science · 29 December 1888 · 136 years ago
In 1888, the Great Nebula in Andromeda, M31, was photographed by Isaac Roberts, with a 4-hour exposure using a 20-inch aperture reflecting telescope. It was undoubtedly his finest achievement, for it provided one of the best defined images up to that time. He was a pioneer in the technique of guiding an optical telescope to remain steadily pointed at a particular point in the sky, to compensate during a long-exposure for the rotation of the earth. In the same month, he presented his photographic evidence to a meeting of the Royal Astronomical Society, showing that Andromeda was of the spiral type, illustrating the main idea of the nebula hypothesis. He had made earlier photographs on 15 Nov 1887, and the first two days of 1887. He also successfully photographed other galaxies.
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