III. 2MASS Facilities and Operations

1. Facilities

c. Observatory Site Conditions

Presented here are plots which track a number of critical quantities showing the variable quality of the observing sites as a function of time (Julian date minus 2450000). During the Quality Assurance procedure, quality scores were given to each Survey scan based in part on these data. Only scans included in the All-Sky Release are shown in these plots. The data are averages over the duration of each scan (about 7 minutes), and are color-coded by band as blue (J), green (H) and red (Ks). The obvious gaps in the northern data correspond to shutdowns during the Arizona monsoon season.

i. Backgrounds

Figure 1, plotting the background levels in DN, shows most notably the strong seasonal dependence of the Ks backgrounds, The emission is primarily from the optical elements and support structure of the telescope, and there is also a significant airglow component in the Ks-band. The variation is most pronounced at the northern facility, consistent with the larger seasonal temperature swing at that site. The H and J backgrounds are dominated by airglow (OH) emission, which shows considerable night-to-night variation, and variations within a night can be more than a factor of two. The northern H-band array was replaced in August 1999. The apparent dip in the northern H-background levels beginning on JD=2451434 is due to the slightly lower quantum efficiency of the replacement array.

ii. Image Quality

Scan-averaged values of seeing are plotted in Figure 2. It should be kept in mind that the camera pixels are 2´´ in size, and the seeing values are derived from the individual camera frames. The best images for the Mt. Hopkins facility have seeing of ~2.5´´ FWHM, while the average from 1997 to the 1998 monsoon shutdown was 2.8´´ FWHM. Image sharpness, particularly at Ks, improved after telescope adjustments were made at the end of the 1998 monsoon season (JD=2451005). The FWHM increased during the summer, which may have been a combination of the atmospheric conditions and the poorer focus behavior for the Mt. Hopkins telescope at high temperatures. Adjustments made at the end of the 1999 monsoon further improved the image quality, and the average seeing improved to 2.6´´ FWHM. The CTIO facility consistently achieved ~2.5´´-2.7´´ FWHM.

iii. Sensitivity

Analysis of repeat observations of the calibrator fields has been used to determine empirical correlations describing the magnitude at which SNR=10 is achieved as a function of background and image sharpness. Figure 3 show the derived magnitudes corresponding to SNR=10 for each night, with the Level 1 Science Requirements indicated by horizontal lines for each band. The combination of high backgrounds and poor seeing compromises the Mt. Hopkins July data at Ks; the Mt. Hopkins data taken during the winter is typically 0.6 mag deeper. Sensitivity at H is driven primarily by OH airglow. The J sensitivity nearly always achieves SNR=10 at J>16. The CTIO facility shows less extreme seasonal variations for the Ks band, but can also suffer from high airglow. The All-sky Release contains only scans whose sensitivity surpass the requirement levels. Therefore points that would have fallen below the lines are not shown on the plots.

iv. Zero-Point Offsets

The zero-point, stated in magnitudes, is a measure of atmospheric transparency and system throughput. It measures the difference between instrumental and true magnitudes in the sense that more positive zero-points indicate more sensitivity. The zero-point offsets are derived from the nightly photometric calibration (c.f. IV.8). Figure 4 shows that the J zero-point varies more strongly on a night-to-night basis than H or Ks at both facilities. This is consistent with how the zero-point was fit as a function of time during the night. In J band, the zero-point data were fit in a piecewise manner, while H and Ks data had slower varying linear fits applied. The zero-points show evidence that the atmosphere at Mt. Hopkins is less transmissive near the monsoon season, consistent perhaps with the high humidities and resulting water absorption. The jumps in the CTIO zeropoints are associated with cleaning the camera window and washing the primary mirror of the telescope.

Figure 1Figure 2Figure 3Figure 4

[Last Updated: 2003 Mar 04; by B. Nelson]

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