The world’s largest solar telescope, part of the National Solar Observatory, opened in 1962 at an altitude of 2100 m on Kitt Peak, Arizona. The main telescope is housed in a diagonal shaft, 152 m long, aligned parallel with the Earth’s axis. A heliostat on a tower 30.5 m high reflects sunlight down this shaft to a 1.6-m mirror, some 50 m below ground level. Rays are bounced from this mirror part-way up the shaft again to a flat mirror, which diverts the light vertically downwards to form a solar image 0.8 m wide in an observation room below ground. It is named after the American solar physicists Robert Raynolds McMath (1891–1962) and A(ustin) Keith Pierce (1918–2005).