Twin Keck telescopes (left) and venerable Palomar dome (right)

The mounting of the mask at the secondary mirror

Taken duing 90mph (yes mph) winds at the summit of Mauna Kea..
This is a Lenticular or Wave Cloud formed by the insane windspeeds. These are a good indication to astronomers that it would be a good night to go for a few beers instead. For those keen on atmospheric phenomena: also see here the triangular mountain shadow cast by Mauna Kea, complete with Anti-Crepuscular rays from the sunset (below left from the cloud).