A mirror system that reflects light from a chosen part of the sky into a fixed instrument, such as a telescope, with no rotation of the field of view. A single mirror, driven to turn once in 48 hours, will do this for a single declination only. To observe objects at a wide range of declinations, another mirror is needed to feed the light from the driven mirror into the telescope. This second mirror’s position and tilt are altered to point at different declinations. Such an arrangement is useful with large or delicate apparatus which can therefore be mounted in a fixed position. See also heliostat; siderostat.