heliometer

A heliometer is a type of optical telescope designed to measure small angular separations between celestial objects. It achieves this by splitting the telescope’s objective into two halves and shifting them relative to one another, producing two images of the same object whose angular separation can be varied and measured precisely.

The core principle involves a split objective, typically cut along a diameter, with the two halves moved in opposite directions. The instrument is calibrated so that a given linear displacement of the optical element corresponds to a known angular separation on the sky. By adjusting the halves until the two images coincide or reach a defined alignment, the angular distance between the images can be read from a micrometer scale or reticle.

Historically, the heliometer was developed in the early 19th century, with Fraunhofer among the pioneers. It was used to measure the apparent diameter of the Sun, the angular separations of double stars, and stellar parallaxes. The design favored precise control of small angular measurements, making it a valuable tool for astrometry before the advent of later interferometric methods. Various forms existed, including refracting heliometers with split objectives and, later, alternative split-image arrangements.

Today, heliometers are largely of historical interest, having been supplanted by more modern techniques such as interferometry and CCD-based astrometry. The concept, however, influenced the development of precision angular measurement instruments and contributed to the evolution of astronomical instrumentation.