The first known practical telescopes wererefracting telescopes with glasslenses and were invented in theNetherlands at the beginning of the 17th century. They were used for both terrestrial applications andastronomy.
Thereflecting telescope, which uses mirrors to collect and focus light, was invented within a few decades of the first refracting telescope.
In the 20th century, many new types of telescopes were invented, includingradio telescopes in the 1930s andinfrared telescopes in the 1960s.
The wordtelescope was coined in 1611 by the Greek mathematicianGiovanni Demisiani for one ofGalileo Galilei's instruments presented at a banquet at theAccademia dei Lincei.[2][3] In theStarry Messenger, Galileo had used theLatin termperspicillum. The root of the word is from theAncient Greek τῆλε,tele 'far' and σκοπεῖν,skopein 'to look or see'; τηλεσκόπος,teleskopos 'far-seeing'.[4]
The earliest existing record of a telescope was a 1608 patent submitted to the government in the Netherlands by Middelburg spectacle makerHans Lipperhey for arefracting telescope.[6] The actual inventor is unknown but word of it spread through Europe.Galileo heard about it and, in 1609, built his own version, and made his telescopic observations of celestial objects.[7][8]
The invention of theachromatic lens in 1733 partially corrected color aberrations present in the simple lens[12] and enabled the construction of shorter, more functional refracting telescopes.[13] Reflecting telescopes, though not limited by the color problems seen in refractors, were hampered by the use of fast tarnishingspeculum metal mirrors employed during the 18th and early 19th century—a problem alleviated by the introduction of silver coated glass mirrors in 1857, and aluminized mirrors in 1932.[14] The maximum physical size limit for refracting telescopes is about 1 meter (39 inches), dictating that the vast majority of large optical researching telescopes built since the turn of the 20th century have been reflectors. The largest reflecting telescopes currently have objectives larger than 10 meters (33 feet), and work is underway on several 30–40m designs.[15]
The 20th century also saw the development of telescopes that worked in a wide range ofwavelengths fromradio togamma-rays. The first purpose-built radio telescope went into operation in 1937. Since then, a large variety of complex astronomical instruments have been developed.
In the late 2010s, smart telescopes[16] democratized access to night sky observations[17]. They simplify setup, automate object tracking, and deliver clear, processed images to users, including those in light-polluted environments. Smart telescopes don't have an eyepiece like a traditional telescope. They capture multiple images of an object, stacking the images in real time to display a clear view.
Since the atmosphere is opaque for most of the electromagnetic spectrum, only a few bands can be observed from the Earth's surface. These bands are visible – near-infrared and a portion of the radio-wave part of the spectrum.[18] For this reason there are no X-ray or far-infrared ground-based telescopes as these have to be observed from orbit. Even if a wavelength is observable from the ground, it might still be advantageous to place a telescope on a satellite due to issues such as clouds,astronomical seeing andlight pollution.[19]
The disadvantages of launching a space telescope include cost, size, maintainability and upgradability.[20]
Some examples of space telescopes from NASA are the Hubble Space Telescope that detects visible light, ultraviolet, and near-infrared wavelengths, the Spitzer Space Telescope that detects infrared radiation, and the Kepler Space Telescope that discovered thousands of exoplanets.[21] The latest telescope that was launched was the James Webb Space Telescope on December 25, 2021, in Kourou, French Guiana. The Webb telescope detects infrared light.[22]
Six views of theCrab Nebula at different wavelengths of light
The name "telescope" covers a wide range of instruments. Most detectelectromagnetic radiation, but there are major differences in how astronomers must go about collecting light (electromagnetic radiation) in different frequency bands.
As wavelengths become longer, it becomes easier to use antenna technology to interact with electromagnetic radiation (although it is possible to make very tiny antenna). The near-infrared can be collected much like visible light; however, in the far-infrared and submillimetre range, telescopes can operate more like a radio telescope. For example, theJames Clerk Maxwell Telescope observes from wavelengths from 3 μm (0.003 mm) to 2000 μm (2 mm), but uses a parabolic aluminum antenna.[23] On the other hand, theSpitzer Space Telescope, observing from about 3 μm (0.003 mm) to 180 μm (0.18 mm) uses a mirror (reflecting optics). Also using reflecting optics, theHubble Space Telescope withWide Field Camera 3 can observe in the frequency range from about 0.2 μm (0.0002 mm) to 1.7 μm (0.0017 mm) (from ultra-violet to infrared light).[24]
With photons of the shorter wavelengths, with the higher frequencies, glancing-incident optics, rather than fully reflecting optics are used. Telescopes such asTRACE andSOHO use special mirrors to reflectextreme ultraviolet, producing higher resolution and brighter images than are otherwise possible. A larger aperture does not just mean that more light is collected, it also enables a finer angular resolution.
