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Proceedings Paper

Many segments and few dollars: SALT solutions for ELTs?
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Paper Abstract

The Southern African Large Telescope (SALT) is a little over 18 months away from completion (in early 2005). It is based on the innovative tilted-Arecibo optical analog, first pioneered by the Hobby-Eberly Telescope (HET). By the end of 2003, all major subsystems, including the verification instrument, will be in place and the commissioning of them begun. Tests of a 7-segment subset of the mirror array, including the Shack-Hartmann alignment instrument, the mirror actuators, capacitive edge sensors and active control system has recently started. The first engineering on-sky tests involving the complete light path, from object to detector, have begun. SALT's primary mirror consists of 91 identical segments mounted on a 9 point whiffle tree mount, using three actuators to control tip and tilt, and a foil-type capacitive edge sensor to detect mirror misalignment. These 480 relatively affordable sensors are permanently attached to the segment edges, and are capable of measuring all misalignment modes, including global radius of curvature. This sensing system, used together with a Shack-Hartman wavefront instrument at the center of curvature, controls the primary mirror array, and could be scaled to an array of the size envisaged for an ELT. SALT has developed some innovative designs improvement over the original HET concept. These include a more effective spherical aberration corrector (SAC), interferometric distance sensing and laser auto-collimation of the prime focus payload, the use of newly developed efficient and durable mirror coatings on the SAC optics, and the use of economical low expansion ceramics for the primary mirror segments. These innovative and cost effective solutions used on SALT have potential applications to ELT designs.

Paper Details

Date Published: 7 July 2004
PDF: 12 pages
Proc. SPIE 5382, Second Backaskog Workshop on Extremely Large Telescopes, (7 July 2004); doi: 10.1117/12.566263
Show Author Affiliations
David A. H. Buckley, Southern African Large Telescope Foundation (South Africa)
Jacobus G. Meiring, Southern African Large Telescope Foundation (South Africa)
Jian Swiegers, Southern African Large Telescope Foundation (South Africa)
Gerhard Pieter Swart, Southern African Large Telescope Foundation (South Africa)

Published in SPIE Proceedings Vol. 5382:
Second Backaskog Workshop on Extremely Large Telescopes
Arne L. Ardeberg; Torben Andersen, Editor(s)

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