When was the first telescope invented?

The first telescopes appeared in the Netherlands in 1608 when Middelburg spectacle-maker Hans Lippershey tried to obtain a patent on one. By 1609 Galileo had heard about it and built his own improved version. He probably was not the first person to aim the new invention at the night sky but his was the first systematic (and published) study of celestial bodies using one. One of Galileo's first telescopes had 8x to 10x linear magnification and was made out of lenses that he had ground himself. This was increased to 20x linear magnification in the improved telescope he used to make the observations in "Sidereus Nuncius".

Sidereus Nuncius

Sidereus Nuncius (usually Sidereal Messenger, also Starry Messenger or Sidereal Message) is a short astronomical treatise (or "pamphlet") published in New Latin by Galileo Galilei on March 13, 1610. It was the first published scientific work based on observations made through a telescope, and it contains the results of Galileo's early observations of the imperfect and mountainous Moon, the hundreds of stars that were unable to be seen in either the Milky Way or certain constellations with the naked eye, and the Medicean Stars that appeared to be circling Jupiter.

The Latin word "nuncius" was typically used during this time period to denote "messenger"; however, it was also (though less frequently) rendered as "message". Though the title "Sidereus Nuncius" is usually translated into English as "Sidereal Messenger", many of Galileo's early drafts of the book and later related writings indicate that the intended purpose of the book was "simply to report the news about recent developments in astronomy, not to pass himself off solemnly as an ambassador from heaven." Therefore, the correct English translation of the title is "Sidereal Message" (or often, "Starry Message").

A copy of the original edition is a valuable rare book and as of December 2010 has sold at auction for US$662,500, including premium.

"Sidereus Nuncius" contains more than seventy drawings and diagrams of the Moon, certain constellations such as Orion, Pleiades, and Taurus, and the Medicean Stars of Jupiter. Galileo's text also includes descriptions, explanations, and theories of his observations.

In observing the Moon, Galileo saw that the line separating lunar day from night (the terminator) was smooth where it crossed the darker regions of the Moon but quite irregular where it crossed the brighter areas. From this he deduced that the darker regions are flat, low-lying areas, and the brighter regions rough and mountainous. Basing his estimate on the distance of sunlit mountaintops from the terminator, he judged, quite accurately, that the lunar mountains were at least four miles high. Galileo's engravings of the lunar surface provided a new form of visual representation, besides shaping the field of selenography, the study of physical features on the Moon.

Galileo reported that he saw at least ten times more stars through the telescope than are visible to the naked eye, and he published star charts of the belt of Orion and the star cluster Pleiades showing some of the newly observed stars. With the naked eye observers could see only six stars in the Taurus constellation; through his telescope, however, Galileo was capable of seeing thirty-five – almost six times as many. When he turned his telescope on Orion, he was capable of seeing eighty stars, rather than the previously observed nine – almost nine times more. In "Sidereus Nuncius", Galileo revised and reproduced these two star groups by distinguishing between the stars seen without the telescope and those seen with it.
Also, when he observed some of the "nebulous" stars in the Ptolemaic star catalogue, he saw that rather than being cloudy, they were made of many small stars. From this he deduced that the nebulae and the Milky Way were "congeries of innumerable stars grouped together in clusters" too small and distant to be resolved into individual stars by the naked eye.

In the last part of "Sidereus Nuncius", Galileo reported his discovery of four objects that appeared to form a straight line of stars near Jupiter. On the first night he detected a line of three little stars close to Jupiter parallel to the ecliptic; the following nights brought different arrangements and another star into his view, totalling four stars around Jupiter.
Throughout the text, Galileo gave illustrations of the relative positions of Jupiter and its apparent companion stars as they appeared nightly from late January through early March 1610. That they changed their positions relative to Jupiter from night to night and yet always appeared in the same straight line near it, persuaded Galileo that they were orbiting Jupiter. On January 11 after four nights of observation he wrote:

In his drawings, Galileo used an open circle to represent Jupiter and asterisks to represent the four stars. He made this distinction to show that there was in fact a difference between these two types of celestial bodies. It is important to note that Galileo used the terms "planet" and "star" interchangeably, and “both words were correct usage within the prevailing Aristotelian terminology.”

At the time of "Sidereus Nuncius"' publication, Galileo was a mathematician at the University of Padua and had recently received a lifetime contract for his work in building more powerful telescopes. He desired to return to Florence, and in hopes of gaining patronage there, he dedicated "Sidereus Nuncius" to his former pupil, now the Grand Duke of Tuscany, Cosimo II de' Medici. In addition, he named his discovered four moons of Jupiter the "Medicean Stars," in honor of the four royal Medici brothers. This helped him receive the position of Chief Mathematician and Philosopher to the Medici at the University of Pisa. Ultimately, his effort at naming the moons failed, for they are now referred to as the "Galilean moons".

