Photograph of Steinheil's Equatorial telescope with Zollner photometer taken at the Pulkowa Observatory, St Petersburg, Rusia. Carl August von Steinheil (1801-1870) was a Swis physicist best known for his optical instruments including telescopes. Johann Karl Friedrich Zollner (1834-1882) was a German astrophysicist and astronomer. Photometers are used to accurately measure the amount of light received from an object and can be applied to telescopes to determine the orbit and size of planets.
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