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Proceedings Paper

Re-aluminising the primary mirror of the South African Astronomical Observatory's 74-inch telescope
Author(s): Lisa A. Crause; John Stoffels; Willie Koorts; Brendt Christian; Wilhelmina de Water; Timothy Fransman; Denville Gibbons; Nelson Machete; Ramotholo R. Sefako; Sinethemba Taaibos
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Paper Abstract

Telescope mirrors reside in harsh environments and thus require periodic re-aluminisation to maintain their reflectivity. The SAAO’s Sutherland field station suffers from dust and frequent bouts of high humidity. Dust settling on the mirrors adheres to the upward-facing optical surfaces and is not removed by CO2 cleaning. The 74-inch primary mirror was unsuccessfully re-aluminised in April 2015. Parts of the mirror proved difficult to clean and the resulting coating included hazy, white patches in those problem areas. Cotton wool soaked with ferric chloride was used to strip small patches of coating, confirming that no optical surface damage had occurred. The 55 year-old aluminising equipment for the 74-inch required an extensive overhaul and the spruced up system was then used to re-coat the primary mirror in November 2015. We used the same de-ionised water, potassium hydroxide, sodium lauryl sulphate, cotton wool, safety gear and cleaning techniques employed by the mirror coating team at the neighbouring Southern African Large Telescope, as well as their Ocean Optics reflectometer to quantify the improvement in reflectivity. Measurements at 320 nm on different parts of the dirty primary ranged between 10 % and 70 %, while the new coating exceeded 95 % over the entire surface.

Paper Details

Date Published: 27 July 2016
PDF: 9 pages
Proc. SPIE 9906, Ground-based and Airborne Telescopes VI, 99063W (27 July 2016); doi: 10.1117/12.2232675
Show Author Affiliations
Lisa A. Crause, South African Astronomical Observatory (South Africa)
John Stoffels, South African Astronomical Observatory (South Africa)
Willie Koorts, South African Astronomical Observatory (South Africa)
Brendt Christian, South African Astronomical Observatory (South Africa)
Wilhelmina de Water, SALT Foundation (South Africa)
Timothy Fransman, SALT Foundation (South Africa)
Denville Gibbons, SALT Foundation (South Africa)
Nelson Machete, South African Astronomical Observatory (South Africa)
Ramotholo R. Sefako, South African Astronomical Observatory (South Africa)
Sinethemba Taaibos, South African Astronomical Observatory (South Africa)

Published in SPIE Proceedings Vol. 9906:
Ground-based and Airborne Telescopes VI
Helen J. Hall; Roberto Gilmozzi; Heather K. Marshall, Editor(s)

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