A large solar telescope in which the optics are mounted on a high tower to avoid air turbulence near the ground. Sunlight is directed vertically downwards by a movable flat mirror termed a heliostat into an observing room, usually sited underground. Examples are the two towers at Mount Wilson in California. Tower telescopes can be cooled to reduce convection currents. In a vacuum solar tower the tower is evacuated to prevent air turbulence, although practical considerations limit these to maximum apertures of about 1 m.