Replica of Newton's reflecting telescope
Replica of Newton’s reflecting telescope

The telescope was invented in Holland in about 1608. It is usually ascribed to Hans Lippershey, a spectacle maker, who applied for a patent based on the observation that placing a concave and a convex lens in a tube magnified distant objects With at least two other Dutch inventors of similar devices at about the same time, the States General ruled that the device was too easily copied to be patentable. The first recorded demonstration of a telescope was by Galileo in 1609.

Johannes Kepler discovered the principle of the astronomical telescope, which uses a different combination of lenses to give a brighter but inverted image, and his design was constructed by Christoph Scheimer, a German Jesuit astronomer, in 1630. Because of problems of spherical aberration, astronomical telescopes had to be very long – sometimes more than 60 metres.

To overcome the problem of lens aberration, attention turned to the possibility of using mirrors rather than lenses in telescopes. Isaac Newton constructed the first reflecting telescope in 1668 but viewing with it was difficult because the eyepiece and the head of the viewer blocked a lot of the incoming light. William Herschel solved the problem by tilting the mirror in his telescope (which was about 12 metres long).

Lens were greatly improved in the 1750s by the invention of optical flint glass in 1754 and the achromatic lens (which solved the problem of aberration by combining two different types of glass) by John Dollond in 1757. Towards the end of the 18th century, the Swiss optician Pierre Louis Guinard devised a technique for making large disks of flint glass. This allowed the manufacture of telescopes with a diameter of up to 25cm compared to about 10cm in the 1750s.

Refracting telescope with viewfinder (ca 1900 )
Refracting telescope with viewfinder (ca 1900 )

Before the invention of achromatic glass by John Dollond in 1760, telescopes which worked well in daylight failed to show colours at night. This was particularly important on ships where differently coloured signalling lamps needed to be distinguished. As a result “day glasses” and “night glasses”, which gave an accurately coloured but inverted image, were usually carried on ships.  

Until the early 19th century, telescopes were usually made with a single mahogany tube. From this time, two or more extending brass tubes were used. From about 1850, the outer brass tube usually had a protective covering of leather, canvas, sting or plaited horsehair.

Early reflector telescopes had mirrors made of speculum metal (an alloy of copper and tin) which had to be frequently repolished. Glass mirrors were first used in 1856 by Carl August von Steinheil and Jean Foucalt. The glass surface had silver deposited on it by a chemical process. This was superseded in 1934 by vacuum-deposited aluminium.

Binoculars were invented early in the 17th century but, as no convenient way of focusing them could be devised, they were not manufactured commercially until the 1830s. These consisted of twin brass tubes with a central focusing knob. Prismatic binoculars, which are more compact yet more powerful, were introduced in the 1880s.
 

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