Physics – Optics
Scientific paper
Dec 2005
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2005phdt........12k&link_type=abstract
PhD Thesis, Combined Faculties for the Natural Sciences and for Mathematics of the University of Heidelberg, Germany. VIII+129 p
Physics
Optics
4
Adaptive Optics
Scientific paper
Since more than 15 years, Adaptive Optics (AO) is a proven concept to reach diffraction limited imaging at modern astronomical telescopes. In the case of next generation telescopes (Extremely Large Telescopes (ELTs)) with aperture diameters of up to 100m, sodium laser guide star based multi-conjugated-AO systems will be a basic requirement to exploit their full capability in terms of resolution and light concentration. A drawback of such an approach emerges in the finite distance and vertical extent of the sodium beacon in the mesosphere with respect to the telescope. This induces effects such as perspective elongation, where conventional wavefront sensing mechanisms will fail. Although several engineering concepts are under development to counteract these constraints at the cost of overall light efficiency and increased system complexity, this thesis proposes a novel kind of wavefront sensing technique to overcome the imposed limitations in a more natural way. The sensing technique is composed of two independently working sensors, a reflective rod and a mask with circular slits, each a representative of a novel wavefront sensor class, the so called z-invariant and Inverse Bessel Beam technique. Both are discussed in this thesis with a focus on the Inverse Bessel Beam technique. The latter is compared to alternative concepts such as temporal gating, with respect to the photon efficiency. Furthermore, the reflective rod was tested for its feasibility in laboratory conditions and in a more realistic environment at the William Herschel Telescope (WHT) at La Palma. With this test run its sensing principle has been verified. A novel technique already intensively used at 8m class telescopes is Simultaneous Differential Imaging. The direct detection of giant extra-solar planets is and will be a major science driver for galactic astronomy in the coming years. Modern telescope facilities such as the VLT reach, by means of adaptive optics, potentially the capability in terms of resolution and contrast required to detect planetary companions orbiting their host star. The recently implemented Simultaneous Differential Imaging upgrade for the near-infrared camera system NAOS-CONICA (NACO), is optimized in tracing the methane absorption feature of giant extra-solar planets. In the framework of this thesis a pipeline was developed to reduce NACO-SDI data. It was calibrated against the most promising candidate (Epsilon Eridani) hosting planetary companions by evaluating the deepest high contrast image data ever taken of this source.
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