Preparation for aluminization

A New Coat for a Large Mirror — Ensuring a Highly Reflecting Surface

The final touch before aluminization. (Photo obtained on March 16, 2000).

Astronomical mirrors must be regularly coated in order to retain their ability to reflect light efficiently. ANTU and KUEYEN, the two first VLT Unit Telescopes to enter into operation did so while construction work was still ongoing at the top of Paranal. During this period, there was unsually much dust in the air, some of which was deposited on the large mirrors of these two telescopes. It was therefore decided to re-aluminize these mirrors when the work was over. This was done in February and March 2000. For this delicate operation, the mirror cell with the 22-tonnes, 8.2-m Zerodur mirror is removed from the telescope and wrapped in a protective cover. It is then moved out of the telescope enclosure and placed on a carriage that is hauled down the mountain to the Mirror Maintenance Building (MMB). Here the old aluminium layer is removed and the mirror is carefully washed, before it is placed in the Coating Tank. A new and clean aluminium layer is deposited by the sputtering technique. After careful checking, the mirror is brought back to the telescope and mounted. This photo was obtained at the time when KUEYEN's mirror was undergoing this process.