Hobby-Eberly telescope. Astronomer with the primary mirror of the Hobby-Eberly Telescope (HET). Commissioned in late 1997, the HET has the world's largest primary mirror. The 11-metre mirror consists of 91 identical 1-metre hexagonal segments. The use of a segmented mirror greatly reduces construction costs. Unlike most telescopes, the HET tracks objects across the sky not by rotating the entire telescope but by moving just a few focusing instruments suspended above the primary mirror. This reduces by more than ten times the amount of mass that must be moved under precise control. Photographed at the McDonald Observatory in the Davis mountains of Texas, USA.