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 136-foot-tall (41 m) 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.