Nasmyth's 20 inch reflecting telescope, 1850-52.
© Science Museum / Science & Society Picture Library
James Nasmyth (1808-1890), a succesful entrepreneur, built this reflecting telescope at his foundry in Patricroft near Manchester around 1850. With a 20 inch (508mm) speculum (metal) mirror, it originally had an optical configuration with the eyepiece perilously located at the top of the tube. To improve matters, Nasmyth added an extra mirror which lengthened the light path so that it was directed through the trunnion axis of the turntable stand. This optical configuration is now used on modern telescopes and is called the Nasmyth focus. From his drawings of the Moon, Nasmyth made relief models of the lunar surface. These were then photographed with suitable lighting, producing more lifelike results than the direct lunar photography of the time.