Size ranking: 57th
Origin: The southern constellations of Nicolas Louis de Lacaille
One of the faint and obscure constellations of the southern sky introduced by the Frenchman Nicolas Louis de Lacaille after his sky-mapping trip to the Cape of Good Hope in 1751–52. It represents the type of long, unwieldy refractor suspended from a pole known as an aerial telescope, as used by J. D. Cassini at Paris Observatory. The reason for the great length was to reduce chromatic aberration (false colour) produced by the crude lenses of that time.
Lacaille originally depicted Telescopium as extending northwards between Sagittarius and Scorpius, as shown on the accompanying illustration by Bode, but modern astronomers have cut off the top of the telescope’s tube and mounting so that it is now restricted to a rectangular area of sky south of Sagittarius and Corona Australis. As a result, Lacaille’s Beta Telescopii, positioned in the pulley at the top of the mast, is now Eta Sagittarii, Gamma Telescopii, in the upper part of the refractor’s tube, is G Scorpii, and Lacaille’s Theta Telescopii, which marked the telescope’s objective lens, is humble 45 Ophiuchi (also known as d Ophiuchi). See here for a modern chart of this area. Telescopium contains no stars brighter than fourth magnitude.
Telescopium, shown under the name Tubus Astronomicus in the Uranographia of Johann Bode (1801), was envisaged as a long-tubed refractor operated by ropes and pulleys. For Lacaille’s original depiction, see here.
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