McMath–Pierce solar telescope is a 1.6 m reflecting solar telescope at Kitt Peak National Observatory in Arizona, United States. It is the largest solar telescope and the largest unobstructed aperture telescope in the world. It is named for astronomers Robert McMath and Keith Pierce.
In 1955 a special advisory panel appointed by the National Science Foundation advocated construction of a solar telescope. During the Fall of 1957, Detroit engineer W. Zabiskie prepared detailed drawings of three possible structures for the solar telescope. In all three of these designs the telescope was an immense vertical triangle with its hypotenuse along the polar axis. The telescope therefore appeared much like the gnomon of an incredibly large sundial.
Designing the solar telescope was a difficult task. The structure supporting the heliostat mirror had to be so rigid that even when a 25-mile-an-hour wind slammed against it the image of the sun at the end of the 780 foot optical path would not deflect by more that 1/60th of an inch. Additionally, to avoid the thermal effects on the optical path, the air inside the structure would have to be maintained at a temperature equal to the air outside. Therefore, a design criterion was that all surfaces exposed to sunlight had to be temperature controlled. In the final design the heliostat sits atop a massive concrete cylinder 26 feet in diameter with steel reinforced walls which are 4 feet thick.