Astronomical Seeing at the South Pole

R. F. Loewenstein, Chris Bero(*), James P. Lloyd(**), Fred Mrozek
University of Chicago, Yerkes Observatory, 373 W. Geneva Street, Williams Bay, WI 53191
John Bally and David Theil
Astrophysical, Planetary, and Atmospheric Sciences Department University of Colorado, Boulder, CO 80909

(*) also Wyoming Infrared Observatory, University of Wyoming
(**) also Dept. of Astronomy, University of California, Berkeley

Abstract

A Hartmann differential image motion monitor (H-DIMM) was used on two different telescopes situated at the South Pole to measure astronomical seeing. A series of observations conducted 12 m above the surface during two Austral winters indicates that the median ``ground level'' seeing at 500 nm was ~ 1.7 arcseconds. This is in good agreement with microthermal measurements from balloon flights which indicate that the seeing is dominated by a temperature inversion which appears within the first 220 m above the surface during the winter. Above this layer, the mean ``free atmosphere'' seeing is 0.37±0.07 arcseconds.

1  Introduction

The Center for Astrophysical Research in Antarctica (CARA) began various astronomical site characterizations of the South Pole in 1992. One of the important measurements was the astronomical seeing. To do this, we implemented a differential image motion monitor, as described by Sarazin and Roddier (1990), but modified the design by eliminating the prism, adding a Hartmann aperture mask, and defocusing the telescope to image each sub-aperture of the mask. We call this arrangement a Hartmann differential motion monitor (H-DIMM), and described the technique in Bally et al. (1996). With a mask of n apertures, the number of baselines is given as 1/2n(n-1), permitting a highly redundant calculation of the seeing.

During the Austral winter of 1994-95 we operated a 48 aperture H-DIMM on a 60 cm f/10 telescope (SPIREX) situated on a 12 m tower upwind of the observatory building. The following winter we mounted a Celestron 28 cm f/10 telescope (C-11) equipped with a 12 aperture H-DIMM mask onto the SPIREX tube so that both telescopes shared a common mount.

The detector was a Panasonic CCTV camera, capable of good signal-to-noise imaging of 2nd magnitude stars through the 3.8 cm sub-apertures at 30 Hz resolution. Image sequences were captured on video tape, as well as frame grabbed in real time for analysis.

In addition to these observations, CARA sponsored South Pole microthermal measurements from a 27 m tower (Marks et al. 1996) and balloons (Marks et al. 1997).

2  Observations

At winter temperatures, which can reach below -80° C, radiative cooling of optical surfaces often results in a thin layer of ice coating the surfaces. Blowing snow can also adhere exposed surfaces. In order to prevent these accumulations from happening, the surfaces are kept at a few degrees above ambient by the use of heaters. Additionally, any electronics packages on the telescope must be heated to temperatures closer to 0° C. Our first season's SPIREX data was at times contaminated by turbulence within the SPIREX tube as a result of some of these heaters. We experimented with ways of managing these heaters so that the internal temperatures were near ambient for the observations. After all observations, the heaters could be turned back on to keep the surfaces snow free. Typical science observations with SPIREX now use this type of heat management, as well as the use of fans to occasionally blow off any light snow accumulation on the optical components during long periods of observing.

The C-11 telescope has a corrector plate so that its tube, unlike the SPIREX tube, is basically sealed. Fewer heaters were necessary for the C-11, so the telescope helped serve as a verification of the proper heat management. Because the heaters were properly managed for the entire 1995-96 winter, we restrict the discussion to these data.

The telescopes were mounted on a 12 m tower for several reasons: to minimize blowing surface snow accumulation on the equipment, to be isolated from the laboratory building, and to eliminate as much of the surface seeing as possible. Results from tower microthermal measurements (Marks, et al. 1996) indicated that the first 27 m above the surface contributes an average of 0.64 arcseconds to the total seeing.

