Refracting telescope
The first record of a refracting telescope appeared in the Netherlands about 1608, when a spectacle maker from Middelburg named Hans Lippershey unsuccessfully tried to patent one.
[2] News of the patent spread fast and Galileo Galilei, happening to be in Venice in the month of May 1609, heard of the invention, constructed a version of his own, and applied it to making astronomical discoveries.
The combination of an objective lens 1 and some type of eyepiece 2 is used to gather more light than the human eye is able to collect on its own, focus it 5, and present the viewer with a brighter, clearer, and magnified virtual image 6.
[7] Galileo had to work with the poor lens technology of the time, and found he had to use aperture stops to reduce the diameter of the objective lens (increase its focal ratio) to limit aberrations, so his telescope produced blurry and distorted images with a narrow field of view.
[citation needed] The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design.
The design also allows for use of a micrometer at the focal plane (to determine the angular size and/or distance between objects observed).
Huygens built an aerial telescope for Royal Society of London with a 19 cm (7.5″) single-element lens.
[18] In the late 19th century, the Swiss optician Pierre-Louis Guinand[19] developed a way to make higher quality glass blanks of greater than four inches (10 cm).
[citation needed] In the Royal Observatory, Greenwich an 1838 instrument named the Sheepshanks telescope includes an objective by Cauchoix.
[citation needed] The long achromats, despite having smaller aperture than the larger reflectors, were often favored for "prestige" observatories.
[citation needed] For example, the Nice Observatory debuted with 77-centimeter (30.31 in) refractor, the largest at the time, but was surpassed within only a couple of years.
[citation needed] Such telescopes contain elements of fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration.
[31] Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture.
[33][34] The Cooke triplet can correct, with only three elements, for one wavelength, spherical aberration, coma, astigmatism, field curvature, and distortion.
Most of these problems are avoided or diminished in reflecting telescopes, which can be made in far larger apertures and which have all but replaced refractors for astronomical research.
In the 19th century, refracting telescopes were used for pioneering work on astrophotography and spectroscopy, and the related instrument, the heliometer, was used to calculate the distance to another star for the first time.
[38][39] In 1861, the brightest star in the night sky, Sirius, was found to have smaller stellar companion using the 18 and half-inch Dearborn refracting telescope.
By the 18th century refractors began to have major competition from reflectors, which could be made quite large and did not normally suffer from the same inherent problem with chromatic aberration.
[45] Jupiter's moon Amalthea was discovered on 9 September 1892, by Edward Emerson Barnard using the 36 inches (91 cm) refractor telescope at Lick Observatory.
[48] The astronomer Professor Hartmann determined from observations of the binary star Mintaka in Orion, that there was the element calcium in the intervening space.
[48] Planet Pluto was discovered by looking at photographs (i.e. 'plates' in astronomy vernacular) in a blink comparator taken with a refracting telescope, an astrograph with a 3 element 13-inch lens.
