Dunn Solar Telescope, solar telescope in Sunspot, New Mexico, United States

The Richard B. Dunn Solar Telescope is a vertical-axis solar telescope at Sacramento Peak in Sunspot, New Mexico. Its concrete tower rises 136 feet (41 m) above ground, while the lowest excavated point lies 228 feet (69 m) below. Inside is a vertical vacuum tube with 3-foot-thick walls; sunlight enters through a heliostat and window at the top, is reflected down to the 64-inch primary mirror 193 feet (59 m) underground, and returns to six quartz windows in the floor of the ground-level optical laboratory.

The facility was built in 1969 as the Vacuum Tower Telescope and renamed in 1998 to honor solar astronomer Richard B. Dunn. This recognition celebrated his important contributions to studying the sun and advancing solar physics research.

Scientists and researchers from multiple universities gather at this facility to study solar phenomena through advanced instruments and collaborative projects.

The telescope is reached via Sunspot, New Mexico, on Sacramento Peak. The tower rises 136 feet (41 m) above ground while the lowest excavated point, the bottom of the sump, lies 228 feet (69 m) below it, so most of the instrument is underground. The 250-ton vacuum tube hangs from a mercury-float bearing holding 10 tons of mercury, itself suspended on three bolts only 76 mm across. Much of the telescope is run from a single control room beside the main observing table.

The 250-ton vacuum tube hangs from a mercury-float bearing holding 10 tons of mercury, itself suspended on three bolts just 76 mm across. This bearing rotates the whole tube to counter the apparent rotation of the solar image as the Sun rises. In February 2026 the NSF announced the telescope would be dismantled after a liquid mercury leak was found.