27 July 2016 Re-aluminising the primary mirror of the South African Astronomical Observatory's 74-inch telescope
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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.
© (2016) COPYRIGHT Society of Photo-Optical Instrumentation Engineers (SPIE). Downloading of the abstract is permitted for personal use only.
Lisa A. Crause, Lisa A. Crause, John Stoffels, John Stoffels, Willie Koorts, Willie Koorts, Brendt Christian, Brendt Christian, Wilhelmina de Water, Wilhelmina de Water, Timothy Fransman, Timothy Fransman, Denville Gibbons, Denville Gibbons, Nelson Machete, Nelson Machete, Ramotholo R. Sefako, Ramotholo R. Sefako, Sinethemba Taaibos, 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); doi: 10.1117/12.2232675; https://doi.org/10.1117/12.2232675


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