Telescope

The telescope was the first scientific instrument to extend one of the human senses, and it became one of the central instruments of the Scientific Revolution of the seventeenth century. Its revelations of unsuspected phenomena in the heavens bore directly on the controversy between followers of traditional geocentric cosmology and those who favored the heliocentric system of Copernicus.

The instrument was not the invention of scientists but the product of craftsmen, which is why much of its origin is inaccessible to historians.

A telescope exists to perform two essential tasks: to collect the faint light arriving from an astronomical source, and to focus all of that light into a point or an image. Because its light-gathering ability is set by the area of the device that acts as the light-gathering bucket, instruments are compared by their apertures — the diameters of the opening through which light travels or reflects. The relationship is not linear but geometric: a telescope with a mirror 4 meters in diameter can collect 16 times as much light as one 1 meter in diameter. The aperture of the main mirror or lens is therefore the key characteristic of any telescope, since a larger aperture gathers more light and brings fainter objects within reach of the eye or the photographic plate.

The design that uses a lens as its main optical element to form an image is called a refractor. Galileo's telescopes were refractors, and so are the binoculars and field glasses in use today. The refractor carries a structural burden, however: because light must pass through the lens, the glass has to be perfect all the way through, and producing large pieces of glass free of flaws and bubbles has proven very difficult. A further distortion arises from the material itself, since the optical properties of transparent materials change slightly with the wavelength of light; this is chromatic aberration, in which each wavelength focuses at a slightly different spot and the image is blurred.

Mirrors avoid both of these difficulties. Only the front surface of a mirror must be manufactured to a precise shape, and the body of the mirror can be supported from behind — an arrangement that allows most astronomical telescopes today, whether amateur or professional, to use a mirror rather than a lens.

In October 1608 the States General at The Hague took up patent applications for a device for seeing faraway things as though nearby, first from Hans Lipperhey of Middelburg and then from Jacob Metius of Alkmaar; the instrument consisted of a convex and a concave lens in a tube and magnified three or four times. The priority of invention has never been settled beyond dispute, and the historical record leaves the question of who first devised the instrument open.

Until Galileo turned such an instrument on the sky, astronomical observation had been confined to the unaided eye, with all the attendant limits on how faint and how detailed a sighting could be. His own instruments progressed quickly: a three-powered spyglass in June or July 1609, an eight-powered one presented to the Venetian Senate in August, and a twenty-powered one directed at the heavens in October or November. The typical Galilean telescope paired a plano-convex objective of about 30-40 inches focal length with a plano-concave ocular of about 2 inches, yielding a magnification of 15-20 times.

Galileo's observations of lunar mountains, of four moons of Jupiter and of nebular patches resolved into individual stars were published in Sidereus Nuncius in March 1610. The announcement of 1610 placed before readers a body of observation that contradicted many traditional cosmological assumptions. Thomas Harriot had drawn the Moon earlier, on 26 July 1609, in a manner more cartographical than Galileo's.

As early as 1611 Johannes Kepler had shown that a telescope could also be made by combining a convex objective with a convex ocular, and from 1630 Scheiner's Rosa Ursina helped spread this form. The change removed the Galilean telescope's small field-of-view limit and set off a competitive race in instrument making, with tube lengths rising from 5 or 6 feet to 15 or 20 feet by mid-century.

Newton is often credited with inventing the reflecting telescope, but contemporaries like James Gregory were experimenting with similar designs at this time.

The largest refracting telescope ever built was a 49-inch instrument constructed for the Paris 1900 Exposition. It was dismantled after the event.

The largest refracting telescope currently in operation is the 40-inch refractor at Yerkes Observatory in Wisconsin.

In 1904 George Ellery Hale, seeking clearer skies than existed near his native Chicago, obtained support from the newly formed Carnegie Institution of Washington to found the Mount Wilson Solar Observatory in the San Gabriel mountains near Pasadena, California. The massive glass disk for the 100-inch telescope was ordered from the French Plate Glass Companies in St. Gobain, France, in September 1906; a Hooker building with rooms for grinding and figuring the mirror and a 100-foot-long testing hall was built at the Observatory headquarters in Pasadena.

Edwin Hubble joined the Mount Wilson staff in 1919 and used the 100-inch telescope to attack the spiral nebulae problem, believing them to be island universes and looking for variable stars in the Andromeda nebula.

In 1928, astronomers at Carnegie and Mount Wilson persuaded the Rockefeller Foundation to fund a 200-inch telescope on Palomar Mountain. The instrument saw first light in 1949, and with that it replaced the 100-inch Hooker telescope as the largest in the world. The ambition behind it had deep roots: with the completion of the 60-inch telescope in 1908, Mount Wilson became the home of the largest astronomical instrument in the world, yet George Ellery Hale was already making plans for a much larger instrument of unheard-of dimensions.

Before the 1980s, telescope mirrors were limited to diameters of 6 metres (236 inches) because larger mirrors would sag under their own weight, until a solution using a thin mirror whose shape could be controlled was found.

Radio astronomy studies celestial objects that give off radio waves, revealing phenomena that are often invisible or hidden in other portions of the electromagnetic spectrum. Radio telescopes are the instruments built to solve these great cosmic mysteries.

Karl Jansky, an engineer at Bell Telephone Laboratories, reported extraterrestrial radio emission in 1933 while searching for the cause of shortwave interference. He not only detected interference from distant thunderstorms but also located a source of radio noise from the centre of the Milky Way Galaxy. That first detection of cosmic radio waves received much public attention but only passing notice from the astronomical community.

Grote Reber, a radio engineer and amateur radio operator, built a 9.5-metre parabolic reflector in his backyard in Wheaton, Illinois, to continue Jansky's investigation of cosmic radio noise. After World War II ended, the technology that had been developed for military radar was applied to astronomical research. Radio telescopes of increasing size and sophistication were built first in Australia and Great Britain and later in the United States and other countries.