Radio telescopes aredirectionalradio antennas that typically employ a large dish to collect radio waves. The dishes are sometimes constructed of a conductive wire mesh whose openings are smaller than thewavelength being observed.
Unlike an optical telescope, which produces a magnified image of the patch of sky being observed, a traditional radio telescope dish contains a single receiver and records a single time-varying signal characteristic of the observed region; this signal may be sampled at various frequencies. In some newer radio telescope designs, a single dish contains an array of several receivers; this is known as afocal-plane array.
By collecting and correlating signals simultaneously received by several dishes, high-resolution images can be computed. Such multi-dish arrays are known asastronomical interferometers and the technique is calledaperture synthesis. The 'virtual' apertures of these arrays are similar in size to the distance between the telescopes. As of 2005, the record array size is many times the diameter of the Earth – using space-basedvery-long-baseline interferometry (VLBI) telescopes such as the JapaneseHALCA (Highly Advanced Laboratory for Communications and Astronomy) VSOP (VLBI Space Observatory Program) satellite.[25]
Radio telescopes are also used to collectmicrowave radiation, which has the advantage of being able to pass through the atmosphere and interstellar gas and dust clouds.
An optical telescope gathers andfocuses light mainly from the visible part of the electromagnetic spectrum.[29] Optical telescopes increase the apparentangular size of distant objects as well as their apparentbrightness. 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 glasslenses and/ormirrors, to gather light and other electromagnetic radiation to bring that light or radiation to a focal point. Optical telescopes are used forastronomy and in many non-astronomical instruments, including:theodolites (includingtransits),spotting scopes,monoculars,binoculars,camera lenses, andspyglasses. There are three main optical types:
Most ultraviolet light is absorbed by the Earth's atmosphere, so observations at these wavelengths must be performed from the upper atmosphere or from space.[37][38]
Higher energy X-ray and gamma ray telescopes refrain from focusing completely and usecoded aperture masks: the patterns of the shadow the mask creates can be reconstructed to form an image.
X-ray and Gamma-ray telescopes are usually installed on high-flying balloons[47][48] or Earth-orbitingsatellites since theEarth's atmosphere is opaque to this part of the electromagnetic spectrum. An example of this type of telescope is theFermi Gamma-ray Space Telescope which was launched in June 2008.[49][50]
The detection of very high energy gamma rays, with shorter wavelength and higher frequency than regular gamma rays, requires further specialization. Such detections can be made either with theImaging Atmospheric Cherenkov Telescopes (IACTs) or with Water Cherenkov Detectors (WCDs). Examples of IACTs areH.E.S.S.[51] andVERITAS[52][53] with the next-generation gamma-ray telescope, the Cherenkov Telescope Array (CTA), currently under construction.HAWC andLHAASO are examples of gamma-ray detectors based on the Water Cherenkov Detectors.
A discovery in 2012 may allow focusing gamma-ray telescopes.[54] At photon energies greater than 700 keV, the index of refraction starts to increase again.[54]
^Brennan, Pat; NASA (26 July 2022)."Missons/Discovery".NASA's exoplanet-hunting space telescopes. Retrieved17 September 2023.
^Space Telescope Science Institution; NASA (19 July 2023)."Quick Facts".Webb Space Telescope. Retrieved17 September 2023.
^ASTROLab du parc national du Mont-Mégantic (January 2016)."The James-Clerk-Maxwell Observatory".Canada under the stars.Archived from the original on 5 February 2011. Retrieved16 April 2017.
^Wolter, H. (1952), "Verallgemeinerte Schwarzschildsche Spiegelsysteme streifender Reflexion als Optiken für Röntgenstrahlen",Annalen der Physik,10 (4–5):286–295,Bibcode:1952AnP...445..286W,doi:10.1002/andp.19524450410.
Sabra, A. I.; Hogendijk, J. P. (2003).The Enterprise of Science in Islam: New Perspectives.MIT Press. pp. 85–118.ISBN978-0-262-19482-2.
Wade, Nicholas J.; Finger, Stanley (2001), "The eye as an optical instrument: from camera obscura to Helmholtz's perspective",Perception,30 (10):1157–1177,doi:10.1068/p3210,PMID11721819,S2CID8185797
Watson, Fred (2007).Stargazer : the life and times of the telescope. Crows Nest, New South Wales, Australia: Allen & Unwin.ISBN978-1-74176-392-8.OCLC173996168.
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