History of the telescope

Percival Lowell observing Mars from the observer's chair of the 61-centimeter (24-inch) refracting telescope in Flagstaff, Arizona (USA)

The properties of lenses had been known since antiquity and had been studied widely in the centuries preceding the telescopes development, however the earliest known working telescopes in the modern sense appeared in 1608 and are credited to three individuals, Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar also known as Jacob Adriaanszoon. The design of this early refracting telescope consisted of a convex objective lens and a concave eyepiece, and Galileo greatly improved upon this design the following year.

In 1611, Johannes Kepler described how a telescope could be made with a convex objective and eyepiece lens. Such telescopes allowed for many improvements in observations techniques but suffered from chromatic aberration. The only way to overcome the aberrations was to fabricate objectives with very long focal lengths. By 1655 astronomers such as Christiaan Huygens were building powerful but extremely large and unwieldy Keplerian telescopes with compound eyepieces.

Concurrent with the history of refracting telescope employing lenses is the history of the reflecting telescope that used mirrors to form an image. The advantage of reflectors was that they did not suffer from chromatic aberration. Early reflectors did suffered from a server loss in the light they gathered due to the fast tarnishing speculum metals used to construct mirrors at that time. Niccolò Zucchi is credited with constructing the first reflecting telescope in 1616, but since the observers head had to block the incoming light to view the image formed, it was relatively impractical. Isaac Newton’s 1668 design for a reflector overcame the problems of Zucchi’s reflector by adding a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. Laurent Cassegrain in the same year described the design of a reflector with a small convex secondary mirror, to reflect light through a central hole in the main mirror.

A step forward in the evolution of the refracting telescope came with the invention of the achromatic lens that corrected some of the color aberration of simple lenses, allowing for more functional shorter telescopes. This type of lens first appeared in a 1733 telescope made by Chester Moore Hall, who did not publicize it. John Dollond independently developed achromatic lenses, and produced telescopes using them in commercial quantities starting in 1758.

It wasn't until 1721, when John Hadley was able to produce much larger paraboloidal mirrors, that Newtonian reflectors began to proliferate. The process of silvering glass mirrors introduced by by Léon Foucault in 1857 greatly improved the light gathering efficiency of reflectors and lead to further development of the reflector as the predominate type of optical astronomical telescope. The adoption of long lasting aluminized coatings on reflector mirrors in 1932 and the maximum physical size limit of the refracting telescope objectives, at around 1 meter (40 inches), means almost all of the large research telescopes used today are reflectors.

The 20th century saw the construction of giant telescopes, with many types of telescopes being developed for a wide range of wavelengths from radio to gamma-rays. The first radio telescope went into operation in 1937. Since then a tremendous variety of complex astronomical instruments have been developed.

Optical telescopes

Invention

See also: History of optics

Lenses and their properties were know well before the invention of the optical telescope. Simple lenses made from rock crystal have been known from before recorded history. Ptolemy in his work Optics, written in the 2nd century AD, wrote about properties of light, including reflection, refraction, and color. The work is a significant part of the early history of optics.

The effects of pinhole and concave lenses were described by Arabian astronomer Ibn al-Haytham (known as Alhazen in the West) around 1020. The Latin translation of his main work, Kitab al-Manazir (Book of Optics), influenced European scientist such as Johannes Kepler and the work of Roger Bacon, who cites Ibn Al-Haytham by name. Later on, tremendous advances in telescopic technology—which was to be quickly applied and crafted in Europe—reveal the influence of his research in the field of refraction and magnification.

It was from approximately the 11th century in Europe that 'reading stones' (magnifying lenses placed on the reading material) were well documented—as well as the use of lenses as burning glasses . It is generally considered that in Europe spectacles for correcting long sightedness with convex lenses were invented in Northern Italy in the late 13th to early 14th century. The invention of the use of concave lenses to correct near-sightedness is ascribed to Nicholas of Cusa in 1451. Thus, early knowledge of lenses, and the availability of lenses for spectacles from the 13th century onwards through the 16th century, means that it was possible for many individuals to discover the principles of a telescope using a combination of concave or concave and convex lenses.

Pre-17th century telescopes?