The reactions to "Sidereus Nuncius", ranging from appraisal and hostility to disbelief, soon spread throughout Italy and England. Many poems and texts were published expressing love for the new form of astronomical science. Three works of art were even created in response to Galileo's book: Adam Elsheimer’s "The Flight into Egypt" (1610), Lodovico Cigoli’s "Assumption of the Virgin" (1612), and Andrea Sacchi’s "Divine Wisdom" (1631). In addition, the discovery of the Medicean Stars fascinated other astronomers, and they wanted to view the moons for themselves. Their efforts “set the stage for the modern scientific requirement of experimental reproducibility by independent researchers. Verification versus falsifiability…saw their origins in the announcement of "Sidereus Nuncius."”

But many individuals and communities were sceptical. A common response to the Medicean Stars was simply to say that the telescope had a lens defect and was producing illusory points of light and images; those saying this completely denied the existence of the moons. That only a few could initially see and verify what Galileo had observed supported the supposition that the optical theory during this period “could not clearly demonstrate that the instrument was not deceiving the senses.” By naming the four moons after the Medici brothers and convincing the Grand Duke Cosimo II of his discoveries, the defence of Galileo's reports became a matter of State. Moran notes, “the court itself became actively involved in pursuing the confirmation of Galileo’s observations by paying Galileo out of its treasury to manufacture spyglasses that could be sent through ambassadorial channels to the major courts of Europe."

The first astronomer to publicly support Galileo's findings was Johannes Kepler, who published an open letter in April 1610, enthusiastically endorsing Galileo's credibility. It wasn't until August 1610 that Kepler was able to publish his independent confirmation of Galileo's findings, due to the scarcity of sufficiently powerful telescopes.

Several astronomers, such as Thomas Harriot, Joseph Gaultier de la Vatelle, Nicolas-Claude Fabri de Peiresc, and Simon Marius, published their confirmation of the Medicean Stars after Jupiter became visible again in the autumn of 1610. Marius, a German astronomer who had studied with Tycho Brahe, was the first to publish a book of his observations. Marius attacked Galileo in "Mundus Jovialis" (published in 1614) by insisting that he had found Jupiter's four moons before Galileo and had been observing them since 1609. Marius believed that he therefore had the right to name them, which he did: he named them after Jupiter's love conquests: Io, Europa, Ganymede, and Callisto. But Galileo was not confounded —- he pointed out that being outside the Church, Marius had not yet accepted the Gregorian calendar and was still using the Julian calendar. Therefore, the night Galileo first observed Jupiter's moons was January 7, 1610 on the Gregorian calendar -— December 28, 1609 on the Julian calendar (Marius claimed to have first observed Jupiter's moons on December 29, 1609) Although Galileo did indeed discover Jupiter's four moons before Marius, Io, Europa, Ganymede, and Callisto are now the names of Galileo's four moons.

Galileo's drawings of an imperfect moon directly contradicted Ptolemy’s and Aristotle’s cosmological descriptions of perfect and unchanging heavenly bodies made of quintessence (the fifth element in ancient and medieval philosophy of which the celestial bodies are composed).

Before the publication of "Sidereus Nuncius", the Catholic Church accepted the Copernican heliocentric system as strictly mathematical and hypothetical. However, once Galileo began to speak of the Copernican system as fact rather than theory, it introduced “a more chaotic system, a less-than-godly lack of organization.” In fact, the Copernican system that Galileo believed to be real challenged the scripture, "which referred to the sun ‘rising’ and the earth as ‘unmoving.’"

The conflict between Galileo and the church ended in 1633 with his being sentenced to a form of house arrest by the Catholic Church.

The earliest known telescope appeared in 1608 in the Netherlands when an eyeglass maker named Hans Lippershey tried to obtain a patent on one. Although Lippershey did not receive his patent, news of the new invention soon spread across Europe. The design of these early refracting telescopes consisted of a convex objective lens and a concave eyepiece. Galileo improved on this design the following year and applied it to astronomy. In 1611, Johannes Kepler described how a far more useful telescope could be made with a convex objective lens and a convex eyepiece lens and by 1655 astronomers such as Christiaan Huygens were building powerful but unwieldy Keplerian telescopes with compound eyepieces.

History of the telescope

Isaac Newton is credited with building the first reflector in 1668 with a design that incorporated a small flat diagonal mirror to reflect the light to an eyepiece mounted on the side of the telescope. Laurent Cassegrain in 1672 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 learned of Hall's invention and began producing telescopes using it 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 Léon Foucault in 1857; and the adoption of long-lasting aluminized coatings on reflector mirrors in 1932. The Ritchey-Chretien variant of Cassegrain reflector was invented around 1910, but not widely adopted until after 1950; many modern telescopes including the Hubble Space Telescope use this design, which gives a wider field of view than a classic Cassegrain.

