#Y2021 #天文光学 A heliostat (from helios, the Greek word for sun, and stat, as in stationary) is a device that includes a mirror, usually a plane mirror, which turns so as to keep reflecting sunlight toward a predetermined target, compensating for the Sun's apparent motions in the sky. [[Coelostat定天镜]] Coelostat, device consisting of a flat mirror that is turned slowly by a motor to reflect a portion of the sky continuously into a fixed telescope.