CARA Science:
Seeing: Optical and Infrared

Both differential image motion observations and direct measurements of turbulence show that the seeing during the winter at the South Pole is approximately 1.7 arcseconds at visual wavelengths at a height of 12 meters above the snow surface. Assuming Kolmogarov turbulence, this implies seeing of approximately 1.2-1.1 arcseconds at thermal infrared wavelengths between 2.4 and 5 µm. Almost all of the turbulence causing the seeing is located in the lowest 200 meters of the atmosphere, where wind velocities are typically less than 7 meters/sec when the sky is clear. In these conditions, simple tip-tilt adaptive optics should allow 2-meter telescopes to achieve diffraction-limited imagery at 2.4-5 µm over large fields of view (Wild et al. 1998).

Differential Image Motion Measurements

Two telescopes made HDIMM (Hartmann Differential Image Motion Measurement) observations at a wavelength of 500 nm during the winter of 1995-96. The measurement technique is similar to the differential image motion method described by Sarazin and Roddier (1990), but employs multiple apertures to increase the amount of data which can be accumulated during a given measurement period (Bally et al. 1996). The telescopes were the 60-cm SPIREX telescope (using a 48-aperture HDIMM mask) and a 28-cm Celestron telescope (using a 12-aperture HDIMM mask). The 28-cm telescope was attached to the side of the 60-cm telescope. They were mounted on a tower approximately 12 m above the snow surface. A set of observations consisted of measurements of five stars, Alpha Centauri, Beta Centauri, Sirius, Alpha Eridani, and Alpha Carina. Each set took about 75 minutes. There were 28 sets of observations during the course of the winter, comprising a total of 274 individual stellar measurements (Loewenstein et al. 1998). The differential-motion measurements yield a robust measure of the seeing. The two telescopes give similar values, with medians of 1.71 arcseconds for the 60-cm telescope and 1.64 arcseconds for the 28-cm telescope. Plotting the seeing data against wind speed and direction revealed no strong correlations. The HDIMM data can also be analyzed to obtain the common-mode motion of the images. The common-mode motion is comprised of a component due to common-mode seeing and a component due to vibrations of the structure supporting the telescope. These results set upper limits on the separate contributions of the two components.

The mean rms common-mode motion for the 60-cm telescope was 0.84 arcseconds. Power spectra of the motions show that most of the power is concentrated at frequencies below 5 Hz. The common-mode motion correlates weakly with wind speed, with higher values observed above 14.5 mph (the mean winter wind speed). There was no correlation with date of observation over the May-September duration of the experiment.

The figure below shows the accumulated probability distribution based on the individual measurements for each telescope. 50% of the time the seeing was better than 1.64 arcseconds in the C-11 and 1.71 arcseconds in SPIREX. The average seeing values of each telescope are 1.53 +/- 0.053 arcsec and 1.42 +/- 0.058 arcsec, respectively. The median seeing for each telescope is 1.64 arcseconds and 1.71 arcseconds, respectively.

Microthermal Experiments

Direct measurements with microthermal sensors on a 27-m tower (Marks et al. 1996) and during 15 balloon flights (Marks et al. 1997) yielded results which are consistent with the HDIMM observations. In addition, the data show where the turbulence responsible for the seeing is located. The mean value of the seeing calculated by integrating the balloon microthermal measurements from the surface to the maximum balloon altitude was 1.86 arcseconds. The data show that the turbulence causing the seeing is strongly concentrated in the lowest few hundred meters of the atomosphere, as expected from the altitude profiles of temperature and wind shear. The mean seeing above approximately 200 meters is 0.37 arcseconds. The balloon microthermal measurements were made and analyzed using techniques identical to those employed by Jean Vernin and his collaborators at a number of other astronomical sites, including Parenal in Chile. Above 200 meters, the Polar seeing is better than the seeing above Parenal. Microthermal observations from a 27-m tower (Marks et al. 1996) indicate that the contribution of the lowest 27 meters of the boundary layer to the total seeing is approximately 0.6 arcseconds.

Measurement Mean Std Dev Median Best 25% Best Worst
Seeing (arcsec): total 1.86±0.02 0.75 1.6 1.0 0.8±0.1 3.1±0.2
Seeing (arcsec): free atmosphere 0.37 0.07 0.32 0.29 0.23 0.52
r0 (cm): total 5.48±0.05 3.40 6.4 9.9 12.3 3.3
r0 (cm): free atmosphere 27.20 7.16 28.18 34.77 44.58 23.33
Boundary layer height (m) 220 70 190 165 120 275
Summary of integrated seeing and boundary layer data, from 16 balloon launches between 20 June and 18 August 1995. The "free atmosphere" refers to the entire atmosphere excluding the boundary layer. Values are quoted for a wavelength of 0.5 µm. For more information, see Marks et al. 1999.

Acoustic Backscatter Measurements

Acoustic backscatter (also called echosonde or SODAR) measurements also give information about the vertical distribution of turbulence. Measurements at the South Pole by Neff in 1993-94 (personal communication) confirm the result that most of the turbulence lies below an altitude of 200 m. During the summer of 1998-99, the University of New South Wales group installed an acoustic sounder in the AASTO (Automated Astrophysical Site Testing Observatory) at the South Pole. This instrument measures the altitude profile of turbulence, wind speed, and wind direction to a height of 300-600 m. It will continuously monitor the structure of the boundary-layer turbulence at the South Pole during the coming winter. After characterizing the seeing and sky brightness at the South Pole, the AASTO will be deployed to make similar measurements at other sites on the Antarctic Plateau.