4 February 2003 Demonstrating the suitability of Sitall for the SALT primary mirror
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Abstract
Methods to limit image degradation due to temperature variations in telescopes include reducing the coefficient of thermal expansion (CTE) of the optical elements. In segmented mirror telescopes, not only does the average CTE have to be as close as possible to zero, but, more importantly, a high level of homogeneity is essential to avoid temperature related changes to mirror figure. The Southern African Large Telescope (SALT) has selected Sitall optical glass-ceramic from the Russian company LZOS for the manufacturing of its 91 primary mirror segments. A detailed specification, including strict requirements for CTE, was developed from basic principles. LZOS, together with the Mendeleev Metrological Institute in St. Petersburg, have developed CTE measuring equipment (dilatometers) to demonstrate that individual segments meet specification. A theoretical analysis of the allowable measurement uncertainty was conducted which accounted for the inaccuracy of the dilatometers, resulting from uncertainties due to sample length, fringe fraction reading and laser wavelength instability. Developments include operating in a dynamical temperature mode to reduce testing time and utilizing computer controlled units to read and process interferograms. Fizeau and heterodyne interferometric methods were implemented in separate dilatometers. CTE measurements of the first batch of SALT segments demonstrate that the material complies with the SALT specification. These results are presented in this paper.
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Oleg V. Ponin, Oleg V. Ponin, Alexander A. Sharov, Alexander A. Sharov, Igor Galyavov, Igor Galyavov, Tatyana A. Kompan, Tatyana A. Kompan, Jian Swiegers, Jian Swiegers, Arek Swat, Arek Swat, } "Demonstrating the suitability of Sitall for the SALT primary mirror", Proc. SPIE 4837, Large Ground-based Telescopes, (4 February 2003); doi: 10.1117/12.457965; https://doi.org/10.1117/12.457965
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