If the 17th century is notable for the huge leaps that were made in optics, it is mostly because the art of lens griding improved dramatically between 1610 and 1660. Until this happened, telescopes were used mostly for terrestrial purposes: for naval exploration and spying. Astronomers like the Dutchman Christiaan Huygens was persistent and outspoken in his quest for the perfectly crafted lens. During a trip to England in 1661, Huygens was so determined to perfect his telescopes that he skipped the event of the year–the coronation of Charles II–in order to spend the day in the workshop of John Reeves, a top telescope maker.
With improvements in lens quality, telescopes got much longer and much more effective. Between 1645-1650, a “good” telescope measured 6-8 feet. By 1660, they had grown to about 25 feet. And 1670, upwards of 40 t0 50 feet.
For great overview of the history of early telescopes, see: Albert Van Helden, “The Telescope in the Seventeenth Century.” Isis (1974): 38-58. Older, but very informative.
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