an adequate description of the H and Ks data , while a zero-point that varies linearly with time is needed for the J data. The zero-points show evidence that the atmosphere is less transmissive near the monsoon season, consistent perhaps with the high humidities and resulting water absorption.
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