<P> For reflecting telescopes, which use a curved mirror in place of the objective lens, theory preceded practice . The theoretical basis for curved mirrors behaving similar to lenses was probably established by Alhazen, whose theories had been widely disseminated in Latin translations of his work . Soon after the invention of the refracting telescope Galileo, Giovanni Francesco Sagredo, and others, spurred on by their knowledge that curved mirrors had similar properties as lenses, discussed the idea of building a telescope using a mirror as the image forming objective . The potential advantages of using parabolic mirrors (primarily a reduction of spherical aberration with elimination of chromatic aberration) led to several proposed designs for reflecting telescopes, the most notable of which was published in 1663 by James Gregory and came to be called the Gregorian telescope, but no working models were built . Isaac Newton has been generally credited with constructing the first practical reflecting telescopes, the Newtonian telescope, in 1668 although due to their difficulty of construction and the poor performance of the speculum metal mirrors used it took over 100 years for reflectors to become popular . Many of the advances in reflecting telescopes included the perfection of parabolic mirror fabrication in the 18th century, silver coated glass mirrors in the 19th century, long - lasting aluminum coatings in the 20th century, segmented mirrors to allow larger diameters, and active optics to compensate for gravitational deformation . A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera, which uses both a lens (corrector plate) and mirror as primary optical elements, mainly used for wide field imaging without spherical aberration . </P> <P> The late 20th century has seen the development of adaptive optics and space telescopes to overcome the problems of astronomical seeing . </P> <P> The basic scheme is that the primary light - gathering element the objective (1) (the convex lens or concave mirror used to gather the incoming light), focuses that light from the distant object (4) to a focal plane where it forms a real image (5). This image may be recorded or viewed through an eyepiece (2), which acts like a magnifying glass . The eye (3) then sees an inverted magnified virtual image (6) of the object . </P> <P> Most telescope designs produce an inverted image at the focal plane; these are referred to as inverting telescopes . In fact, the image is both turned upside down and reversed left to right, so that altogether it is rotated by 180 degrees from the object orientation . In astronomical telescopes the rotated view is normally not corrected, since it does not affect how the telescope is used . However, a mirror diagonal is often used to place the eyepiece in a more convenient viewing location, and in that case the image is erect, but still reversed left to right . In terrestrial telescopes such as spotting scopes, monoculars and binoculars, prisms (e.g., Porro prisms) or a relay lens between objective and eyepiece are used to correct the image orientation . There are telescope designs that do not present an inverted image such as the Galilean refractor and the Gregorian reflector . These are referred to as erecting telescopes . </P>

The part of a telescope that gathers the light from a distant object