History of the telescope

The earliest known working telescopes appeared in 1608 in the Netherlands and are credited to Hans Lippershey. Among many others who claimed to have made the discovery were Zacharias Janssen, a spectacle-maker in Middelburg, and Jacob Metius of Alkmaar. The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. Galileo used this design the following year. In 1611, Johannes Kepler described how a telescope could be made with a convex objective lens and a convex eyepiece lens and by 1655 astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces. Hans Lippershey is the earliest person documented to have applied for a patent for the device.

Isaac Newton is credited with building the first "practical" reflector in 1668 with a design that incorporated a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. Laurent Cassegrain in 1672 described the design of a reflector with a small convex secondary mirror to reflect light through a central hole in the main mirror.

The achromatic lens, which greatly reduced color aberrations in objective lenses and allowed for shorter and more functional telescopes, first appeared in a 1733 telescope made by Chester Moore Hall, who did not publicize it. John Dollond learned of Hall's invention and began producing telescopes using it in commercial quantities, starting in 1758.

Important developments in reflecting telescopes were John Hadley's production of larger paraboloidal mirrors in 1721; the process of silvering glass mirrors introduced by Léon Foucault in 1857; and the adoption of long-lasting aluminized coatings on reflector mirrors in 1932. The Ritchey-Chretien variant of Cassegrain reflector was invented around 1910, but not widely adopted until after 1950; many modern telescopes including the Hubble Space Telescope use this design, which gives a wider field of view than a classic Cassegrain.

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