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(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.

Figure 1. Diagram of a coelostat

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.


A device consisting of a clockwork-driven mirror that enables a fixed telescope to continuously keep the same region of the sky in its field of view.
References in periodicals archive ?
I came to believe the universe is a critical part of the environment, as much as some people would accept clouds," he said, before walking down the circular stairs beneath the 74-year-old coelostat.
Although inconspicuous from the street, the house-size building had a motorized dome and coelostat, a 76-foot-deep pit for spectrograph optics to provide a huge path for the Sun's rays, and state-of-the-art equipment.
Hale designed the telescope's optical path so that sunlight piped into the building from the dome-covered coelostat would form a solar image for his inspection in the first-floor entry hall.
At the Boyden Observatory near Bloemfontein, there is a 20cm coelostat with instrumentation for making narrow-band Ha observations.
At the Boyden Observatory near Bloemfontein, there is a 20-cm coelostat with instrumentation for making narrow-band H[alpha] observations.