During the period 1850–1900, reflectors suffered from problems with speculum metal mirrors, and a considerable number of "Great Refractors" were built from 60 cm to 1 metre aperture, culminating in the Yerkes Observatory refractor in 1897; however, starting from the early 1900s a series of ever-larger reflectors with glass mirrors were built, including the Mount Wilson 60-inch (1.5 metre), the 100-inch (2.5 metre) Hooker Telescope (1917) and the 200-inch (5 metre) Hale telescope (1948); essentially all major research telescopes since 1900 have been reflectors. A number of 4-metre class (160 inch) telescopes were built on superior higher altitude sites including Hawaii and the Chilean desert in the 1975–1985 era. The development of the computer-controlled alt-azimuth mount in the 1970s and active optics in the 1980s enabled a new generation of even larger telescopes, starting with the 10-metre (400 inch) Keck telescopes in 1993/1996, and a number of 8-metre telescopes including the ESO Very Large Telescope, Gemini Observatory and Subaru Telescope.

The era of radio telescopes (along with radio astronomy) was born with Karl Guthe Jansky's serendipitous discovery of an astronomical radio source in 1931. Many types of telescopes were developed in the 20th century for a wide range of wavelengths from radio to gamma-rays. The development of space observatories after 1960 allowed access
to several bands impossible to observe from the ground, including X-rays and longer wavelength infrared bands.

Objects resembling lenses date back 4000 years although it is unknown if they were used for their optical properties or just as decoration.
Greek accounts of the optical properties of water filled spheres (5th century BC) followed by many centuries of writings on optics, including Ptolemy (2nd century) in his "Optics", who wrote about the properties of light including reflection, refraction, and color, followed by Ibn Sahl (10th century) and Ibn Al-Haytham (11th century).

Actual use of lenses dates back to the widespread manufacture and use of eyeglasses in Northern Italy beginning in the late 13th century. The invention of the use of concave lenses to correct near-sightedness is ascribed to Nicholas of Cusa in 1451.

The first record of a telescope comes from the Netherlands in 1608. It is in a patent filed by Middelburg spectacle-maker Hans Lippershey with the States General of the Netherlands on 2 October 1608 for his instrument ""for seeing things far away as if they were nearby"". A few weeks later another Dutch instrument-maker, Jacob Metius also applied for a patent. The States General did not award a patent since the knowledge of the device already seemed to be ubiquitous but the Dutch government awarded Lippershey with a contract for copies of his design.

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 this date of "invention", and rapidly found their way all over Europe.

In 1655 Dutch diplomat William de Boreel tried to solve the mystery of who invented the telescope. He had a local magistrate in Middelburg follow up on Boreel's childhood and early adult recollections of a spectacle maker named "Hans" who he remembered as the inventor of the telescope. The magistrate was contacted by a then unknown claimant, Middelburg spectacle maker Johannes Zachariassen, who testified that his father, Zacharias Janssen invented the telescope and the microscope as early as 1590. This testimony seemed convincing to Boreel, who now recollected that Zacharias and his father, Hans Martens, must have been who he remembered. Boreel's conclusion that Zacharias Janssen invented the telescope a little ahead of another spectacle maker, Hans Lippershey, was adopted by Pierre Borel in his 1656 book "De vero telescopii inventore". Discrepancies in Boreel's investigation and Zachariassen's testimony (including Zachariassen misrepresenting his date of birth and role in the invention) has led some historians to consider this claim dubious. The "Janssen" claim would continue over the years and be added on to with Zacharias Snijder in 1841 presenting 4 iron tubes with lenses in them claimed to be 1590 examples of Janssen's telescope and historian Cornelis de Waard's 1906 claim that the man who tried to sell a broken telescope to astronomer Simon Marius at the 1608 Frankfurt Book Fair must have been Janssen.

In 1682, the minutes of the Royal Society in London Robert Hooke noted Thomas Digges' 1571 "Pantometria", (a book on measurement, partially based on his father Leonard Digges' notes and observations) seemed to support an English claim to the invention of the telescope, describing Leonard as having a "fare seeing glass" in the mid 1500s based on an idea by Roger Bacon. Thomas described it as ""by proportional Glasses duly situate in convenient angles, not only discovered things far off, read letters, numbered pieces of money with the very coin and superscription thereof, cast by some of his friends of purpose upon downs in open fields, but also seven miles off declared what hath been done at that instant in private places"." Comments on the use of proportional or "perspective glass" are also made in the writings of John Dee (1575) and William Bourne (1585). Bourne was asked in 1580 to investigate the Diggs device by Queen Elizabeth I's chief advisor Lord Burghley. Bourne's is the best description of it, and from his writing it seemed to consist of peering into a large curved mirror that reflected the image produced by a large lens. The idea of an "Elizabethan Telescope" has been expanded over the years, including astronomer and historian Colin Ronan concluding in the 1990s that this reflecting/refracting telescope was built by Leonard Digges between 1540 and 1559. This "backwards" reflecting telescope would have been unwieldy, it needed very large mirrors and lens to work, the observer had to stand backwards to look at an upside down view, and Bourne noted it had a very narrow field of view making it unsuitable for military purposes. The optical performance required to see the details of coins lying about in fields, or private activities seven miles away, seems to be far beyond the technology of the time and it could be the "perspective glass" being described was a far simpler idea, originating with Bacon, of using a single lens held in front of the eye to magnify a distant view.