Optical diagram showing light being refracted by a spherical glass container full of water, from Roger Bacon

There are some early references to devices that may have been telescopes. Robert Grosseteste wrote several scientific treatises between 1230 and 1235 including De Iride (Concerning the Rainbow) in which he said:

This part of optics, when well understood, shows us how we may make things a very long distance off appear as if placed very close, and large near things appear very small, and how we may make small things placed at a distance appear any size we want, so that it may be possible for us to read the smallest letters at incredible distances...

Roger Bacon was a pupil of Grosseteste at Oxford, and is frequently stated as having described a telescope in the 13th century, however it is not certain if he built a working model. There is some documentary evidence, but no physical evidence, that the principles of telescopes were known in the late 16th century. Writings by John Dee and Thomas Digges in England in 1570 and 1571 respectively ascribe the use of both reflecting and refracting telescopes to Thomas' father, Leonard Digges, and this is independently confirmed by a report by William Bourne in approximately 1580. They may have been an experimental devices and were never widely reported or reproduced. In the Ottoman Empire, Taqi al-Din seems to describe another early telescope in a 1574 optical treatise, but his earlier work on how to construct the instrument has been lost. In Italy, Giambattista della Porta also described a possible telescope in his Natural Magic published in 1589. These early attempts at constructing telescopes may have been crude, since we hear so little about them. It was not until the early 17th century in the Netherlands that the knowledge of construction and use of telescopes became widespread.

The first telescopes

The practical exploitation of the instrument was certainly achieved in the Netherlands about 1608, but the credit of the original invention has been claimed on behalf of three individuals, Hans Lippershey and Sacharias Jansen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar, also known as Jacob Adriaanszoon. Hans Lippershey was credited with creating and disseminating designs for the first practical telescope, applying to General Estates of The Hague on October 2, 1608 for a patent for an instrument "for seeing things far away as if they were nearby" (beating Jacob Metius's patent by a few weeks). Lippershey failed to receive a patent but was handsomely rewarded by the Dutch government for copies of his design. Sacharias Jansen's design for a telescope may have pre-dated Lippershey and Metius but it wasn't known at the time and the invention was never widely publicized.

The original Dutch telescopes were composed of a convex and a concave lens, and telescopes so constructed do not invert the image. Lippershey's original design only had a 3x magnification. Telescopes seem to have been made in the Netherlands in considerable numbers soon after the date of their invention, and rapidly found their way all over Europe.

Galileo happened to be in Venice in about the month of May 1609, and there heard of the "Dutch perspective glass" by means of which distant objects appeared nearer and larger. Galileo states that he solved the problem of the construction of a telescope the first night after his return to Padua from Venice, and made his first telescope the next day by fitting a convex lens in one extremity of a leaden tube and a concave lens in the other one. A few days afterwards, having succeeded in making a better telescope than the first, he took it to Venice, where he communicated the details of his invention to the public, and presented the instrument itself to the doge Leonardo Donato, sitting in full council. The senate, in return, settled him for life in his lectureship at Padua and doubled his salary. Galileo may thus claim to have invented the telescope independently, but not until he had heard that others had done so.

Portrait of Galileo Galilei

Galileo devoted his time to improving and perfecting the telescope, and soon succeeded in producing telescopes of greatly increased power. His first telescope magnified three diameters; but he soon made instruments which magnified eight diameters, and finally one that magnified thirty-three diameters. With this last instrument he discovered in 1610 the satellites of Jupiter, and soon afterwards the spots on the sun, the phases of Venus, and the hills and valleys on the Moon. He demonstrated the revolution of the satellites of Jupiter around the planet, and gave rough predictions of their configurations, proved the rotation of the Sun on its axis, established the general truth of the Copernican system as compared with that of Ptolemy, and fairly routed the fanciful dogmas of the philosophers. Galileo’s instrument was the first to be given the name “telescope”. The name was invented by an unidentified Greek poet/theologian, present at a banquet held in 1611 by Prince Federico Cesi to make Galileo Galilei a member of the Accademia dei Lincei. The word was created from the Greek tele = 'far' and skopein = 'to look or see'; teleskopos = 'far-seeing'.

These brilliant achievements, together with Galileo's immense improvement of the instrument, overshadowed to a great degree the credit due to the original inventor, and led to the universal adoption of the name of the Galilean telescope for the form of the instrument invented by Lippershey.

