Heliostat or Coelostat by Prazmouski

Catalogue number #1147112828

Heliostat or Coelostat by Prazmouski 1	Heliostat or Coelostat by Prazmouski 2	Heliostat or Coelostat by Prazmouski 3	Heliostat or Coelostat by Prazmouski 4	Heliostat or Coelostat by Prazmouski 5

Description:

Ceolostat or Heliostat signed on the enamel dial by A. Prazmouski 1 Rue Bonaparte, Paris. Beautifully made instrument with integral winding key in fitted case. The Coelostat is possibly the earliest form of heliostat being sketched out by Borelli in about 1665 and re-discovered and built as a working instrument by a college teacher E. F. August before 1839. It is unique in employing a mirror fixed with its plane in the polar axis. A beam of sunlight will be sent at a fixed altitude (normally horizontal)and azimuth throughout the day if the mirror is rotated in a direction to follow the Sun at a rate of one revolution per 48 hours, since the angle turned through by the reflected ray is twice the angle turned by the mirror. The changing declination of the Sun throughout the year will, however, cause a slow change in the azimuth of the reflected beam; so either the receiving apparatus (camera, microscope) must be moved or a second mirror must be interposed. This correction is facilitated by the additional enamel scale. Unlike all other heliostats this design produces a non-rotating exit beam. So, if a lens or concave mirror is inserted to produce an image of a celestial object or star field, then the real image does not rotate about its centre in the course of a day or night. Astronomers immediately realised it was just what they needed for high-resolution studies of the Sun's disc and corona. This is one of the most intellectually satisfying instruments I have ever encountered. This instrument made made in 1877, Prazmouski recommended it for photography both for the reason above and because it could swing a comparatively large mirror to produce a wide fixed beam. Reference Annals of Science Vol.43 No.4 July 1986 article by Allan Mills on the history, theory and types of heliostats. Base diameter 9" (23 cm) Mirror 6.25" (16 cm) x 5.75 (12 cm)

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