An optical telescope is a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct view, or to make aphotograph, or to collect data through electronic image sensors.
There are three primary types of optical telescope:
•    refractors, which use lenses (dioptrics)
•    reflectors, which use mirrors (catoptrics)
•    catadioptric telescopes, which combine lenses and mirrors
A telescope's light gathering power and ability to resolve small detail is directly related to the diameter (or aperture) of its objective (the primary lens or mirror that collects and focuses the light). The larger the objective, the more light the telescope collects and the finer detail it resolves.
People use telescopes and binoculars for activities such as observational astronomy, ornithology, pilotage and reconnaissance, and watching sports or performance arts.


History of the telescope
The telescope is more a discovery of optical craftsmen than an invention of a scientist.[1][2] The lens and the properties of refracting and reflecting light had been known since antiquity and theory on how they worked were developed by ancient Greek philosophers, preserved and expanded on in the medieval Islamic world, and had reached a significantly advanced state by the time of the telescope's invention in early modern Europe.[3][4] But the most significant step cited in the invention of the telescope was the development of lens manufacture for spectacles,[2][5][6] first in Venice and Florence in the thirteenth century,[7] and later in the spectacle making centers in both the Netherlands and Germany.[8]It is in the Netherlands in 1608 where the first recorded optical telescopes (refracting telescopes) appeared. The invention is credited to the spectacle makers Hans Lippershey and Zacharias Janssen in Middelburg, and the instrument-maker and optician Jacob Metius of Alkmaar.[9]
Galileo greatly improved[citation needed] on these designs the following year, and is generally credited as the first to use a telescope for astronomy. Galileo's telescope used Hans Lippershey's design of a convex objective lens and a concaveeye lens, and this design is now called a Galilean telescope. Johannes Kepler proposed an improvement on the design[10] that used a convex eyepiece, often called the Keplerian Telescope.
The next big step in the development of refractors was the advent of the Achromatic lens in the early 18th century,[11] which corrected the chromatic aberration in Keplerian telescopes up to that time—allowing for much shorter instruments with much larger objectives.
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.[12] 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.[13] 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,[14] the most notable of which was published in 1663 by James Gregory and came to be called the Gregorian telescope,[15][16] but no working models were built. Isaac Newtonhas been generally credited with constructing the first practical reflecting telescopes, the Newtonian telescope, in 1668[17] 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,[18] silver coated glass mirrors in the 19th century, long-lasting aluminum coatings in the 20th century,[19] 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.
The late 20th century has seen the development of adaptive optics and space telescopes to overcome the problems of astronomical seeing.