COELOSTAT, a mirror driven by clock-work so as to reflect continually the same region of the sky into the field of view of a fixed telescope. The mirror is mounted so as to rotate about an axis in its own plane which points to the pole of the heavens, and is driven at the rate of one revolution in 48 (sidereal) hours. The image of a star seen in the mirror is then stationary. The device is particularly useful in eclipse expeditions when elaborate equatorial mounting of the telescopes is impossible. Other in struments for somewhat similar purposes are the heliostat and siderostat (q.v.).