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February 3, 2023



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The tertiary mirror


We have already mentioned the Gran Telescopio Canarias’ (GTC) tertiary mirror, especially its capability to level the weight automatically, allowing for the movement of the mirror. Its function is to "invade" the path of the light, in order to redirect it to the different foci, in our case the 4 folded Cassegrains and the 2 Nasmyths.

The automatic scaling allows for it to automatically slide on the guides, getting in the way of the light and redirecting it when necessary. The rest of the time, if its use is not precise, it remains “parked.”

The mirror is inside of a tower with a 1,8 m diameter and a height of 7 m, which moves rotating about its structure and directs the mirror towards the chosen focus.

The mirror is elliptical, flat and very thin, with a width of only 70 mm, manufactured in Zerodur™ with measurements 1521x1073 cm. It is supported by a tree of levers with 18 fastening points.

The company hired to supply this mirror and the elements associated with its mount was the Belgian company AMOS which hired ZEISS to make the mirror. SHOTT was the supplier of the block of Zerodur™ and the Russian company LZOS carried out the polishing process of the mirror.

LZOS also tests the GTC’s tertiary mirror. In order to carry out measurements related to the mirror’s flat surface, a vertical test bench was created, allowing for very precise surface measurements.

For the polishing process, the mirror has been introduced in a “postizo of Sitall CO-115M (Astrositall)”, a type of “vitrocerámica” manufactured by LZOS since 1959, with a very low thermal expansion coefficient similar to Zerodur™ (material used by Schott). This facilitates the polishing of the edges, which is usually the most complicated part of the process.

Natalia R. Zelman

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