Fields of heliostat mirrors focus sunlight on receivers located on centralized solar power towers. Each heliostat tracks the Sun along two axes (azimuth and elevation). One heliostat mirror is a reflecting surface, giving a total of per heliostat. The model HE54 heliostat has 54 mirrors with a total reflective surface of . There was a heliostat mirror and a meridian room for a transit instrument. The designer described the Solar Beacon as a "heliostat", not a "heliograph". Many other types of heliostat have also occasionally been used. This conventional heliostat is often referred to as a glass/metal heliostat. The Mangin apparatus with heliostat was still in service in 1917. Another type of heliograph was the heliostat fitted with a Colomb shutter. See article "Heliostat" for more information on solar mirrors used for terrestrial energy. The heliostat can be set to capture moonlight at night. A tower resides in the center of the heliostat field. The term "heliostat" is sometimes used as a synonym for "heliograph" because of this origin.