heliometer

300px|thumb|right|Heliometer at the Kuffner observatory (Vienna, Austria) alt=Brass Heliometer Split lens (glass lens held in brass housing, with split down the middle, and screw mechanism to move one half relative to the other)|thumb|Heliometer split lens, displayed at Custer Observatory, Long Island, New York. A heliometer (from Greek 'sun' and 'measure') is an instrument originally developed from the mid-eighteenth to early nineteenth centuries for measuring the variation of the Sun's diameter at different seasons of the year, but the modern version of the instrument is capable of much wider use.

==Description== thumb|Heliometer by Carl Bamberg, Berlin, around 1880/1890. Diameter of the lens 4.2 cm. thumb|Double image of the solar disk The basic concept is to introduce a split element into a telescope's optical path so as to produce a double image. If one element is moved using a screw micrometer, precise angle measurements can be made. The simplest arrangement is to split the object lens in half, with one half fixed and the other attached to the micrometer screw and slid along the cut diameter. To measure the diameter of the Sun, for example, the micrometer is first adjusted so that the two images of the solar disk coincide (the "zero" position where the split elements form essentially a single element). The micrometer is then adjusted so that diametrically opposite sides of the two images of the solar disk just touch each other. The difference in the two micrometer readings so obtained is the (angular) diameter of the Sun. Similarly, a precise measurement of the apparent separation between two nearby stars, A and B, is made by first superimposing the two images of the stars and then adjusting the double image so that star A in one image coincides with star B in the other. The difference in the two micrometer readings so obtained is the apparent separation or angular distance between the two stars.