I. Nightly Photometric Intervals

Photometric calibration observations were made hourly during each night at the two 2MASS observatories. A time interval was considered photometric if the zero point offsets measured between subsequent calibration observations had an RMS of <0.04 magnitudes. Figure 1 shows the number of contiguous photometric hours achieved at each observatory plotted versus the running survey day number (day 1 = 1 March 1997 UT).

The long periods of non-photometric nights in the northern distributions near days 170, 550, 900, and 1275 are the summer monsoon shutdown periods. These are considered lost due to weather and are counted in the non-photometric time.

Figure 1 - Photometric hours per night at 2MASS/Mt. Hopkins (top) and 2MASS/Cerro Tololo (bottom)

II. Photometric Statistics

The following table lists for the operational date range specified for each observatory: the total number of calendar nights, the number of nights during which 2MASS has found at least one photometric interval, and the total number of photometric hours on those nights. The final line gives the net photometric fraction for each site defined to the total number of photometric hours divided by the total number of possible hours.

Notes to Table 1:

(1) - These numbers reflect photometric conditions only. 2MASS may have rejected data acquired during nominally photometric conditions because of other factors such as atmospheric seeing, background level, etc.

(2) - Time lost due to non-weather-related reasons is not factored into the photometric fractions, but the impact is negligible. Engineering, telescope and instrument problems or other reasons resulted in a loss of only 21 nights at Mt. Hopkins and 4 nights at Cerro Tololo.

Tables 2 and 3 contain listings of the maximum number of hours available and the contiguous photometric hours achieved for each night of the survey at the northern and southern observatories, respectively. The first five columns in these tables give the running survey day number, the UT observation date (YYMMDD), the number of possible hours for observations during the night, the number of photometric hours measured, and the ratio of photometric to possible hours (i.e. the nightly photometric fraction). Nights lost due to non-weather related problems are included in the tables, including the northern summer monsoon shutdown. These nights are annotated with the cause of the loss in the sixth column.

Table 2 - Number of photometric hours and total hours possible per night at Mt. Hopkins

Table 3 - Number of photometric hours and total hours possible per night at Cerro Tololo

Figure 2 shows the mean photometric fraction by calendar month for Mt. Hopkins (top panel) and Cerro Tololo (bottom panel), derived from the statistics in Tables 2 and 3. The mean photometric fraction for each month is defined to be the sum of the photometric hours for all days in the month, divided by the total number of hours possible for the month. The dashed horizontal red lines show the survey mean photometric fraction for each site. The July and August Mt. Hopkins photometric fractions are artificially low because we treat the summer monsoon shutdowns as non-photometric time.

Tables 4 and 5 contain listings of the mean photometric fractions by calendar month that were used in Figure 2. The three columns in each of those tables contain the year/month (YYMM), mean photometric fraction and the number of days going into the computation. The first and last months for each observatory are partial months.

Table 4 - Photometric fraction by calendar month for Mt. Hopkins

Table 5 - Photometric fraction by calendar month for Cerro Tololo

Figure 2 - Photometric fraction by month at 2MASS/Mt. Hopkins (top) and 2MASS/Cerro Tololo (bottom)