Heliometer

The Heliometer is a historic measuring instrument of astronomy for the precise measurement of very small angle. It was initially intended for the determination of possible variations of the solar diameter or long suspected Sonnenabplattung, but was also used for determining the diameter of other celestial bodies, and for the distance and position angle measurements of double stars.

The Heliometer is an equatorial ( equatorially ) mounted telescope, the objective lens is cut along a diameter apart. The two halves can be slightly tilted against each other, something like a parallel plate micrometer applied to the observed stellar images. To measure an angle between two nearby stars, the two sub-images are brought into register and the displacement of the lens halves measured with a vernier. In the sun it coincides, however, the two opposite edges of the sun.

The first Heliometer (double image micrometer ) were Servington 1743 by Savary in England and in 1748 by Pierre Bouguer built in France, and later - and particularly precise - by Joseph Fraunhofer. With such a device Friedrich Wilhelm Bessel in 1838 could determine the first stellar parallax of 61 Cygni and calculate the distance of this star.

A 1755 by the London workshop of John Dollond built for the observatory Gotha Heliometer is issued in astronomy exhibition of the Deutsches Museum in Munich. Even still functioning is the 1893 by the company Repsold and Sons (mechanics) and Steinheil (optics) built Heliometer the Kuffner Observatory in Vienna. It has a lens diameter of 217 mm and a focal length of 3 m, the largest instrument of its kind