Long focal length refractors History of the telescope

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woodcut illustration of 45 m (148 ft) focal length keplerian astronomical refracting telescope built johannes hevelius. book, machina coelestis (first part), published in 1673.


the sharpness of image in kepler s telescope limited chromatic aberration introduced non-uniform refractive properties of objective lens. way overcome limitation @ high magnifying powers create objectives long focal lengths. giovanni cassini discovered saturn s fifth satellite (rhea) in 1672 telescope 35 feet (11 m) long. astronomers such johannes hevelius constructing telescopes focal lengths long 150 feet (46 m). besides having long tubes these telescopes needed scaffolding or long masts , cranes hold them up. value research tools minimal since telescope s frame tube flexed , vibrated in slightest breeze , collapsed altogether.








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