Definitions

Herschelian telescope

History of the telescope

The earliest known working telescopes appeared in 1608 and are credited to Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar. The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. 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 and by 1655 astronomers such as Christiaan Huygens were building powerful but extremely large and unwieldy Keplerian telescopes with compound eyepieces.

Niccolò Zucchi is credited with constructing the first reflecting telescope in 1616, but since the observer's 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.

The achromatic lens, which greatly reduced color aberrations in objective lenses and allowed for shorter and more functional telescopes, 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.

Important developments in reflecting telescopes were John Hadley's production of larger paraboloidal mirrors in 1721; the process of silvering glass mirrors introduced by by Léon Foucault in 1857; and the adoption of long lasting aluminized coatings on reflector mirrors in 1932. Almost all of the large optical research telescopes used today are reflectors.

The first radio telescope was built by Grote Reber in 1937, and many types of telescopes were developed in the 20th century for a wide range of wavelengths from radio, to gamma-rays.

Optical telescopes

Invention

Optical foundations

Lenses and their properties were known 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 the properties of light including: reflection, refraction, and color. The effects of pinhole and the magnifying properties of concave lenses were described by the Arabian astronomer Ibn al-Haytham around 1020. The Latin translation of his main work the Kitab al-Manazir (Book of Optics) influenced European scientists such as Johannes Kepler—and the work of Roger Bacon. From the descriptions found in the Book of Optics in respect to the camera obscura, it was indeed the famous Kepler who was the first to craft the inverted image principle that was later applied to Sheiner's telescope.

While possibly in Egypt, Alhazan once wrote:

"If an object object is placed in a dense spherical medium of which the curved surface is turned towards the eye and is between the eye and the center of the sphere, the object will appear magnified.

It was approximately from the 12th 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 spectacles for correcting long sightedness with convex lenses were invented in Northern Italy in the late 13th to early 14th century, and 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; in the 13th century, Robert Grosseteste wrote several scientific treatises between 1230 and 1235, including De Iride (Concerning the Rainbow), in which he said:

Roger Bacon was a pupil of Grosseteste at Oxford, and is frequently stated as having described a magnifying device in the 13th century, however it is not certain if he built a working model.

Pre 17th century developments

There is some documentary evidence, but no surviving designs or 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 it is independently confirmed by a report by William Bourne in approximately 1580. They may have been experimental devices and were never widely reported or reproduced. Thomas Digges describes his father's device as follows:

In the Ottoman Empire, Taqi al-Din seems to describe another early telescope in a 1574 optical treatise, Kitab Nūr hadaqat al-ibsār wa-nūr haqīqat al-anzār (Book of the Light of the Pupil of Vision and the Light of the Truth of the Sights), but his earlier work on how to construct the instrument has been lost. He described his device as follows:

In Italy, Giambattista della Porta also described a possible telescope in his Natural Magic published in 1589:

In 1959 the Spanish optometrist and amateur historian, Simon de Guilleuma, investigated a reference in a book published in 1609 by the Italian Girolamo Sirtori. In this book, Sirtori describes a meeting in Girona, Catalonia, with an old Burgundian spectacle maker called Juan Roget, who he described as the real inventor of the telescope.

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 known telescopes

The practical exploitation of the instrument was certainly achieved and came to public attention in the Netherlands at 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—later 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 since the same claim for invention had been made by other spectacle-makers. Lippershey 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 the invention was never widely publicized.

The original Dutch telescopes were composed of a convex and a concave lens- telescopes that are constructed this way do not invert the image. Lippershey's original design had only 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, who was 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.

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 commanded a chief advantage of the form of telescope suggested by Kepler: that a small material object could be placed at the common focal plane of the objective and the eyepiece. This led to his invention of the micrometer, and his application of telescopic sights to precision astronomical instruments. It was not till about the middle of the 17th century that Kepler's telescope came into general use: 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: 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; in 1659, he published his "Systema Saturnium" which for the first time, gave a true explanation of Saturn's ring—founded on observations made with the same instrument.

Long focal length refractors

The sharpness of the image in Kepler's telescope was 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) in 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; he presented a 7.5 inch (190mm) diameter 123 ft (37.5 m) focal length objective to the Royal Society of London. Adrien Auzout 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

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. It consisted 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's reflecting telescope overcame the problem of chromatic aberration but used a spherical mirror that introduced the problem of spherical aberration.

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 seen in lenses simply arose from errors in the spherical figure of their surfaces. This lead opticians to try to overcome this by constructing lenses with 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 like the part of a conic section, which would correct spherical aberration as well as the chromatic aberration seen in refractors. The design he came up with bears his name: the "Gregorian telescope"; but according to his own confession, Gregory had no practical skill and 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.

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. However, 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. He later devised means for grinding and polishing them, but did not attempt the formation of a parabolic figure on account of the probable mechanical difficulties: 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 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.

