The Blanco 4-m telescope primary mirror, cleaned of its old coating of aluminum prior to installation in the vacuum chamber used to apply a new coating. The 15-ton mirror is made of a glass called Cer-Vit, which has a very low coefficient of thermal expansion (and is no longer available). On Wednesday 30th March, we applied a very high quality, 100nm thick layer of pure aluminum. In this condition, the mirror is more reflective than it has been in over a decade and will provide a good start to the forthcoming Dark Energy Survey.
Links
[1] http://www.ctio.noao.edu/noao/content/Blanco-4-m-Telescope-Primary-Mirror-aluminization
[2] http://www.ctio.noao.edu/noao/sites/default/files/field/image/Clean mirror.jpg