A 1959 research paper by Simon de Guilleuma claimed that evidence he had uncovered pointed to the French born spectacle maker Juan Roget (died before 1624) as another possible builder of an early telescope that predated Hans Lipperhey's patent application.

Lippershey's application for a patent was mentioned at the end of a diplomatic report on an embassy to Holland from the Kingdom of Siam sent by the Siamese king Ekathotsarot: "Ambassades du Roy de Siam envoyé à l'Excellence du Prince Maurice, arrivé à La Haye le 10 Septemb. 1608" ("Embassy of the King of Siam sent to his Excellency Prince Maurice, arrived at The Hague on 10 September 1608"). This report was issued in October 1608 and distributed across Europe, leading to experiments by other scientists, such as the Italian Paolo Sarpi, who received the report in November, and the English mathematician and astronomer Thomas Harriot, who used a six-powered telescope by the summer of 1609 to observe features on the moon.

The Italian polymath Galileo Galilei was in Venice in June 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 spent his time to improving the telescope, producing telescopes of increased power. His first telescope had a 3x magnification, but he soon made instruments which magnified 8x and finally, one nearly a meter long with a 37mm objective (which he would stop down to 16mm or 12mm) and a 23x magnification. With this last instrument he began a series of astronomical observations in October or November 1609, discovering the satellites of Jupiter, hills and valleys on the Moon, the phases of Venus and observed spots on the sun (using the projection method rather than direct observation). Galileo noted that the revolution of the satellites of Jupiter, the phases of Venus, rotation of the Sun and the tilted path its spots followed for part of the year pointed to the validity of the sun-centered Copernican system over other Earth-centered systems such as the one proposed by Ptolemy. Galileo's instrument was the first to be given the name "telescope". The name was invented by the Greek poet/theologian Giovanni Demisiani at a banquet held on April 14, 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 observations, together with Galileo's improvement of the instrument, led to the adoption of the name of the Galilean telescope for these early forms of telescope that employed a negative lens.

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 and a 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.

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 long. Astronomers such as Johannes Hevelius were constructing telescopes with focal lengths as long as. Besides having really long tubes these telescopes needed scaffolding or long masts and cranes to hold them up. Their value as research tools was minimal since the telescope's frame "tube" flexed and vibrated in the slightest breeze and sometimes collapsed altogether.

In some of the very long refracting telescopes constructed after 1675, no tube was employed at all. The objective was mounted on a swiveling ball-joint on top of a pole, tree, or any available tall structure and aimed by means of string or connecting rod. The eyepiece was handheld or mounted on a stand at the focus, and the image was found by trial and error. These were consequently termed aerial telescopes. and have been attributed to Christiaan Huygens and his brother Constantijn Huygens, Jr. although it is not clear that they invented it. Christiaan Huygens and his brother made objectives up to diameter and focal length and others such as Adrien Auzout made telescopes with focal lengths up to. Telescopes of such great length were naturally difficult to use and must have taxed to the utmost the skill and patience of the observers. Aerial telescopes were employed by several other astronomers. Cassini discovered Saturn's third and fourth satellites in 1684 with aerial telescope objectives made by Giuseppe Campani that were in focal length.

The ability of a curved mirror to form an image may have been known since the time of Euclid and had been extensively studied by Alhazen in the 11th century. Galileo, Giovanni Francesco Sagredo, and others, spurred on by their knowledge that curved mirrors had similar properties to lenses, discussed the idea of building a telescope using a mirror as the image forming objective. Niccolò Zucchi, an Italian Jesuit astronomer and physicist, wrote in his book "Optica philosophia" of 1652 that he tried replacing the lens of a refracting telescope with a bronze concave mirror in 1616. Zucchi tried looking into the mirror with a hand held concave lens but did not get a satisfactory image, possibly due to the poor quality of the mirror, the angle it was tilted at, or the fact that his head partially obstructed the image.

In 1636 Marin Mersenne proposed a telescope consisting of a paraboloidal primary mirror and a paraboloidal secondary mirror bouncing the image through a hole in the primary, solving the problem of viewing the image. James Gregory went into further detail in his book "Optica Promota" (1663), pointing out that a reflecting telescope with a mirror that was shaped like the part of a conic section, 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.

