Refracting telescopes History of the telescope

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.

aerial telescopes

in of long refracting telescopes constructed after 1675, no tube employed @ all. objective mounted on swiveling ball-joint on top of pole, tree, or available tall structure , aimed means of string or connecting rod. eyepiece handheld or mounted on stand @ focus, , image found trial , error. these consequently termed aerial telescopes. , have been attributed christiaan huygens , brother constantijn huygens, jr. although not clear invented it. christiaan huygens , brother made objectives 8.5 inches (220 mm) diameter , 210 ft (64 m) focal length , others such adrien auzout made telescopes focal lengths 600 ft (180 m). telescopes of such great length naturally difficult use , must have taxed utmost skill , patience of observers. aerial telescopes employed several other astronomers. cassini discovered saturn s third , fourth satellites in 1684 aerial telescope objectives made giuseppe campani 100 , 136 ft (30 , 41 m) in focal length.