Telescopes are optical instruments that aids in the observation of remote objects by collecting electromagnetic radiation (such as visible light, IR, UV). The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s, by using glass lenses. They found use in both terrestrial applications and astronomy.


A replica of Newton's second reflecting telescope which was presented to the Royal Society in 1672. image: wikipedia

Within a few decades, reflecting telescopes were invented, which used mirrors to collect and focus the light. In the 20th century many new types of telescopes were invented, including radio telescopes in the 1930s and infrared telescopes in the 1960s. 


The Anglo-Australian is modern telescope at Siding Springs Observatory, Australia. image: wikipedia

The word telescope now refers to a wide range of instruments capable of detecting different regions of the electromagnetic spectrum, and in some cases other types of detectors.

Origin of the word telescope

The word telescope (from the Ancient Greek τῆλε, tele "far" and σκοπεῖν, skopein "to look or see"; τηλεσκόπος, teleskopos "far-seeing") was coined in 1611 by the Greek mathematician Giovanni Demisiani for one of Galileo Galilei's instruments presented at a banquet at the Accademia dei Lincei.  In the Starry Messenger, Galileo had used the term perspicillum.

The most familiar telescope is the optical telescope

Optical telescopes gather and focuses light mainly from the visible part of the electromagnetic spectrum (although some work in the infrared and ultraviolet). 

Optical telescopes increase the apparent angular size of distant objects as well as their apparent brightness. In order for the image to be observed, photographed, studied, and sent to a computer, telescopes work by employing one or more curved optical elements, usually made from glass lenses and/or mirrors, to gather light and other electromagnetic radiation to bring that light or radiation to a focal point. 

Optical telescopes are used for astronomy and in many non-astronomical instruments, including: theodolites (including transits), spotting scopes, monoculars, binoculars, camera lenses, and spyglasses. There are three main optical types:

  • The refracting telescope which uses lenses to form an image.

Schematic of Keplerian refracting telescope. image:Szőcs Tamás / wikipedia

  • The reflecting telescope which uses an arrangement of mirrors to form an image.

A Newtonian telescope.  

  • The catadioptric telescope which uses mirrors combined with lenses to form an image.

Schmidt–Cassegrain type telescope . image: Griffenjbs/ wikipedia

Beyond these basic optical types there are many sub-types of varying optical design classified by the task they perform such as astrographs, comet seekers, solar telescope, etc.

Telescopes are often classified by the wavelengths of light they detect:

  • Radio telescopes, using very long wave lengths, usually expressed in GHz rather than wavelength
  • X-ray telescopes, using shorter wavelengths than ultraviolet light
  • Ultraviolet telescopes, using shorter wavelengths than visible light
  • Optical telescopes, using visible light
  • Infrared telescopes, using longer wavelengths than visible light
  • Submillimetre telescopes, using longer wavelengths than infrared light
  • Fresnel Imager, an optical lens technology
  • X-ray optics, optics for certain X-ray wavelengths

Securing land based telescope

Telescope mounts are designed to support the mass of the telescope and allow for accurate pointing of the instrument. 

Many sorts of mounts have been developed over the years, with the majority of effort being put into systems that can track the motion of the stars as the Earth rotates. The two main types of tracking mount are:

  • Altazimuth mount
    • a simple two-axis mount for supporting and rotating an instrument about two perpendicular axes – one vertical and the other horizontal
  • Equatorial mount
    • a mount that compensates for Earth's rotation by having one rotational axis parallel to the Earth's axis of rotation

Source adapted from: Telescope. (2017, March 23). In Wikipedia, The Free Encyclopedia. Retrieved 06:03, March 26, 2017, from