Richard B. Dunn Solar Telescope

Lore & Background

The telescope was inaugurated in 1969 as the world's premier high spatial resolution optical solar telescope. Its design was started by architect and engineer Charles W. Jones in 1963, with construction from 1966 to 1967 under the U.S. Army Corps of Engineers at a cost of about $3 million. The architectural firm was Roghlin and Baran, Associates. Richard B. Dunn, for whom the instrument was eventually dedicated, wrote an article in Sky & Telescope about its completion in 1969, describing how the design eliminated the dome and evacuated the entire telescope inside the tower to reduce local seeing problems. The tower telescope was originally dedicated on October 15, 1969, and renamed in 1998 after Richard B. Dunn.

The whole building from top to bottom is a single instrument, with more than half underground. Enclosed within the concrete tower is a vertical vacuum tube with 3-foot-thick walls. The optical path starts at a heliostat on top of the tower, with an entrance window and two mirrors reflecting sunlight down the vacuum tube to the 64-inch primary mirror 193 feet underground. The primary mirror focuses light and reflects it back up, exiting through six quartz optical windows in the floor of an optical laboratory at ground level. The interior vacuum tube is suspended from the top of the tower by a mercury float bearing containing 10 tons of mercury, allowing the entire tube to rotate to compensate for the apparent rotation of the Sun's image.

In February 2026, the U.S. National Science Foundation announced that the telescope would be dismantled and the Sacramento Peak site restored following the discovery of a liquid mercury leak in the facility and subsequent safety and environmental concerns.

Reader's Guide

The Dunn Solar Telescope helped astrophysicists worldwide better understand the Sun and how it affects Earth. Its observations allowed solar astronomers to investigate a range of solar activities, often in concert with satellites or rocket launches, and to develop new technologies for the 4-meter Daniel K. Inouye Solar Telescope. The telescope's high-order adaptive optics, installed in 2004, compensated for blurring caused by Earth's atmosphere. Its rotating optical bench could be configured to multiple observing setups, supporting instruments such as the Facility Infrared Spectropolarimeter (FIRS), the Spectro-Polarimeter for Infrared and Optical Regions (SPINOR), the Interferometric Bidimensional Spectro-polarimeter (IBIS), and the Rapid Oscillations in the Solar Atmosphere (ROSA) instrument. Scientific work at the telescope included inferring telescope polarization properties through spectral lines, adapting the telescope for Jovian Doppler spectro imaging, deriving solar coronal magnetic fields using seismology techniques applied to sunspot waves, and developing solar multi-conjugate adaptive optics and speckle interferometry. The telescope's legacy includes its role as a test platform for new instrumentation and its contributions to high-resolution solar physics until its planned dismantling in 2026 due to a mercury leak.

Did You Know?

• The lowest excavated point of the telescope is 228 feet (69 m) below ground, while the tower extends 136 feet (41 m) above ground.
• The telescope was upgraded with high-order adaptive optics in 2004.
• In February 2026, the U.S. National Science Foundation announced the telescope would be dismantled due to a liquid mercury leak and subsequent safety and environmental concerns.