Adaptive-optics performance of Antarctic telescopes

Abstract

The performance of natural guide star adaptive-optics systems for telescopes located on the Antarctic plateau is evaluated and compared with adaptive-optics systems operated with the characteristic mid-latitude atmosphere found at Mauna Kea. A 2-m telescope with tip-tilt correction and an 8-m telescope equipped with a high-order adaptive-optics system are considered. Because of the large isoplanatic angle of the South Pole atmosphere, the anisoplanatic error associated with an adaptive-optics correction is negligible, and the achievable resolution is determined only by the fitting error associated with the number of corrected wave-front modes, which depends on the number of actuators on the deformable mirror. The usable field of view of an adaptive-optics equipped Antarctic telescope is thus orders of magnitude larger than for a similar telescope located at a mid-latitude site; this large field of view obviates the necessity for multiconjugate adaptive-optics systems that use multiple laser guide stars. These results, combined with the low infrared sky backgrounds, indicate that the Antarctic plateau is the best site on Earth at which to perform high-resolution imaging with large telescopes, either over large fields of view or with appreciable sky coverage. Preliminary site-testing results obtained recently from the Dome Concordia station indicate that this site is far superior to even the South Pole.

Site testing in summer at Dome C, Antarctica

“…This corresponds to a gain of a factor 10 in the field used to find calibrator stars, which therefore increases the observable piece of sky, as discussed by Coudé du Foresto et al (2004). Another advantage of a large isoplanatic domain is the uselessness of multi-conjugate adaptive optics for high-resolution wide field imaging (Lawrence 2004). …”

Astronomy from Antarctica

“…That fraction goes as the seeing to the -6.5th power! Clearly, with median seeing between two and three times better than the best existing temperate sites, Dome C offers a quite extraordinary opportunity for astronomers (Lawrence 2004b).…”

Science Programs for a 2-m Class Telescope at Dome C, Antarctica: PILOT, the Pathfinder for an International Large Optical Telescope

“…This, combined with a lack of high altitude winds, results in significant improvements for the performance of any wavefront correction system compared to a typical midlatitude site. Based on the measurements of the seeing and turbulence profile at Dome C, as measured with a SODAR and MASS , we have calculated in Figure 5 what the Strehl ratio (the ratio of the peak flux to that obtained with diffraction-limited performance) would be for a tip-tilt system on the PILOT telescope (see Lawrence 2004b for further details). and an 8-m telescope (dotted lines) at both Dome C and Mauna Kea, as a function of wavelength.…”

“…The performance of an AO system is a function of many parameters, including the number of actuators in use, the size of the telescope, the brightness of the guide star being used for correction, and its angular distance from the object under study, in addition to the site seeing and isoplanatic angle. In Figure 6 we compare the Strehl ratio obtainable with an on-axis AO system with 45 actuators on a 2-m telescope at Dome C to that of the same-sized telescope on Mauna Kea (see Lawrence 2004b for details). The Strehl ratio is shown as a function of the magnitude of the star being used for the correction; i.e.…”