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coelostat(see -lŏ-stat) A flat mirror that can be driven by a clock mechanism so as to rotate from east to west about an axis parallel to the Earth's rotational axis, thereby compensating for the west-to-east rotation of the Earth. The mirror may thus continuously reflect light from the same area of the sky into the field of view of an instrument that is fixed in position, usually by means of an additional optical system. See also heliostat; siderostat; solar telescope.
an auxiliary astronomical instrument with a flat rotating mirror that makes it possible to observe celestial bodies moving as a result of the apparent diurnal rotation of the celestial sphere by means of stationary instruments, such as horizontal and solar tower telescopes.
A flat mirror M (see Figure 1) is fixed to the OO axis, which is parallel to the plane of the mirror and to the axis of the earth. The coelostat’s axis is rotated by a clockwork mechanism C at a rate of one rotation per 48 hours (solar or stellar time, depending on whether the sun or stars are being observed). By virtue of the design, the normal to the mirror slides along the celestial equator, while the direction of the reflected ray of a celestial body remains fixed. By turning the mirror about its axis, the reflected ray of the celestial object with declination δ can be directed at any point of the parallel with declination – δ. The horizontal direction, which is different for bodies of different declinations, proves to be most convenient. Introducing an additional fixed flat mirror makes it possible to direct in any direction a ray reflected from the coelostat’s mirror.
The image produced by a coelostat does not rotate (in its plane), which is an advantage over heliostats and siderostats, of whose design the coelostat is an improvement.