Our Astronomical Column

Abstract

THE CŒLOSTAT.—The name cœlostat has been given by M. G. Lippmann to a modified form of siderostat which he has devised (Comptes rendus, No. 19, 1895, and Observatory, August). The special feature of the instrument is that it gets rid of the rotation of the field of view which disqualifies the siderostat for some purposes, such, for instance, as long-exposure photography. It consists simply of a mirror with its plane parallel to the earth's axis, and turning on a polar axis once in forty-eight hours in the same direction as the apparent diurnal motion of the heavens. It is easily demonstrated that the image of any star whatever will be seen stationary in a mirror so mounted, and a telescope pointed at the mirror in any direction will have a constant field of view. The telescope being directed to the cœlostat in a given position, to observe other objects having the same declination as that in view, it will only be necessary to turn the mirror; but for objects with different declinations the telescope must also be moved. If it be desired to use a horizontal telescope, it must be directed to the point on the horizon where the object rises, and the mirror must be started in a position suited to the hour-angle; but there is a limit to the use of a horizontal telescope. It is pointed out that the simplicity of the instrument makes it possible to turn it into one of great precision; stability being readily attained, while the possibility of flexure can be reduced to a minimum.