1) Optical device 
device consisting of a flat mirror that is turned slowly by a motor to reflect the Sun continuously into a fixed telescope. The mirror is mounted to ... [1 related articles]
The coelstat at Mt. Wilson Observatory, California A device that uses two movable mirrors to reflect sunlight along a particular path – for instance, down a fixed telescope tube – so that the image at the other end of the telescope doesn't appear to move or rotate when the Sun moves...
[n] - optical device used to follow the path of a celestial body and reflect its light into a telescope
noun optical device used to follow the path of a celestial body and reflect its light into a telescope; has a movable and a fixed mirror
an apparatus consisting of a mirror driven by clockwork, used to enable a fixed telescope to keep the same area of sky in its field of view by compensating for the apparent rotation of the celestial sphere. Cf. siderostat.