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 until 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 a 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 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). Molyneux communicated these methods to two London opticians —Scarlet and Hearn— who started a business manufacturing telescopes.

It was however 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 the professor of mathematics at the local university, Colin Maclaurin who permitted him in about 1732 to make use of his rooms in the college buildings for experiments in the construction of prototypes. 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

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 practical achromatic refracting telescope was Chester Moore Hall from Essex, England. He argued that the different humours of the human eye refract rays of light 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. Unfortunately, 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.

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 that 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 the Swedish mathematician and astronomer, Samuel Klingenstierna, 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.

As a practical man, Dollond at once put his doubts to the test of experiment: he 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 aberration—and afterwards—the spherical aberration were corrected.

Dollond was aware of the conditions necessary for the attainment of achromatism in refracting telescopes, but relied on the accuracy of experiments made by Newton. His writings show that with the exception of his bravado, 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.

In 1765 Peter Dollond (son of John Dollond) introduced the triple objective, which consisted of a combination of two convex lenses of crown glass with a concave flint lens between them. 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 the lenses found in the achromatic telescope. 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) in aperture.

Adaptive optics

Adaptive optics (AO) is the latest technology used to improve the performance of telescopes. It reduces the effects of rapidly changing optical distortion due to the motion of air currents in the Earth's atmosphere. It is especially used in astronomical telescopes to remove the effects of atmospheric distortion. Adaptive optics works by measuring the distortions in a wavefront usually with a laser and then compensating for them by rapid changes of actuators applied to a deformable mirror or with a liquid crystal array filter. AO was first envisioned by Horace W. Babcock in 1953, but did not come into common usage in astronomical telescopes until advances in computer technology during the 1990s made it possible to calculate the compensation needed in real time.

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 in a research telescope with the construction of the Yerkes Observatorys' 40 inch (101.6 cm) refractor (although a larger refractor Great Paris Exhibition Telescope of 1900 with an objective of 49.2 inch (1.25 m) diameter was temporarily exhibited at the Paris 1900 Exposition). No larger refractors could be built because of gravity's 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.

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 formed image 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.

The 20th century saw the construction of 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 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; three years later the and telescopes became the first large astronomical telescopes to have their mirrors aluminized. The rise of 1948 saw the completion of the 200 inch (508 cm) Hale reflector at Mount Palomar which was the largest telescope in the world up until the completion of the massive 605 cm (238 in) Large Altazimuth Telescope in Russia seventeen years later. 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.

All large earth based telescopes now have adaptive optics fitted to them.

Other wavelengths

The twentieth century saw the construction of telescopes which could produce images using wavelengths other than visible light. The first radio telescope was built by Grote Reber in 1937; 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

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 was built in 1937 by Grote Reber, with a 31.4 ft (9.6 m) dish; using this, 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. However, not every radio telescope is of the dish type. For example, 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

Gamma rays are absorbed high in the Earth's atmosphere so most gamma-ray astronomy is conducted with satellites. Gamma-ray telescopes use 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: the HEGRA (1987), STACEE (2001), HESS (2003), and MAGIC (2004).

X-ray telescopes

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 (Wolter 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 the Uhuru (1970) which discovered 300 sources. More recent X-ray satellites include: the EXOSAT (1983), ROSAT (1990), Chandra (1999), and Newton (1999).

Ultra-violet telescopes

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, using 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

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

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 transformation was well understood and could be performed mathematically on paper, he noted that by 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: the diameters of Jupiter's satellites (Michelson 1891). Thirty years later, a direct interferometric measurement of a stellar diameter was finally realized by Michelson & Pease (1921) which was applied by 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) located a number of new cosmic radio sources by constructing a radio analogue of the Michelson interferometer. 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 as well as 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 also 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

Fast Fourier transform telescope

In 2008, Max Tegmark realized that the lenses and mirrors could be dispensed with altogether when computers become fast enough to perform all the necessary transforms.

Notes

References

  • (1966). The Construction of Large Telescopes. International Astronomical Union. Symposium no. 27, London, New York: Academic Press.
  • Fizeau, H. 1868 C. R. Hebd. Seanc. Acad. Sci. Paris 66, 932
  • (1955). The History of the Telescope. London: Charles Griffin & Co. Ltd.
  • Lindberg, D. C. (1976), Theories of Vision from al-Kindi to Kepler, Chicago: University of Chicago Press
  • Michelson, A. A. 1891 Publ. Astron. Soc. Pac. 3, 274
  • Michelson, A. A. & Pease, F. G. 1921 Astrophys. J. 53, 249
  • Ryle, M. & Vonberg, D., 1946 Solar radiation on 175Mc/s, Nature 158 pp 339
  • (2004). Star Gazer: The Life and History of the Telescope. Sydney, Cambridge: Allen & Unwin, Da Capo Press.

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