In 1666 Isaac Newton, based on his theories of refraction and color, perceived that the faults of the refracting telescope were due more to a lens's varying refraction of light of different colors than to a lens's imperfect shape. He concluded that light could not be refracted through a lens without causing chromatic aberrations, although he incorrectly concluded from some rough experiments that "all" refracting substances would diverge the prismatic colors in a constant proportion to their mean refraction. From these experiments Newton concluded that no improvement could be made in the refracting telescope. Newton's experiments with mirrors showed that they did not suffer from the chromatic errors of lenses, for all colors of light the angle of incidence reflected in a mirror was equal to the angle of reflection, so as a proof to his theories Newton set out to build a reflecting telescope. Newton completed his first telescope in 1668 and it is the earliest known functional reflecting telescope. After much experiment, he chose 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 chose a spherical shape for his mirror instead of a parabola to simplify construction. He added to his reflector what is the hallmark of the design of a ""Newtonian telescope"", a secondary "diagonal" mirror near the primary mirror's focus to reflect the image at 90° angle to an eyepiece mounted on the side of the telescope. This unique addition allowed the image to be viewed with minimal obstruction of the objective mirror. He also made all the tube, mount, and fittings. Newton's first compact reflecting telescope had a mirror diameter of 1.3 inches and a focal ratio of f/5. With it he found that he could see the four Galilean moons of Jupiter and the crescent phase of the planet Venus. Encouraged by this success, he made a second telescope with a magnifying power of 38x 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 for another 50 years until John Hadley (best known as the inventor of the octant) developed ways to make precision aspheric and parabolic speculum metal mirrors. In 1721 he showed the first parabolic Newtonian reflector to the Royal Society. It had a diameter, focal length speculum metal objective mirror. The instrument was examined by James Pound and James 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 a diameter aerial telescope originally presented to the Royal Society by Constantijn Huygens, Jr. 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 speculum metal, succeeded in producing large reflecting telescopes of their own, one of which had a focal length of. These methods of fabricating mirrors were passed on by Molyneux to two London opticians —Scarlet and Hearn— who started a business manufacturing telescopes.

The British mathematician, optician James Short began experimenting with building telescopes based on Gregory's designs in the 1730s. He first tried making his mirrors out of glass as suggested by Gregory, but he later switched to speculum metal mirrors creating Gregorian telescopes with original designers parabolic and elliptic figures. Short then adopted telescope-making as his profession which he practised first in Edinburgh, and afterward in London. All Short's telescopes were of the Gregorian form. Short died in London in 1768, having made a considerable fortune selling telescopes.

Since speculum metal mirror secondaries or diagonal mirrors greatly reduced the light that reached the eyepiece, several reflecting telescope designers tried to do away with them. In 1762 Mikhail Lomonosov presented a reflecting telescope before the Russian Academy of Sciences forum. It had its primary mirror tilted at four degrees to telescope's axis so the image could be viewed via an eyepiece mounted at the front of the telescope tube without the observer's head blocking the incoming light. This innovation was not published until 1827, so this type came to be called the Herschelian telescope after a similar design by William Herschel.
About the year 1774 William Herschel (then a teacher of music in Bath, England) began to occupy his leisure hours with the construction of reflector telescope mirrors, finally devoted himself entirely to their construction and use in astronomical research. In 1778, he selected a reflector mirror (the best of some 400 telescope mirrors which he had made) and with it, built a focal length telescope. Using this telescope, he made his early brilliant astronomical discoveries. In 1783, Herschel completed a reflector of approximately in diameter and focal length. He observed the heavens with this telescope for some twenty years, replacing the mirror several times. In 1789 Herschel finished building his largest reflecting telescope with a mirror of and a focal length of, (commonly known as his 40-foot telescope) at his new home, at Observatory House in Slough, England. 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. He discovered Saturn's sixth known moon, Enceladus, the first night he used it (August 28, 1789), and on September 17, its seventh known moon, Mimas. This telescope was world's largest telescope for over 50 years. However, this large scope was difficult to handle and thus less used than his favorite 18.7-inch reflector.

In 1845 William Parsons, 3rd Earl of Rosse built his Newtonian reflector called the "Leviathan of Parsonstown" with which he discovered the spiral form of galaxies.

All of these larger reflectors suffered from the poor reflectivity and fast tarnishing nature of their speculum metal mirrors. This meant they need more than one mirror per telescope since mirrors had to be frequently removed and re-polished. This was time consuming since the polishing process could change the curve of the mirror so it usually had to be "re-figured" to the correct shape.

From the time of the invention of the first refracting telescopes it was generally supposed that chromatic errors seen in lenses simply 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 an objective measuring with a relatively short focal length of.

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. However, it was ruled by Lord Mansfield that it was not the original inventor who ought to profit from such invention, but the one who brought it forth for the benefit of mankind.

In 1747, Leonhard Euler sent to the Prussian 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 lesser 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.