Further refinements

Refracting telescopes

Johannes Kepler first explained the theory and some of the practical advantages of a telescope constructed of two convex lenses in his Catoptrics (1611). The first person who actually constructed a telescope of this form was the Jesuit Christoph Scheiner, who gives a description of it in his Rosa Ursina (1630).

William Gascoigne was the first who practically appreciated a chief advantage of the form of telescope suggested by Kepler, that a small material object could be placed at the common focal plane os the objective and the eyepiece. This led to his invention of the micrometer and his application of telescopic sights to precision astronomical instruments. But it was not till about the middle of the 17th century that Kepler's telescope came into general use, and then, not so much because of the advantages pointed out by Gascoigne, but because its field of view was much larger than in the Galilean telescope.

The first powerful telescopes of Keplerian construction were made by Christiaan Huygens, after much labor, in which his brother assisted him. With one of these, with an objective diameter of 2.24 inches (57mm) and a 12 ft (3.7 m) focal length, he discovered the brightest of Saturn's satellites (Titan) in 1655, and in 1659 he published his Systema Saturnium, in which was given for the first time a true explanation of Saturn's ring, founded on observations made with the same instrument.

Long focal length refractors

Woodcut illustration of a 45 m (150 ft) focal length Keplerian astronomical refracting telescope built by Johannes Hevelius. From his book Machina coelestis (first part), published in 1673.

The sharpness of the image in Kepler's telescope limited by the chromatic aberration introduced by the non-uniform refractive properties of the objective lens. The only way to overcome this limitation at high magnifying powers was to create objectives with very long focal lengths. Giovanni Cassini discovered Saturn's fifth satellite (Rhea) In 1672 with a telescope 35 ft (10.7 m) long, and the third and fourth satellites in 1684 with telescopes made by Campani that were 100 and 136 ft (30.5 and 41.5 m) focal length. Christian Huygens states that he and his brother made objectives of 8 inch (200mm) and 8.5 inch (220mm) diameter and 170 and 210 ft (52 and 64 m) focal length respectively, and he presented a 7.5 inch (190mm) diameter 123 ft (37.5 m) focal length objective to the Royal Society of London. Adrien Auzout (died in 1691) and others are said to have made telescopes of from 300 to 600 ft (90 to 180 m) focal length, but it does not appear that they were ever able to use them in practical observations. James Bradley, on December 27, 1722, actually measured the diameter of Venus with a telescope whose objective had a focal length of 212 ft (65 m).

In some of the very long telescopes constructed after 1675 no tube was employed at all. The objective was mounted on a pole or building on a swiveling ball-joint and aimed by means of string or connecting rod. The eyepiece would be mounted on a stand at the focus and the image was found by trial and error. These were consequently termed aerial telescopes. Huygens contrived some ingenious arrangements for directing such telescopes towards any object visible in the heavens - the focal adjustment and centering of the eyepiece being preserved by a braced rod connecting the objective lens and eyepiece. Other contrivances for the same purpose are described by Philippe de la Hire and by Nicolaus Hartsoeker. Telescopes of such great length were naturally difficult to use, and must have taxed to the utmost the skill and patience of the observers.

Reflecting telescopes

See also: Reflecting telescope

The ability of a curved mirror to form an image hade been known since the time of Euclid and had been extensively studied by Ibn al-Haytham in the 11th century. Niccolò Zucchi, an Italian Jesuit astronomer and physicist, is credited with producing the first telescope using mirrors, a reflecting telescope, in 1616, consisting of a curved mirror where the observer directly viewed the focal plane. Since the observers head had to block the incoming light to view the image formed, it was relatively impractical although Zucchi did use it in 1630 to discover the belts of Jupiter. Zucchi reflector overcame the problem of chromatic aberration but used a spherical mirror that introduced the problem of spherical aberration.

