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A telescope (from the Greek τηλε (tele) meaning “far” and σκοπεῖν (skopein) meaning “look, see”) is an optical instrument that increases the brightness and apparent size of the objects to be observed. Its role as a light receiver is often more important than its optical magnification, it allows to see punctual celestial objects that are difficult to perceive or invisible to the naked eye.
Telescopes are mainly used in astronomy because their settings only make them suitable for observing objects far away and moving relatively slowly.
There are two major types of telescopes, depending on how the light is focused by the lens:
In a refracting telescope the light is focused by a dioptric system composed of a set of lenses (refraction);
In a reflecting telescope light is focused by a catadioptric system composed of mirrors (reflection) but also diopters (e. g. correctors).
History of the telescope
The telescope’s precursor would have been designed in Italy around 1586 most probably by the Italian optician Giambattista della Porta, or perhaps as early as around 1550 by Leonard Digges.
The telescope appeared in Holland where the States General of 1608 successively received Hans Lippershey’s patent application on October 2 and a few weeks later Jacob Metius’; the magnification factor of these telescopes was 3 to 4. Very quickly used for multiple purposes, in particular as an astronomical telescope, the telescope interested the Italian astronomer Galileo who obtained a copy probably during the visit in 1609 of a Dutch merchant who came to propose the object to the Serenissima, then Galileo perfected the model, increasing its magnification by a factor of 2 to 3. At the top of the bell tower, he caused a sensation by offering an astronomical telescope of his own making to Doge Leonardo Donato and the members of the Senate. His German colleague Johannes Kepler perfected the principle, by proposing an optical formula with two convex lenses.
In a telescope, a concave mirror is used to form the image. In 1663, the Scottish mathematician James Gregory was the first to propose the telescope formula with a secondary magnification. Nevertheless, Marin Mersenne had anticipated a system in which the primary and secondary were parabolic, the exit pupil was located on the secondary, which thus served as an eyepiece. But the field was very weak.
The English mathematician and physicist Isaac Newton built a first version in 1668. In this type of instrument, the light reflected from the concave primary mirror must be brought to an observation position, either under or to the side of the instrument. Henry Draper, one of the very first American astronomers to build a telescope, used a full reflection prism two centuries later instead of the plane mirror of the Newton telescope.
The pioneer was the 2.53-metre telescope at the Mount Wilson Observatory in California, which remained famous for its use in the 1920s for the work of the American astronomer Edwin Hubble, and was discontinued under financial pressure from 1985 to 1992.
The design of the Keck telescopes marks an important innovation: the reflective surface of the mirror of each of the two telescopes is composed of a mosaic of thirty-six hexagonal mirrors, all of which can be individually adjusted using three cylinders. It is equivalent to a primary mirror 10 m in diameter, without having the weight. Active optics techniques are used to play on the cylinders to optimize the profile of the total reflective surface.
The Very Large Telescope (VLT) of the European Southern Observatory (ESO), on the other hand, consists of four telescopes, each with an 8.20 m mirror. It is located in Chile, at the top of Cerro Paranal, at an altitude of 2,600 m. In 2002, it was equipped with the NAOS adaptive optics system, enabling it to be twice as accurate as the Hubble Space Telescope.
It is also possible today to use the principles of interferometry in the optical domain to improve resolution. This is the principle used by the two Kecks, but especially by the VLT whose four mirrors, maximum 130 m apart, have the same theoretical resolution as a single mirror 130 m in diameter. However, the sensitivity is not improved, and the interferometry technique remains quite special, often used in very special cases.