The difficulties with the impractical metal mirrors of reflecting telescopes led to the construction of large refracting telescopes. By 1866 refracting telescopes had reached in aperture with many larger ""Great refractors"" being built in the mid to late 19th century. In 1897, the refractor reached its maximum practical limit in a research telescope with the construction of the Yerkes Observatorys' refractor (although a larger refractor Great Paris Exhibition Telescope of 1900 with an objective of 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.

In 1856–57, Karl August von Steinheil and 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 of being able to be removed and re-deposited without changing the shape of the glass substrate. Towards the end of the 19th century very large silver on glass mirror reflecting telescopes were built.

The beginning of the 20th century saw construction of the first of the "modern" large research reflectors, designed for precision photographic imaging and located at remote high altitude clear sky locations such as the 60-inch Hale telescope of 1908, and the Hooker telescope in 1917, both located at Mount Wilson Observatory. 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. 1948 saw the completion of the Hale reflector at Mount Palomar which was the largest telescope in the world up until the completion of the massive BTA-6 in Russia twenty-seven years later. The Hale reflector introduced several technical innovations used in future telescopes, including hydrostatic bearings for very low friction, the Serrurier truss for equal deflections of the two mirrors as the tube sags under gravity, and the use of Pyrex low-expansion glass for the mirrors. The arrival of substantially larger telescopes had to await the introduction of methods other than the rigidity of glass to maintain the proper shape of the mirror.

The 1980s saw the introduction of two new technologies for building larger telescopes and improving image quality,
known as active optics and adaptive optics. In active optics, an image analyser senses the aberrations of a star image
a few times per minute, and a computer adjusts many support forces on the primary mirror and the location of the secondary mirror
to maintain the optics in optimal shape and alignment. This is too slow to correct for atmospheric blurring effects, but enables the use of thin single mirrors up to 8 m diameter, or even larger segmented mirrors. This method was pioneered by the ESO New Technology Telescope in the late 1980s.

The 1990s saw a new generation of giant telescopes appear using active optics, beginning with the construction of the first of the two 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.

Adaptive optics uses a similar principle, but applying corrections several hundred times per second to
compensate the effects of rapidly changing optical distortion due to the motion of turbulence in the Earth's atmosphere. Adaptive optics works by measuring the distortions in a wavefront and then compensating for them by rapid changes of actuators applied to a small 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 and detector technology during the 1990s made it possible to calculate the compensation needed in real time. In adaptive optics, the high-speed corrections needed mean that a fairly bright star is needed very close to the target of interest (or an artificial star is created by a laser). Also, with a single star or laser the corrections are only effective over a very narrow field (tens of arcsec), and current systems operating on several 8-10m telescopes work mainly in near-infrared wavelengths for single-object observations.

Developments of adaptive optics include systems with multiple lasers over a wider corrected field, and/or working above kiloHertz rates for good correction at visible wavelengths; these are currently in progress but not yet in routine operation as of 2015.

The twentieth century saw the construction of telescopes which could produce images using wavelengths other than visible light starting in 1931 when Karl Jansky discovered astronomical objects gave off radio emissions; 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 astronomy began in 1931 when Karl Jansky discovered that the Milky Way was a source of radio emission while doing research on terrestrial static with a direction antenna. Building on Jansky's work, Grote Reber built a more sophisticated purpose-built radio telescope in 1937, with a 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 Jodrell bank telescope (1957), the Green Bank Telescope (1962), and the Effelsberg telescope (1971). The huge 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 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.

A telescope like the Large Millimeter Telescope (active since 2006) observes from, bridging between the far-infrared/submillimeter telescopes and longer wavelength radio telescopes including the microwave band from about to in wavelength.

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 UKIRT, and the 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 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.

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 alternative coatings such as magnesium fluoride or lithium fluoride are used instead. The Orbiting Solar Observatory 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 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 operating at these wavelengths.

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).

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).

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 Albert A. 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 & Francis G. 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 [https://www.webcitation.org/5kmngkBFy?url=http://www.geocities.com/CapeCanaveral/2309/page1.html

In 2008, Max Tegmark and Matias Zaldarriaga proposed a "Fast Fourier Transform Telescope" design in which the lenses and mirrors could be dispensed with altogether when computers become fast enough to perform all the necessary transforms.

Refractors were the earliest type of optical telescope. The first practical refracting telescopes appeared in the Netherlands about 1608, and were credited to three individuals, Hans Lippershey and Zacharias Janssen, spectacle-makers in Middelburg, and Jacob Metius of Alkmaar. Galileo Galilei, happening to be in Venice in about the month of May 1609, heard of the invention and constructed a version of his own. Galileo then communicated the details of his invention to the public, and presented the instrument itself to the Doge Leonardo Donato, sitting in full council.