Light path in a Gregorian telescope

James Gregory in his book Optica Promota (1663), pointed out that the surfaces of the lenses or mirrors are portions of spheres. It was generally supposed that chromatic errors scene in lenses simply arose from errors in the spherical figure of their surfaces. This lead opticians to try to overcome via construction of lenses of other forms of curvature. Gregory was well aware of the failures of all attempts to perfect telescopes by employing lenses of various forms of curvature, and accordingly proposed the form of reflecting telescope with a mirror that was shaped the part of a conic section would correct spherical aberration as well as the chromatic aberration seen in refractors. The design he cam up with bears his name: the Gregorian telescope. But Gregory, according to his own confession, had no practical skill; he could find no optician capable of realizing his ideas, and after some fruitless attempts was obliged to abandon all hope of bringing his telescope into practical use.

A replica of Isaac Newton's first reflecting telescope

When in 1666 Isaac Newton made his discovery of the varying refraction of light of different colors, he soon perceived that the faults of the refracting telescope were due much more to this cause than to the spherical figure of the lenses. He over-hastily concluded from some rough experiments that all refracting substances diverged the prismatic colors in a constant proportion to their mean refraction; and he drew the natural conclusion that light could not be refracted through a lens without causing chromatic aberrations, and therefore that no improvement could be made in the refracting telescope. But, having ascertained by experiment that for all colors of light the angle of incidence reflected in a mirror was equal to the angle of reflection, he turned his attention to the construction of reflecting telescopes. After much experiment he selected an alloy (speculum metal) of tin and copper as the most suitable material for his objective mirror, and he devised means for grinding and polishing them. He did not attempt the formation of a parabolic figure on account of the probable mechanical difficulties, and he had besides satisfied himself that the chromatic and not the spherical aberration formed the chief faults of previous telescopes. He added to his design a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. Newton's found that with the aid of his new reflector he could see the satellites of Jupiter and the crescent phase of the planet Venus. Encouraged by this success, he made a second telescope, with a magnifying power of 38 diameters, which he presented to the Royal Society of London in December 1672. This type of telescope is still called a Newtonian telescope.

Light path in a Cassegrain telescope

A third form of reflecting telescope, the Cassegrain reflector, was devised in 1672 by Laurent Cassegrain. The telescope had a small convex hyperboloidal secondary mirror placed near the prime focus to reflect light through a central hole in the main mirror.

No further practical advance appears to have been made in the design or construction of the reflecting telescopes till the year 1721, when John Hadley (best known as the inventor of the octant) presented to the Royal Society a Newtonian reflector with a metallic speculum objective mirror of 6 inch (15 cm) aperture and 62 3/4 inch (159 cm) focal length. The instrument was examined by Pound and Bradley. After remarking that Newton's telescope had lain neglected for fifty years, they stated that Hadley had sufficiently shown that the invention did not consist in bare theory. They compared its performance with that of the refractor of 123 ft (37 m) focal length presented to the Royal Society by Huygens, and found that Hadley's reflector "will bear such a charge as to make it magnify the object as many times as the latter with its due charge," and that it represents objects as distinct, though not altogether so clear and bright.

Bradley and Samuel Molyneux, having been instructed by Hadley in his methods of polishing specula, succeeded in producing some telescopes of considerable power, one of which had a focal length of 8 ft (2.4 m); and, Molyneux having communicated these methods to Scarlet and Hearn, two London opticians, the manufacture of telescopes as a matter of business was begun by them.

But it was reserved for James Short of Edinburgh to give practical effect to Gregory's original idea. Born at Edinburgh in 1710 and originally educated for the church, Short attracted the attention of Colin Maclaurin, professor of mathematics at the university, who permitted him about 1732 to make use of his rooms in the college buildings for experiments in the construction of telescopes. In Short's first telescopes the objective mirrors were made of glass, as suggested by Gregory, but he later used speculum metal mirrors only, and succeeded in giving to them true parabolic and elliptic figures. Short then adopted telescope-making as his profession, which he practised first in Edinburgh and afterwards in London. All Short's telescopes were of the Gregorian form. Short died in London in 1768, having made a considerable fortune selling telescopes.

About the year 1774 William Herschel, then a teacher of music in Bath, began to occupy his leisure hours with the construction of reflector telescope mirrors, and finally devoted himself entirely to their construction and use. In 1778 he selected a 6 1/4 inch (16 cm) reflector mirror, the best of some 400 telescope mirrors which he had made, and with it built a 7 foot (2.1 m) focal length telescope. Using this telescope, he made his early brilliant astronomical discoveries. In 1783 Herschel completed a reflector of approximately 18 inches (46 cm) in diameter and 20 ft (6 m) focal length. He observed the heavens with this telescope for some twenty years, replacing the mirror several times.

