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Proceedings Paper

The use of primary mirrors as Hartmann masks for in situ alignment of segmented mirror telescopes
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Paper Abstract

Segmented primary mirrors dominate the current generation of 10m class telescopes as well as the designs for the next generation of Extremely Large Telescopes (ELT's). The complex nature of these telescopes is demonstrated by the long time periods associated with their commissioning and the difficulty of performing high precision optical alignments. However, additional tools to provide in situ measurements of their optical alignment can be provided by making use of the individual mirrors of a segmented primary; with the ability to move in six degrees of freedom, the individual mirrors can be deployed to trace multiple optical paths through the telescope. In this paper we describe how it is possible to use the segments themselves to create a number of Hartmann masks that allow focus and other aberrations to be measured using a standard imaging camera rather than a dedicated wavefront sensor. The Southern African Large Telescope (SALT), with a primary mirror composed of 91 1m segments, is used as an example. The segments were arranged to create eight Hartmann masks to measure the optical alignment. Through imaging data obtained at the telescope, the sensitivity of this method to changes in focus along with aberrations inherent in the system is demonstrated through Zernike polynomial fits to the observed patterns. Finally, we present simulations of possible patterns for use on future ELT's.

Paper Details

Date Published: 10 July 2008
PDF: 9 pages
Proc. SPIE 7012, Ground-based and Airborne Telescopes II, 70123P (10 July 2008); doi: 10.1117/12.788840
Show Author Affiliations
Steven M. Crawford, South African Astronomical Observatory (South Africa)
Martyn Wells, UK Astronomy Technology Ctr., Royal Observatory (United Kingdom)
Hitesh Gajjar, South African Astronomical Observatory (South Africa)

Published in SPIE Proceedings Vol. 7012:
Ground-based and Airborne Telescopes II
Larry M. Stepp; Roberto Gilmozzi, Editor(s)

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