ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY ROYAL ASTRONOMICAL SOCIETY / SCIENCE PHOTO LIBRARY
Great Melbourne Telescope speculum. This is the back of the mirror used in the Great Melbourne Telescope. It is made from speculum, an alloy of copper and tin. The telescope was constructed by optician Thomas Grubb in 1868 in Dublin for the Melbourne Observatory, Australia. The mirror is 122 centimetres in diameter with a focal length of 9.3 metres. The equatorial mount of the telescope compensated for the rotation of the Earth, easily allowing it to follow an object across the sky. The Great Melbourne Telescope remained the largest equatorially mounted instrument for several decades and was used to observe the skies of the Southern Hemisphere.
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