Refracting telescope

A refracting telescope (also called a refractor) is a type of optical telescope that uses a lens as its objective to form an image (also referred to a dioptric telescope). The refracting telescope design was originally used in spy glasses and astronomical telescopes but is also used for long focus camera lenses. Although large refracting telescopes were very popular in the second half of the 19th century, for most research purposes the refracting telescope has been superseded by the reflecting telescope which allows larger apertures. A refractor's magnification is calculated by dividing the focal length of the objective lens by that of the eyepiece.

All refracting telescopes use the same principles. The combination of an objective lens 1 and some type of eyepiece 2 is used to gather more light than the human eye is able to collect on its own, focus it 5, and present the viewer with a brighter, clearer, and magnified virtual image 6.

The objective in a refracting telescope refracts or bends light. This refraction causes parallel light rays to converge at a focal point; while those not parallel converge upon a focal plane. The telescope converts a bundle of parallel rays to make an angle α, with the optical axis to a second parallel bundle with angle β. The ratio β/α is called the angular magnification. It equals the ratio between the retinal image sizes obtained with and without the telescope.

Refracting telescopes can come in many different configurations to correct for image orientation and types of aberration. Because the image was formed by the bending of light, or refraction, these telescopes are called "refracting telescopes" or "refractors".

The design Galileo Galilei used in 1609 is commonly called a Galilean telescope. It used a convergent (plano-convex) objective lens and a divergent (plano-concave) eyepiece lens (Galileo, 1610). A Galilean telescope, because the design has no intermediary focus, results in a non-inverted and upright image.

Galileo’s best telescope magnified objects about 30 times. Because of flaws in its design, such as the shape of the lens and the narrow field of view, the images were blurry and distorted. Despite these flaws, the telescope was still good enough for Galileo to explore the sky. The Galilean telescope could view the phases of Venus, and was able to see craters on the Moon and four moons orbiting Jupiter.

Parallel rays of light from a distant object (y) would be brought to a focus in the focal plane of the objective lens (F′ L1 / y′). The (diverging) eyepiece (L2) lens intercepts these rays and renders them parallel once more. Non-parallel rays of light from the object traveling at an angle α1 to the optical axis travel at a larger angle (α2 > α1) after they passed through the eyepiece. This leads to an increase in the apparent angular size and is responsible for the perceived magnification.

The final image (y″) is a virtual image, located at infinity and is the same way up as the object.

The Keplerian telescope, invented by Johannes Kepler in 1611, is an improvement on Galileo's design. It uses a convex lens as the eyepiece instead of Galileo's concave one. The advantage of this arrangement is that the rays of light emerging from the eyepiece are converging. This allows for a much wider field of view and greater eye relief, but the image for the viewer is inverted. Considerably higher magnifications can be reached with this design, but to overcome aberrations the simple objective lens needs to have a very high f-ratio (Johannes Hevelius built one with a focal length, and even longer tubeless "aerial telescopes" were constructed). The design also allows for use of a micrometer at the focal plane (used to determine the angular size and/or distance between objects observed).

The achromatic refracting lens was invented in 1733 by an English barrister named Chester Moore Hall, although it was independently invented and patented by John Dollond around 1758. The design overcame the need for very long focal lengths in refracting telescopes by using an objective made of two pieces of glass with different dispersion, 'crown' and 'flint glass', to limit the effects of chromatic and spherical aberration. Each side of each piece is ground and polished, and then the two pieces are assembled together. Achromatic lenses are corrected to bring two wavelengths (typically red and blue) into focus in the same plane. The era of the 'great refractors' in the 19th century saw large achromatic lenses culminating with the largest achromatic refractor ever built, the Great Paris Exhibition Telescope of 1900.

Apochromatic refractors have objectives built with special, extra-low dispersion materials. They are designed to bring three wavelengths (typically red, green, and blue) into focus in the same plane. The residual color error (tertiary spectrum) can be up to an order of magnitude less than that of an achromatic lens. Such telescopes contain elements of fluorite or special, extra-low dispersion (ED) glass in the objective and produce a very crisp image that is virtually free of chromatic aberration. Due to the special materials needed in the fabrication, apochromatic refractors are usually more expensive than telescopes of other types with a comparable aperture.

Refractors suffer from residual chromatic and spherical aberration. This affects shorter focal ratios more than longer ones. A achromatic refractor is likely to show considerable color fringing (generally a purple halo around bright objects). A 16 has little color fringing.

In very large apertures, there is also a problem of lens sagging, a result of gravity deforming glass. Since a lens can only be held in place by its edge, the center of a large lens sags due to gravity, distorting the images it produces. The largest practical lens size in a refracting telescope is around.

There is a further problem of glass defects, striae or small air bubbles trapped within the glass. In addition, glass is opaque to certain wavelengths, and even visible light is dimmed by reflection and absorption when it crosses the air-glass interfaces and passes through the glass itself. Most of these problems are avoided or diminished in reflecting telescopes, which can be made in far larger apertures and which have all but replaced refractors for astronomical research.

