Re-aluminising the primary mirror of the South African Astronomical Observatory's 74-inch telescope

Lisa A. Crause; John Stoffels; Willie Koorts; Brendt Christian; Wilhelmina de Water; Timothy Fransman; Denville Gibbons; Nelson Machete; Ramotholo R. Sefako; Sinethemba Taaibos

doi:10.1117/12.2232675

27 July 2016 Re-aluminising the primary mirror of the South African Astronomical Observatory's 74-inch telescope

Lisa A. Crause, John Stoffels, Willie Koorts, Brendt Christian, Wilhelmina de Water, Timothy Fransman, Denville Gibbons, Nelson Machete, Ramotholo R. Sefako, Sinethemba Taaibos

Author Affiliations +

Proceedings Volume 9906, Ground-based and Airborne Telescopes VI; 99063W (2016) https://doi.org/10.1117/12.2232675
Event: SPIE Astronomical Telescopes + Instrumentation, 2016, Edinburgh, United Kingdom

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.

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Lisa A. Crause, John Stoffels, Willie Koorts, Brendt Christian, Wilhelmina de Water, Timothy Fransman, Denville Gibbons, Nelson Machete, Ramotholo R. Sefako, and Sinethemba Taaibos "Re-aluminising the primary mirror of the South African Astronomical Observatory's 74-inch telescope", Proc. SPIE 9906, Ground-based and Airborne Telescopes VI, 99063W (27 July 2016); https://doi.org/10.1117/12.2232675

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KEYWORDS

Mirrors

Telescopes

Coating

Reflectivity

Aluminum

Ocean optics

Observatories

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