Report - School of Physics
Report - School of Physics
Report - School of Physics
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during March and October, although with an extended ‘twilight’ period due to the<br />
fact that the Sun does not set very far below the horizon). Table 8 shows that<br />
an Antarctic OWL might achieve comparable performances to Darwin/TPF in the<br />
near-infrared; currently this is viewed as somewhat hypothetical, given the complex<br />
logistics and formidable meteorological conditions for construction and operation.<br />
Dome C (elevation 3250 m, latitude −75 ◦ ) is the site <strong>of</strong> the French-Italian Concordia<br />
station, whose main characteristics have been studied over the last few years. It provides:<br />
(i) an ambient temperature ranging from 195 K (winter) to 235 K (summer),<br />
resulting in low, stable thermal emission; (ii) extremely dry air (250 µm precipitable<br />
water vapour typical), resulting in enlarged and improved infrared transmission<br />
windows; (iii) very low surface winds (median wind speed <strong>of</strong> 2.7 m s −1 , and below<br />
5 m s −1 for more than 90% <strong>of</strong> the time), resulting in very low free-air turbulence,<br />
with a quasi-absence <strong>of</strong> jet streams since Dome C is located inside the polar vortex;<br />
(iv) some 80% <strong>of</strong> clear skies. The combination <strong>of</strong> coldness and dryness for the atmosphere<br />
results in infrared photometric gains that peak at about a factor <strong>of</strong> 25 in<br />
the K and L bands, i.e. an Antarctic 1.8-m telescope is more efficient than an 8-m<br />
telescope at a temperate site. In the H and N bands the gain (<strong>of</strong> order 3) is also<br />
notable although less dramatic.<br />
Exceptional winter conditions were confirmed by teams from the University <strong>of</strong> Nice<br />
(Aristidi et al., 2003) and Australia UNSW (Lawrence et al., 2004): over a 3-month<br />
period, the median seeing was 0.27 arcsec, with 0.15 arcsec achieved for more than<br />
25% <strong>of</strong> the time. As most <strong>of</strong> the turbulence is located near the ground, the isoplanatic<br />
angle is enlarged (5.9 arcsec under normalised conditions, compared with<br />
2.9 arcsec for Paranal) which improves the field <strong>of</strong> view (and the likelihood <strong>of</strong> finding<br />
a reference star) for adaptive optics. This has consequences in the feasibility <strong>of</strong><br />
adaptive optics for ELTs, where multiconjugate systems and laser guide stars may<br />
no longer be needed. Because the turbulence is generated by low-velocity winds, it is<br />
slow, meaning better sensitivity and improved phase correction for adaptive optics<br />
systems. There is still some debate about the normalised coherence time (whose<br />
median value is 2.6 ms on Paranal): indirect measurements with the MASS scintillometer<br />
indicate a median value <strong>of</strong> 5.9 ms, while direct DIMM measurements show<br />
a correlation <strong>of</strong> the image motions beyond 250 ms (as expected from r 0 = 50 cm<br />
and a typical 2.5 m s −1 wind speed).<br />
Even taking the most pessimistic value (5.9 ms), the coherence volume that drives<br />
the sensitivity <strong>of</strong> adaptive optics and interferometric systems is improved by a typical<br />
factor <strong>of</strong> 20, if the AO wavefront sensor operates in the K-band. When combined<br />
with the factor <strong>of</strong> 25 photometric gain, this results in an expected sensitivity for interferometers<br />
and AO systems 500 times better than a similarly equipped temperate<br />
site. For bright sources, higher dynamic range observations (either by coronography<br />
or nulling) can be achieved due to the reduced phase and background noise.<br />
Current adaptive optics mainly concerns the correction <strong>of</strong> the phase <strong>of</strong> the wavefront<br />
to achieve diffraction-limited imaging. Even after a perfectly plane wavefront has<br />
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