The Richard B. Dunn Solar Telescope is a unique vertical-axis solar telescope, located at the National Solar Observatory at Sacramento Peak, New Mexico. The optical path starts at a heliostat on top of a tower and continues 193 feet (58.8 m) more underground to the primary mirror. It then returns to one of six quartz optical windows in the floor of an optical laboratory at ground level. The optics are evacuated to eliminate distortion due to convection in the telescope that would otherwise be caused by the great heat produced by focusing the light of the sun.
A unique feature of the telescope is its approach to image derotation: the entire telescope and optics lab, 250 tons total, rotates suspended from a mercury float bearing at the top of the tower.
Originally the Vacuum Tower Telescope at Sacramento Peak, it was renamed in 1998 in honor of the retiring solar astronomer Richard B. Dunn who was the driving force behind its construction. The lowest excavated point (the bottom of the sump) is 228 feet (69.5 m) below ground.