Achromatic refracting telescopes

See also: Achromatic lens

Light path through an achromatic lens

From the time of the invention of the first refracting telescopes it was generally supposed that chromatic errors seen in lenses simply arose from arose from errors in the spherical figure of their surfaces. Opticians tried to construct lenses of varying forms of curvature to correct these errors. Isaac Newton discovered in 1666 that chromatic colors actually arose from the un-even refraction of light as it passed through the glass medium. This led opticians to experiment with lenses constructed of more than one type of glass in an attempt to canceling the errors produced by each type of glass. It was hoped that this would create an “achromatic lens”, a lens that would focus all colors to a single point and produce instruments of much shorter focal length.

The first person who succeeded in making a reasonably achromatic refracting telescopes was Chester Moore Hall from Essex, England. He argued that the different humours of the human eye so refract rays of light as to produce an image on the retina which is free from color, and he reasonably argued that it might be possible to produce a like result by combining lenses composed of different refracting media. After devoting some time to the inquiry he found that, by combining two lenses formed of different kinds of glass, he could make an achromatic lens where the effects of the unequal refractions of two colors of light (red and blue) was corrected. In 1733 he succeeded in constructing telescope lenses which exhibited much reduced chromatic aberration. One of his instruments had a 2 1/2 inches (6.4 cm) objective with a relatively short 20 inches (51 cm) focal length.

Hall was a man of independent means, and seems to have been careless of fame; at least he took no trouble to communicate his invention to the world. At a trial in Westminster Hall about the patent rights granted to John Dollond (Watkin v. Dollond), Hall was admitted to be the first inventor of the achromatic telescope; but it was ruled by Lord Mansfield that it was not the original inventor who ought to profit from such invention, but he who brought it forth for the benefit of humankind.

In 1747 Leonhard Euler sent to the Berlin Academy of Sciences a paper in which he tried to prove the possibility of correcting both the chromatic and the spherical aberration of a lens. Like Gregory and Hall, he argued that, since the various humours of the human eye were so combined as to produce a perfect image, it should be possible by suitable combinations of lenses of different refracting media to construct a perfect telescope objective. Adopting a hypothetical law of the dispersion of differently colored rays of light, he proved analytically the possibility of constructing an achromatic objective composed of lenses of glass and water.

But all of Euler's efforts to produce an actual objective of this construction were fruitless - a failure which he attributed solely to the difficulty of procuring lenses worked precisely to the requisite curves. John Dollond agreed with the accuracy of Euler's analysis, but disputed his hypothesis on the grounds that it was purely a theoretical assumption, that the theory was opposed to the results of Newton's experiments on the refraction of light, and that it was impossible to determine a physical law from analytical reasoning alone.

In 1754 Euler sent to the Berlin Academy a further paper, in which, starting from the hypothesis that light consists of vibrations excited in an elastic fluid by luminous bodies, and that the difference of color of light is due to the greater or less frequency of these vibrations in a given time, he deduced his previous results. He did not doubt the accuracy of Newton's experiments quoted by Dollond.

Dollond did not reply to this, but soon afterwards he received an abstract of a paper by Samuel Klingenstierna, the Swedish mathematician and astronomer, which led him to doubt the accuracy of the results deduced by Newton on the dispersion of refracted light. Klingenstierna showed from purely geometrical considerations, fully appreciated by Dollond, that the results of Newton's experiments could not be brought into harmony with other universally accepted facts of refraction.

Dollond telescope

A practical man, Dollond at once put his doubts to the test of experiment, confirmed the conclusions of Klingenstierna, discovered a difference far beyond his hopes in the refractive qualities of different kinds of glass with respect to the divergence of colors, and was thus rapidly led to the construction of lenses in which first the chromatic and afterwards the spherical aberration were corrected.

Dollond was aware of the conditions necessary for the attainment of achromatism in refracting telescopes, but long placed implicit reliance on the accuracy of experiments made by so illustrious a philosopher as Newton. His writings show that but for this confidence he would have arrived sooner at a discovery for which his mind was fully prepared. Dollond's paper recounts the successive steps by which he arrived at his discovery independently of Hall's earlier invention, and the logical processes by which these steps were suggested to his mind.