The Very Large Telescope stands among the world's most advanced optical telescopes, built around four Unit Telescopes whose main mirrors measure 8.2m in diameter, accompanied by four movable Auxiliary Telescopes of 1.8m diameter. These instruments can work together to form a giant interferometer, the ESO Very Large Telescope Interferometer, which allows astronomers to pick up much finer details of the cosmos than would be possible with the Auxiliary or Unit Telescopes alone. The facility has stimulated a new age of discoveries with several notable scientific firsts, including the first image of an extrasolar planet and the tracking of individual stars moving around the supermassive black hole at the centre of the Milky Way.

A telescope on Earth must look through a turbulent atmosphere which blurs out the fine cosmic details, so Hubble works high above it, collecting visible light and observing the infrared and ultraviolet wavelengths usually filtered out.

Hubble took the first ultraviolet image of an exoplanet, observing a still-forming, Jupiter-sized planet coalescing from material around a young star, and the first direct image of the surface of a star other than the Sun when it observed the red supergiant Betelgeuse.

The cryogenic telescope assembly was cooled to 5 degrees above absolute zero using 95 gallons (360 liters) of liquid helium, to ensure that the observatory's body heat did not interfere with observations of relatively cold cosmic objects.

• 1608
  Lipperhey's announcement
  The telescope is announced.
• October 1608
  Patent applications at The Hague
  Patent applications are filed.
• 26 July 1609
  Harriot's lunar drawing
  Thomas Harriot draws the Moon.
• June or July 1609
  Galileo's three-powered spyglass
  Galileo builds a three-powered spyglass.
• August 1609
  Eight-powered instrument to the Venetian Senate
  An eight-powered instrument is presented to the Venetian Senate.
• October or November 1609
  Twenty-powered observations of the heavens
  Galileo observes the heavens with a twenty-powered instrument.
• March 1610
  Sidereus Nuncius
  Sidereus Nuncius is published.
• 1611
  Kepler's convex-ocular design
  Kepler sets out the convex-ocular design.
• 1630
  Scheiner's Rosa Ursina
  Scheiner publishes Rosa Ursina.
• 1668
  Newton's first reflector
  Newton builds his first reflecting telescope. ^{13Reflecting telescope by Isaac Newton (RS.8462) · The Royal Society Picture Library ↗}
• 1671
  Newton's second instrument shown to the Royal Society
  Newton's second model enters the Society's collection. ^{13Reflecting telescope by Isaac Newton (RS.8462) · The Royal Society Picture Library ↗}
• 1672
  Newton elected a Fellow of the Royal Society
  Newton is elected a Fellow. ^{13Reflecting telescope by Isaac Newton (RS.8462) · The Royal Society Picture Library ↗}
• 1703–1727
  Newton as President of the Royal Society
  Newton serves as President. ^{13Reflecting telescope by Isaac Newton (RS.8462) · The Royal Society Picture Library ↗}
• 1766
  Newton's telescope presented to the Society
  The telescope is presented to the Society.
• 1781
  Herschel's discovery of Uranus
  Herschel discovers Uranus.
• 1900
  Paris Exposition 49-inch refractor
  A 49-inch refractor is exhibited at the Paris Exposition.
• 1904
  Founding of Mount Wilson Solar Observatory
  The Mount Wilson Solar Observatory is founded.
• September 1906
  Order for the 100-inch disk
  The 100-inch disk is ordered.
• August 1908
  Casting of the 100-inch disk
  The 100-inch disk is cast.
• 1908
  First light of the 60-inch
  The 60-inch sees first light.
• 1916
  Completion of the 100-inch mirror
  The 100-inch mirror is completed.
• 1917–1949
  Hooker telescope as largest in the world
  The Hooker telescope is the largest in the world. ^{5Observatories History · Carnegie Science ↗}
• 1919
  Hubble joins Mount Wilson
  Hubble joins Mount Wilson.
• New Year's Day 1925
  Announcement on the spiral nebulae
  An announcement is made on the spiral nebulae.
• 1928
  Rockefeller grant for the 200-inch
  A Rockefeller grant funds the 200-inch.
• 1933
  Jansky's report
  Jansky reports.
• 1949
  First light of the 200-inch Hale telescope
  The 200-inch Hale telescope sees first light. ^{5Observatories History · Carnegie Science ↗}
• August 1972
  VLA congressional approval
  Congress approves the Very Large Array. ^{14The Very Large Array · National Radio Astronomy Observatory ↗}
• April 1973
  VLA construction begins
  Construction begins. ^{14The Very Large Array · National Radio Astronomy Observatory ↗}
• September 1975
  First VLA antenna
  The first antenna is put in place. ^{14The Very Large Array · National Radio Astronomy Observatory ↗}
• 1977
  Space telescope funding begins
  Funding for the space telescope begins.
• 1980
  VLA dedication
  The array is formally dedicated. ^{14The Very Large Array · National Radio Astronomy Observatory ↗}
• 24 April 1990
  Hubble launch
  Hubble is launched.
• December 1993
  First Hubble servicing mission
  The first servicing mission takes place.
• 23 July 1999
  Chandra launch
  Chandra is launched.
• July 1999
  Chandra's place as flagship X-ray mission
  Chandra becomes the flagship X-ray mission.
• 2003
  Spitzer launch
  Spitzer is launched.
• May 2009
  Final Hubble servicing mission
  The final servicing mission takes place.
• February 2017
  Spitzer's Trappist-1 announcement
  Spitzer announces on Trappist-1.
• 25 December 2021
  Webb launch
  Webb is launched.
• January of the following year
  Webb reaches L2
  Webb reaches L2.