A set of observations consisted of measurements of five stars with each telescope: Alpha Centauri, Beta Centauri, Sirius, Alpha Eridani, and Alpha Carinae. One set would typically last about 75 minutes. There were 28 such observations made during the second winter season, with a total of 274 individual measurements.

3  Discussion

The data are summarized in Figure 1, which shows the seeing statistics from each telescope observation set. There is very good agreement between the two telescopes, perhaps better illustrated in Figure 2 which 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 and 1.42±0.058 respectively.


Figure 1: Seeing Summary. Daily statistics for each observations set. No observations were done during the period between 22 June and 20 July because of other demands on the telescope.


Figure 2: Cumulative Seeing Probability. Data is based on 274 individual measurements for each telescope. The median seeing for the C-11 is 1.64 arcseconds and 1.71 arcseconds for SPIREX


Figure 3: Seeing versus wind speed.


Figure 4: Common mode motion versus wind speed.

During the 1994-95 season, we coordinated observations to coincide with seven microthermal balloon flights. The results of the 15 balloon flights as well as the simultaneous H-DIMM measurements are discussed in Marks et al. (1997), which show reasonably good correlation given the local turbulence within SPIREX caused by the improper heating mentioned above. The mean integrated seeing, averaged over the 15 balloon flights made during that season, was 1.86 arcseconds. However, because of a very low altitude temperature inversion which forms during the winter months and is unique to the Antarctic Plateau, a very turbulent boundary layer between the surface and about 220 m exists. This layer is responsible for practically all of the ``ground level'' seeing measured near the surface. Above this layer, the balloon measurements show an integrated seeing of only 0.37 arcseconds for ``free atmosphere'' seeing.

Anemometer output was stored with the H-DIMM data so that we could explore any correlation to local winds. The data plotted in Figure 3 show that the seeing is not really affected by the winds under 25 mph; however, there is a hint the seeing may be slightly better at higher wind speeds. The cluster of data between 10 and 20 mph reflects the average winter wind speed of about 15 mph.

A byproduct of the H-DIMM technique is the measurement of common mode motion and Figure 4 shows the relationship of common mode motion versus wind speed. We interpret the elevated common mode motion at wind speeds > 14.5 mph to be predominately caused by wind driven tower shake. The mean RMS motion for SPIREX was 0.84 arcseconds. Because the C-11 has an additional mounting structure, we expect its motion to be slightly more than SPIREX.


Click for readable image
Figure 5: SPIREX Radial Common Mode Motion Power Spectrum. A spectrum of the common mode motion results was generated for each of 132 SPIREX observations. The spectra were averaged together in two different wind speed bins ( < 14.5 mph and >14.5 mph) to show the effect of wind on the tower.


Click for readable image
Figure 6: C-11 Radial Common Mode Motion Power Spectrum. A spectrum of the common mode motion results was generated for each of 130 C-11 observations.

The averaged power spectra of the common mode motion for each telescope are plotted in Figure 5 and Figure6. The increased power in the C-11 spectrum over the SPIREX spectrum at high wind speeds is probably caused by the flexure of the extra mounting hardware for the C-11.

In conclusion, the median ``ground level'' seeing at 500 nm during the 1995-96 winter season at the South Pole was ~ 1.7 arsceconds, as measured by H-DIMM technique using two different telescopes on a 12 m tower. This value is consistent with measurements from microthermal probes mounted on a tower and carried on balloon flights, which also indicate that an inversion layer is responsible for most of the turbulence creating the total seeing, and that this layer is within 220 m of the surface. Above this layer, the mean ``free atmosphere'' seeing is 0.37 ± 0.07 arcseconds. Two obvious methods of obtaining excellent seeing is to observe from a platform as high as possible over the surface and to use a tip/tilt mirror system to improve the image quality.

This research was supported by the National Science Foundation under a cooperative agreement with the Center for Astrophysical Research in Antarctica (CARA), grant number NSF OPP 89-20223. We wish to thank Al Harper and Bob Pernic of CARA and the people of Antarctic Support Associates who helped us in this experiment.

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On 9 Apr 1999, 13:48.