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Opacity Measurements

CARA experiments have directly measured both millimeter and submillimeter-wave atmospheric opacity at the South Pole using skydip techniques. Over 1100 skydip observations were made at 492 GHz (609 m) with AST/RO during the 1995 observing season. Even though this frequency is near a strong oxygen line, the opacity was below 0.70 half of the time during the Austral winter and reached values as low as 0.34, better than ever measured at any ground-based site. The stability was also remarkably good: the opacity remained below 1.0 for weeks at a time.

South Pole 492 GHz Opacity during 1995. Shown is the measured 492 GHz zenith opacity at the South Pole plotted as a function of days elapsed since 1 January 1995 (Chamberlin, Lane and Stark, Ap.J. 476,428). For the 1995 winter season the quartiles of the 492 GHz zenith opacity cumulative distribution function are: (25%:50%:75%) = (0.55:0.70:0.81). The "dry air" 492 GHz opacity at Pole is 0.330.02; most of which is from the wings of the O2  line at 487 GHz.

Skydip data at 225 GHz (1.33 mm) were obtained during 1993 by Richard Chamberlin and John Bally using a standard NRAO tipping radiometer similar to the ones used to measure the 225 GHz zenith opacities at Mauna Kea and the ALMA site at Chajnantor. The tight linear relation between 225 GHz skydip data and balloon sonde PWV measurements is discussed by Chamberlin and Bally (Int. J. IR and MM Waves, 16, 907). 


PWV vs. Opacity at Pole. Opacity at various frequencies measured by skydips plotted against precipitable water vapor. Fits to the data show a positive intercept at zero PWV, a measure of "dry air" opacity. The "dry air" component of opacity is less variable than the water vapor component and therefore generates less sky noise. Figure courtesy of R. Chamberlin.

Of the three sites, South Pole consistently has the lowest water vapor and temperature but the highest pressure. This mixture makes the comparison between Pole and other sites more favorable at some wavelengths than at others. All three sites are excellent at millimeter-waves; the 225 GHz data for South Pole and Chajnantor are comparable because at that frequency the lower PWV at Pole is roughly balanced by the lower pressure at Chajnantor. Both are superior to Mauna Kea. 

From early 1998, the 350m band has been continuously monitored at Mauna Kea, Chajnantor, and South Pole by identical tipper instruments developed by S. Radford of NRAO and J. Peterson of Carnegie-Mellon U. and CARA. These instruments measure a broad band that includes the center of the 350m window as well as more opaque nearby wavelengths. Comparison of the opacity values measured by these instruments is tightly correlated with occasional narrow-band skydip measurements made within this band by the CSO and AST/RO; the narrow band opacity values are about a factor of two smaller than those output by the broadband instrument. 


Broadband 350m Opacity, 1998. Quartiles of broadband 350m  opacity by month in 1998, measured by identical NRAO-CMU tippers at South Pole and the ALMA site at Chajnantor, Chile. Several measurements were made each hour, day and night, and binned together by month. The plot shows the quartiles of the resulting distribution of individual measurements for each month. The opacity measured by these instruments is a factor of ~2 higher than simultaneous narrow-band measurements in the same wavelength band made with AST/RO at the South Pole and the CSO on Mauna Kea. Figure courtesy of S. Radford, NRAO.


Noise and Opacity Measurements at 350m from Three Sites.  These plots show data from identical NRAO-CMU 350m broadband tippers located at Mauna Kea, the ALMA site at Chajnantor, and South Pole during 1998. The upper plot of each pair shows the rms deviation in the opacity during a one-hour period---a measure of sky noise on large scales; the lower plot of each pair shows the broadband 350m opacity. The first 100 days of 1998 on Mauna Kea were exceptionally good for that site. During the best weather at the Pole the rms deviation in the opacity was dominated by detector noise rather than sky noise.

The South Pole PWV levels applicable 10%, 25%, 50%, and 75% of the time during winter have been used to compute values of atmospheric transmittance. For comparison, the transmittance for excellent conditions at Chajnantor is shown by the lowest dashed line in the figure. If three identical background-limited instruments operating in the 350m window were placed at Pole, Chajnantor, and Mauna Kea to do identical observations over a long period of time, the observations at Pole would proceed 110 faster than Mauna Kea and 4.5 faster than Chajnantor.

Calculated Atmospheric Transmittance at South Pole.  Atmospheric transmittance at 1.41 airmasses calculated by K. C. Yu using Grossman's AT code for the pressure, temperature and water vapor content of the South Pole and Chajnantor. The upper four lines are for values of precipitable water vapor which are applicable 10%, 25%, 50%, and 75% of the time during winter (0.14 mm, 0.19 mm, 0.25 mm, and 0.32 mm, respectively). Also plotted (bottom dashed line) are values for the best 25% of the time during the better half of the year at Chajnantor (PWV = 0.68 mm). Note that at long wavelengths, the Chajnantor curve converges with the Pole curves.  An explanation of the calculation and additional plots at other wavelengths and for other sites can be found at John Bally's website.


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Last modified: November 16, 1999