The heliostat (A), an elaborate version of the coelostat. The one shown here is installed in the 152-m (500 ft.) telescope at Kitt Peak, Arizona (B), to track the Sun. Sunlight falls on a rotating mirror (1). It reflects down a tube to a concave mirror (2), focuses on to a plane mirror (3), and passes through a spectrograph (4).
A heliostat is a movable flat mirror used to reflect sunlight into a stationary solar telescope. If the telescope is aligned with Earth's axis, only one mirror is needed. A heliostat is similar to a coelostat, but has a simpler design and (unlike the coelostat) makes the image of the Sun rotate as the Sun moves along the sky.