The triple objective, consisting of a combination of two convex lenses of crown glass with a concave flint lens between them, was introduced in 1765 by Peter Dollond, son of John Dollond, and he made many telescopes of this kind.

The difficulty of procuring disks of glass (especially of flint glass) of suitable purity and homogeneity limited the diameter and light gathering power of achromatic telescope lenses. It was in vain that the French Academy of Sciences offered prizes for large perfect disks of optical flint glass. Not until 1866 did refracting telescopes reach 18 inches (45 cm) aperture.

Giant optical telescopes

The development of the achromatic lenses led to a boom in the construction of large refracting telescopes in the late 19th century. In 1897 the refractor reached its maximum practical limit with he construction of the Yerkes Observatory's 40 inch (101.6 cm) refractor. No larger refractors could be built because of gravities effect on the lens. Since a lens can only be held in place by its edge, the center of a large lens will sag due to gravity, distorting the image it produces.

William Herschel's 49-inch (1,200 mm) telescope

The first giant reflecting telescope can be said to be William Herschel's great reflector with a mirror of 49 inches (124 cm) with a 40 ft (12 m) focal length built in 1789. To cut down on the light loss from the poor reflectivity of the speculum mirrors of that day, Herschel eliminated the small diagonal mirror from his design and tilted his primary mirror so he could view the image formed directly. This design has come to be called the Herschelian telescope. The telescope suffered from other problems of scale that were not altogether solved in Herschel's century. This was followed in 1845 by Lord Rosse's 72 inch (183 cm) Newtonian reflector called the Leviathan of Parsonstown with which he discovered the spiral form of the galaxies.

Both telescopes suffered from the poor reflectivity and fast tarnishing nature of their speculum mirrors. This meant the mirrors had to be frequently removed and re-polished. This could change the curve of the mirror so it usually had to be “re-figured” to the correct shape. In 1857 Léon Foucault introduced a process of depositing a layer of silver on glass telescope mirrors. The silver layer was not only much more reflective and longer lasting than the finish on speculum mirrors, it had the advantage being able to be removed and re-deposited without changing the shape of the glass substrate.

File:Palomar.arp.600pix.jpg

The 200 inch (5 m) Hale telescope.

The 20th century saw the construction much larger reflecting telescopes beginning with the completion of Mount Wilson Observatory’s 60-inch (1.5 m) reflector in 1908 and the 100 inch (2.5 m) Hooker telescope in 1917. These and other telescopes of this size it had to have provisions to allow for the removal of their main mirrors for re-silvering every few months. John Donavan Strong, a young physicist at the California Institute of Technology, developed a technique for coating a mirror with a much longer lasting aluminum coating using thermal vacuum evaporation. In 1932 he became the first person to “aluminize” a mirror. In 1935, the 60-inch (1,500 mm) and 100-inch (2,500 mm) telescopes became the first large astronomical telescopes to have their mirrors aluminized. 1948 saw the completion of the 200 inch (508 cm) Hale reflector at Mount Palomar, which was the largest telescope in the world until the completion of the 605 cm (238 in) Large Altazimuth Telescope in Russia in 1975. The 1990s saw a new generation of giant telescopes appear, beginning with the construction of the first of the two 10 m (394 in) Keck telescopes in 1993. Other giant telescopes built since then include the two Gemini telescopes, the four separate telescopes of the Very Large Telescope, and the Large Binocular Telescope.

Other wavelengths

The twentieth century saw the construction of telescopes which could image wavelengths other than visible light. The first radio telescope was built by Grote Reber in 1937, and this prompted a new era of observational astronomy after World War II, with telescopes being developed for other parts of the electromagnetic spectrum from radio to gamma-rays.

Radio telescopes

See also: Radio astronomy and Radio telescope

The 250 ft (76 m) Lovell radio telescope at Jodrell Bank Observatory.

