heliometer

A heliometer is a type of telescope designed specifically for precise measurements of small angles across the sky. The name stems from its initial motivation, which was to measure the angular diameter of the Sun with the precision to determine its variation through the course of a year, yielding information about Earth's orbital eccentricity and the resulting variation in insolation. Subsequently, the same type of instrument came to be used to measure the angle between stars. The instrument is designed to produce two superimposed images of the target, one of which can be shifted a small amount in relation to the other by a micrometer screw in such a way that the angular difference associated with this shift can be read off.