Heliometer with unequal lenses, 1882
Royal Observatory of Belgium
The split objective heliometer was built by the company Grubb according to the plans of Belgian astronomer Louis Niesten (1844-1920). The instrument was made on the proposal of Jean-Charles Houzeau de Lehaie (1820-1888), to observe the transit of Venus in 1882.
A heliometer consists of a lens whose objective has been cut along a diameter and whose juxtaposed halves can slide in relation to each other. A heliometer is usually used to measure the diameter of the Sun, but can also perform other measurements, as is the case here.
The long focal length objective lens was simply an old Cauchoix achromatic lens with a focal length of 4.54 m and a diameter of 22 cm, cut in two. An identical process was used for the short objective half-lens using a lens measuring just under 14 cm.
The long focal length objective projected a bright image of the Sun superimposed on a black disc of Venus. The shorter focal length objective gave an image of the Sun just a bit smaller than that of Venus on the Sun throughout its passage. When the large image of the Sun was perfectly centred, a screw device could be used to move the image given by the small lens so as to cover the small image of the black disc of Venus throughout the passage.
This device made it possible to take measurements throughout the observed phenomenon and use them to deduce a value for the distance between Earth and Sun.
Two almost identical heliometers were built for this occasion. The company Grubb later transformed one of the two heliometers into a photographic telescope. It is not really known what happened to it. The Observatory still owns parts of the other heliometer. The settings that linked the base of the instrument and the telescope tube and made it possible to monitor or track the Sun in the sky during the observation have still not been found.