Radio astronomy began in 1931 when Karl Jansky discovered that the Milky Way was a source of radio emission. The first purpose built radio telescope, a 31.4 ft (9.6 m) dish, was built by Grote Reber in 1937, with which he discovered various unexplained radio sources in the sky. Interest in radio astronomy grew after the Second World War, when much larger dishes were built including the 250 ft (76 m) Jodrell bank telescope (1957), the 300 ft (91 m) Green Bank Telescope (1962), and the 100 m (328 ft) Effelsberg telescope (1971). The huge 1000 ft (305 m) Arecibo telescope (1963) is so large that it is fixed into a natural depression in the ground, the central antenna can be steered to allow the telescope to study objects up to twenty degrees from the zenith. Not every radio telescope is of the dish type, the Mills Cross Telescope (1954) was an early example of an array which used two perpendicular lines of antennae 1500 ft (457 m) in length to survey the sky.

High-energy radio waves are known as microwaves and this has been an important area of astronomy ever since the discovery of the cosmic microwave background radiation in 1964. Many ground-based radio telescopes can study microwaves. Short wavelength microwaves are best studied from space, because water vapor, even at high altitudes, strongly weakens the signal. The Cosmic Background Explorer (1989) revolutionized the study of the microwave background radiation.

Because radio telescopes have low resolution, they were the first instruments to use interferometry, allowing two or more, widely separated, instruments to simultaneously observe the same source. Very long baseline interferometry extended the technique over thousands of kilometers and allowed resolutions down to a few milli-arcseconds.

Gamma-ray telescopes

See also: Gamma-ray astronomy

Gamma rays are absorbed high in the Earth's atmosphere, so most gamma-ray astronomy is conducted with satellites. Gamma-ray telescopes used scintillation counters, spark chambers and, more recently, solid-state detectors. The angular resolution of these devices is typically very poor. There were balloon-borne experiments in the early 1960s, but gamma-ray astronomy really began with the launch of the OSO 3 satellite in 1967. The first dedicated gamma-ray satellites were SAS B (1972) and Cos B (1975). The Compton Gamma Ray Observatory (1991) was a big improvement on previous surveys. Very high-energy gamma-rays (above 200 GeV) can be detected from the ground via the Cerenkov radiation produced by the passage of the gamma-rays in the Earth's atmosphere. Several Cerenkov imaging telescopes have been built around the world including HEGRA (1987), STACEE (2001), HESS (2003), and MAGIC (2004).

X-ray telescopes

See also: X-ray astronomy

X-rays from space do not reach the Earth's surface, so X-ray astronomy has to be conducted above the Earth's atmosphere. The first X-ray experiments were conducted on sub-orbital rocket flights, which enabled the first detection of X-rays from the Sun (1948) and the first galactic X-ray sources: Scorpius X-1 (June 1962) and the Crab Nebula (October 1962). Since then, X-ray telescopes have been built using nested grazing-incidence mirrors, which deflect X-rays to a detector. Some of the OAO satellites conducted X-ray astronomy in the late 1960s, but the first dedicated X-ray satellite was Uhuru (1970) which discovered 300 sources. More recent X-ray satellites include EXOSAT (1983), ROSAT (1990), Chandra (1999), and Newton (1999).

Ultra-violet telescopes

See also: Ultraviolet astronomy

Although optical telescopes can image the near ultraviolet, the ozone layer in the stratosphere absorbs ultraviolet radiation shorter than 300 nm, so most ultra-violet astronomy is conducted with satellites. Ultraviolet telescopes resemble optical telescopes, but conventional aluminium-coated mirrors cannot be used and alternatives coatings such as magnesium fluoride or lithium fluoride are used instead. The OSO 1 satellite carried out observations in the ultra-violet as early as 1962. The International Ultraviolet Explorer (1978) systematically surveyed the sky for eighteen years, it carried a 45 cm (18 in) aperture telescope with two spectroscopes. Extreme-ultraviolet astronomy (10-100 nm) is a discipline in its own right, and involves many of the techniques of X-ray astronomy, the Extreme Ultraviolet Explorer (1992) was a satellite which operated at these wavelengths.

