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CARA Science:
Comparison with Other Sites

Infrared and Optical

The initial expectation that the South Pole site would be a vastly superior site for near infrared observations (the 2 micron K band) were not met due to relatively poor seeing (Lowenstein 1998) and the narrowing of the window by atmospheric emission lines (Phillips et al. 1999), but the results for longer wavelengths have surpassed expectation. Very low backgrounds were predicted for the 2 micron band since it falls on the Wein side of the peak in the thermal terrestial emission and therefore the colder temperatures should lead to a large decrease in sky brightness. Indeed, in the K-dark band (2.3 - 2.4 micron) the South Pole is measured to be 10 to 100 times darker than at temperate sites (Phillips et al. 1999).

Phillips et al. (1999) also demonstrated that the background flux in the L window (2.9 - 4.2 micron) is typically 20 times lower than that at temperate sites. This band is now being exploited by the SPIREX/Abu project which is routinely conducting observations which could not be carried out at any other existing site.

At mid-infrared wavelengths (10 micron) only a factor of two reduction in the background is expected from the lower temperature of the site, but one must consider the decrease in the water content of the atmosphere. Recently the analyses of the Mid-Infrared Sky Monitor (MISM) mounted on the AASTO were completed and submitted for publication (Chamberlain et al. 1999). The results show that the Pole is indeed superior at mid-infrared wavelengths. A comparison of the South Pole mid-IR sky brightness to Mauna Kea is shown below.

Comparison of South Pole, Mauna Kea, and Canberra. Note the exceptionally low sky brightness which is due primarily to the low atmospheric water vapor.

Optical seeing as a function of height of the telescope above the surface. The solid line represents the average results fom launches at the South Pole, while the dashed line is a summary of a similar experiment performed at the ESO-VLT site at Cerro Paranal, northern Chile, in May 1993.

Site Altitude (m) Total seeing (arcsec) Free atmosphere seeing (arcsec) Boundary layer seeing (arcsec) Surface layer seeing/range (arcsec, m) Boundary layer height Reference
South Pole 2835 1.86 0.37 1.78 0.64, 27 220 Marks et al. 1999, 1996
Cerro Paranal, Chile 2500 0.64 (median) Murtagh & Sarazin 1993
0.73 0.4 0.55 2000 Fuchs 1995
La Silla, Chile 2400 0.97 0.31 0.85 0.15,30 800-1000 ESO VLT Report 1987
0.87 Murtaugh & Sarazin 1993
Mauna Kea, Hawaii 4200 0.74 0.46 0.52 Roddier et al. 1990
La Palma, Canary Islands 2100 0.96 0.40 0.73 0.07, 12 1-2000 Vernin & Munoz-Tunon 1992, 1994

Submillimeter

Broadband 350 µm Opacity, 1998. Quartiles of broadband 350 µm Opacity, tau, by month in 1998, measured by identical instruments at South Pole and Chajnantor, Chile. The tau produced by this instrument is a factor of at least 2 higher than simultaneous narrow-band measurements at the same wavelength made with AST/RO at the South Pole and the CSO on Mauna Kea.

The South Pole is a considerably drier site than Mauna Kea or Atacama. During the wettest quartile at the South Pole, the total precipitable water vapor is lower than during the driest quartile at either Mauna Kea or Atacama.

Data from NRAO/CMU 350 micron tipping radiometers located at the South Pole, Mauna Kea and Chajnantor (Chile). The South Pole data are striking, clearly indicating the superiority of the South Pole site for submillimeter wave observations. The upper plot of each pair shown in the Figure shows the rms deviation in the opacity during a one-hour period which is a measure of sky noise on large scales; the lower plot of each pair shows the broadband 350 micron opacity. The first 100 days of 1998 on Mauna Kea were exceptionally good for that site. During the best weather at the Pole, the measurements were dominated by detector noise rather than sky noise. The South Pole is clearly superior to both Mauna Kea and the Atacama site for submillimeter observations.

The CMU/NRAO tippers at Mauna Kea, Atacama, and the South Pole continue to provide directly comparable measurements of submillimeter opacity about five times per hour. A report on the 1998 tipper data in the Figure is being prepared by CARA REU student Michael O'Kelly (CMU).

Millimeter

Values of A h_av^{8/3} / mK^2 m for the South Pole as estimated from Python V data, and for the Atacama Desert in Chile estimated from phase monitor data. The quantity A h_av^{8/3} is proportional to the atmospheric noise power seen by a given instrument at a given wind speed and direction. The data did not constrain well the altitude of the atmospheric fluctuations, so A h_av^{8/3} is tabulated for various possible altitudes of fluctuations at the two sites. For a detailed explanation of the quantity A h_av^{8/3} and its relation to the amplitude of the atmospheric noise power in a given instrumental configuration, see Lay and Halverson (1998). The Chile numbers have a 50 % uncertainty associated with the conversion from refractive index to brightness temperature. Values are appropriate for 40 GHz, but can be scaled to other frequencies based on the emissivity spectrum of water vapor.

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