THROUGH THE INFRARED WITH LOGBOOK AND LANTERN SLIDES - A HISTORY OF INFRARED ASTRONOMY FROM 1868 TO 1960

Abstract

Only nine years after Kirchoff published his radiation laws and eleven years before Stefan published the fourth power of absolute-temperature law, the fourth Earl of Rosse made the first infrared measurements of any astronomical body other than the Sun. He observed the Moon with a thermopile and the three-foot reflector at Birr Castle in 1868 (Rosse 1869, 1870, 1873). I begin this history of infrared astronomy with his work because he introduced two techniques, one of which was used for 80 years and the other, in modified form, is still used today. Filters that isolated infrared wave bands and were opaque to visible light did not exist, and so he conceived of using a plate of glass as a filter. By differencing the galvanometer readings with and without the glass and suitably correcting the glass readings for reflection losses, he measured the infrared flux from the Moon. This technique I will call subtractive filtering, and it was used until 1950. His first measurements were made with a single thermopile, and he discovered immediately that the galvanometer deflections were strongly influenced by changing telescope temperatures and air temperatures. He then conceived of using two identical thermopiles, one connected in opposition to the other, both side by side in the focal plane ofthe telescope. Only one thermopile was exposed to the Moon; the other saw only sky. This compensation technique is replaced today by the ac equivalent of chopping between the object and the sky. Lord Rosse compared his lunar deflections with those from blackened cans containing water of different temperatures, and he made extinction measurements at different elevations ofthe Moon. From these he derived a temperature ofthe full Moon of 400°-500° F, presumably by linear extrapolation ofthe water-can deflections. His same measurements can now be interpreted with Stefan's law and they do give the correct temperature ofthe Moon (Sin ton 1958). He measured the lunation curve of the Moon, and he also observed a total eclipse. He noted that the infrared deflections from the Moon during the eclipse were closely proportional to the visible light, and he deduced that the Moon's surface cooled extremely rapidly. His apparatus was not sufficiently sensitive to