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10 July 2008 The use of primary mirrors as Hartmann masks for in situ alignment of segmented mirror telescopes
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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.
© (2008) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Steven M. Crawford, Martyn Wells, and Hitesh Gajjar "The use of primary mirrors as Hartmann masks for in situ alignment of segmented mirror telescopes", Proc. SPIE 7012, Ground-based and Airborne Telescopes II, 70123P (10 July 2008); doi: 10.1117/12.788840;


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