Infra-red telescopes

See also: Infrared astronomy

Although most infrared radiation is absorbed by the atmosphere, infrared astronomy at certain wavelengths can be conducted on high mountains where there is little absorption by atmospheric water vapor. Ever since suitable detectors became available, most optical telescopes at high-altitudes have been able to image at infrared wavelengths. Some telescopes such as the 3.8 m (150 in) UKIRT, and the 3 m (118 in) IRTF, both on Mauna Kea, are dedicated infrared telescopes. The launch of the IRAS satellite in 1983 revolutionized infrared astronomy from space. This reflecting telescope which had a 60 cm (23 in) mirror, operated for nine months until its supply of coolant (liquid helium) ran out. It surveyed the entire sky detecting 245 000 infrared sources, more than 100 times the number previously known.

Interferometric telescopes

• See main article History of astronomical interferometry

In 1868 Fizeau noted that the purpose of the arrangement of mirrors or glass lenses in a conventional telescope was simply to provide an approximation to a Fourier transform of the optical wave field entering the telescope. As this mathematical transform was well understood and could be performed mathematically on paper, he noted that using an array of small instruments it would be possible to measure the diameter of a star with the same precision as a single telescope which was as large as the whole array, a technique which later became known as astronomical interferometry. It was not until 1891 that Michelson successfully used this technique for the measurement of astronomical angular diameters, those of Jupiter's satellites (Michelson 1891). Finally, 30 years later, a direct interferometric measurement of a stellar diameter was realized by Michelson & Pease (1921) with their 20 ft (6.1 m) interferometer mounted on the 100 inch Hooker Telescope on Mount Wilson.

The next major development came in 1946 when Ryle and Vonberg (Ryle and Vonberg 1946) constructed a radio analogue of the Michelson interferometer and soon located a number of new cosmic radio sources. The signals from two radio antennas were added electronically to produce interference. Ryle and Vonberg's telescope used the rotation of the Earth to scan the sky in one dimension. With the development of larger arrays, and of computers which could rapidly perform the necessary Fourier transforms, the first aperture synthesis imaging instruments were soon developed, which could obtain high resolution images without the need of a giant parabolic reflector to perform the Fourier transform. This technique is now used in most radio astronomy observations. Radio astronomers soon developed the mathematical methods to perform aperture synthesis Fourier imaging using much larger arrays of telescopes, often spread across more than one continent. In the 1980s the aperture synthesis technique was extended to visible light and infrared astronomy providing the first very high resolution optical and infrared images of nearby stars.

In 1995 this imaging technique was demonstrated on an array of separate optical telescopes for the first time, allowing a further improvement in resolution, and allowing even higher resolution imaging of stellar surfaces. The same techniques have now been applied at a number of other astronomical telescope arrays, including the Navy Prototype Optical Interferometer, the CHARA array and the IOTA array. A detailed description of the development of astronomical optical interferometry can be found here.

References

• This article incorporates text from a publication now in the public domain: Chisholm, Hugh, ed. (1911). Encyclopædia Britannica (11th ed.). Cambridge University Press. {{cite encyclopedia}}: Missing or empty |title= (help)
• Crawford, David Livingstone, ed. (1966). The Construction of Large Telescopes (International Astronomical Union. Symposium no. 27 ed.). London, New York: Academic Press. p. 234. {{cite book}}: Cite has empty unknown parameters: |coauthors= and |month= (help)
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• Fizeau, H. 1868 C. R. Hebd. Seanc. Acad. Sci. Paris 66, 932
• King, Henry C., ed. (1955). The History of the Telescope. London: Charles Griffin & Co. Ltd. {{cite book}}: Cite has empty unknown parameters: |coauthors= and |month= (help)
• Michelson, A. A. 1891 Publ. Astron. Soc. Pac. 3, 274
• Michelson, A. A. & Pease, F. G. 1921 Astrophys. J. 53, 249
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• Ryle, M. & Vonberg, D., 1946 Solar radiation on 175Mc/s, Nature 158 pp 339
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• Watson, Fred, ed. (2004). Star Gazer: The Life and History of the Telescope. Sydney, Cambridge: Allen & Unwin, Da Capo Press. {{cite book}}: Cite has empty unknown parameters: |coauthors= and |month= (help)

Related links

• History of astronomy
• History of astronomical interferometry
• Timeline of telescope technology

External links

• Connexions "Galileo's Telescope" - by Albert Van Helden
• 400th Anniversary of the Invention of the Telescope