Telescope

A telescope (from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing') is an astronomical tool which gathers and focuses electromagnetic radiation. Telescopes increase the apparent angular size of ingerso objects, as well as their apparent brightness.

Famous optical telescopes

• The Hubble Space Telescope is in orbit outside of the Earth's atmosphere to allow for observations not distorted by astronomical seeing, in this way they can be diffraction limited, and used for coverage in the ultraviolet (UV) and infrared.
• The Keck telescopes are currently (2005) the largest, but will soon be superseded by the Gran Telescopio Canarias and Southern African Large Telescope.
• The Very Large Telescope array (VLT) is currently (2002) the record holder for total collecting area in an array of telescopes, with four telescopes each 8 metres in diameter. The four telescopes, belonging to ESO and located in the Atacama desert in Chile, are usually operated independently for faint astronomical observations, but can be operated together for aperture synthesis observations of bright objects.
• The Navy Prototype Optical Interferometer is the optical telescope (array) which can currently (2005) produce the highest resolution images at visible wavelengths.
• The CHARA array is the telescope (array) which can currently (2005) produce the highest resolution images at near-infrared wavelengths.
• There are many plans for even larger telescopes. One of them is the Overwhelmingly Large Telescope or OWL, which is intended to have a single aperture of 100 metres in diameter.
• The 200 inch (5.08 m) Hale telescope on Palomar Mountain is a conventional research telescope that was the largest for many years. It has a single borosilicate (Pyrex™) mirror that was famously difficult to construct. The mounting is also unique, an equatorial mount that is not a fork, yet permits the telescope to image near the north celestial pole.
• The 100 inch (2.54 m) Hooker Telescope at the Mount Wilson Observatory was used by Edwin Hubble to discover galaxies, and the redshift. The mirror was made of green glass by Saint-Gobain. In 1919 the telescope was used for the first stellar diameter measurements using interferometry. The telescope now has an adaptive optics system, and is still useful for advanced research.
• The 1.02 m Yerkes Telescope (in Wisconsin) is the largest aimable refractor in use.
• The 0.76 m Nice refractor (in France) that became operational in 1888 was at that time the world's largest telescope. This was the last time the most powerful operational telescope in the world was located in Europe. It was outperformed one year later by the 0.91 m refractor at the Lick Observatory.
• The largest refractor ever constructed was French. It was on display at the 1900 Paris Exposition. Its lens was stationary, prefigured so as to sag into the correct shape. The telescope was aimed by the aid of a Foucault sidérostat, which is a movable plane mirror with a 2 m diameter, mounted in a large cast-iron frame. The horizontal tube was 60 m long and the objective had 1.25 m in diameter. It was a failure.
• The Swedish 1-meter Solar Telescope (SST) on La Palma, currently the largest high-resolution solar telescope in the world.