A list of the largest refracting telescopes over diameter.

The telescope is more a discovery of optical craftsmen than an invention of a scientist. The lens and the properties of refracting and reflecting light had been known since antiquity and theory on how they worked were developed by ancient Greek philosophers, preserved and expanded on in the medieval Islamic world, and had reached a significantly advanced state by the time of the telescope's invention in early modern Europe. But the most significant step cited in the invention of the telescope was the development of lens manufacture for spectacles, first in Venice and Florence in the thirteenth century, and later in the spectacle making centers in both the Netherlands and Germany. It is in the Netherlands in 1608 where the first recorded optical telescopes (refracting telescopes) appeared. The invention is credited to the spectacle makers Hans Lippershey and Zacharias Janssen in Middelburg, and the instrument-maker and optician Jacob Metius of Alkmaar.

Optical telescope

An optical telescope is a telescope that gathers and focuses light, mainly from the visible part of the electromagnetic spectrum, to create a magnified image for direct view, or to make a photograph, or to collect data through electronic image sensors.

There are three primary types of optical telescope:

A telescope's light gathering power and ability to resolve small detail is directly related to the diameter (or aperture) of its objective (the primary lens or mirror that collects and focuses the light). The larger the objective, the more light the telescope collects and the finer detail it resolves.

People use telescopes and binoculars for activities such as observational astronomy, ornithology, pilotage and reconnaissance, and watching sports or performance arts.

Galileo greatly improved on these designs the following year, and is generally credited as the first to use a telescope for astronomy. Galileo's telescope used Hans Lippershey's design of a convex objective lens and a concave eye lens, and this design is now called a Galilean telescope. Johannes Kepler proposed an improvement on the design that used a convex eyepiece, often called the Keplerian Telescope.

The next big step in the development of refractors was the advent of the Achromatic lens in the early 18th century, which corrected the chromatic aberration in Keplerian telescopes up to that time—allowing for much shorter instruments with much larger objectives.

For reflecting telescopes, which use a curved mirror in place of the objective lens, theory preceded practice. The theoretical basis for curved mirrors behaving similar to lenses was probably established by Alhazen, whose theories had been widely disseminated in Latin translations of his work. Soon after the invention of the refracting telescope Galileo, Giovanni Francesco Sagredo, and others, spurred on by their knowledge that curved mirrors had similar properties as lenses, discussed the idea of building a telescope using a mirror as the image forming objective. The potential advantages of using parabolic mirrors (primarily a reduction of spherical aberration with elimination of chromatic aberration) led to several proposed designs for reflecting telescopes, the most notable of which was published in 1663 by James Gregory and came to be called the Gregorian telescope, but no working models were built. Isaac Newton has been generally credited with constructing the first practical reflecting telescopes, the Newtonian telescope, in 1668 although due to their difficulty of construction and the poor performance of the speculum metal mirrors used it took over 100 years for reflectors to become popular. Many of the advances in reflecting telescopes included the perfection of parabolic mirror fabrication in the 18th century, silver coated glass mirrors in the 19th century, long-lasting aluminum coatings in the 20th century, segmented mirrors to allow larger diameters, and active optics to compensate for gravitational deformation. A mid-20th century innovation was catadioptric telescopes such as the Schmidt camera, which uses both a lens (corrector plate) and mirror as primary optical elements, mainly used for wide field imaging without spherical aberration.

The late 20th century has seen the development of adaptive optics and space telescopes to overcome the problems of astronomical seeing.

The basic scheme is that the primary light-gathering element the objective (1) (the convex lens or concave mirror used to gather the incoming light), focuses that light from the distant object (4) to a focal plane where it forms a real image (5). This image may be recorded or viewed through an eyepiece (2), which acts like a magnifying glass. The eye (3) then sees an inverted magnified virtual image (6) of the object.

Most telescope designs produce an inverted image at the focal plane; these are referred to as "inverting telescopes". In fact, the image is both turned upside down and reversed left to right, so that altogether it is rotated by 180 degrees from the object orientation. In astronomical telescopes the rotated view is normally not corrected, since it does not affect how the telescope is used. However, a mirror diagonal is often used to place the eyepiece in a more convenient viewing location, and in that case the image is erect, but still reversed left to right. In terrestrial telescopes such as spotting scopes, monoculars and binoculars, prisms (e.g., Porro prisms) or a relay lens between objective and eyepiece are used to correct the image orientation. There are telescope designs that do not present an inverted image such as the Galilean refractor and the Gregorian reflector. These are referred to as "erecting telescopes".

Many types of telescope fold or divert the optical path with secondary or tertiary mirrors. These may be integral part of the optical design (Newtonian telescope, Cassegrain reflector or similar types), or may simply be used to place the eyepiece or detector at a more convenient position. Telescope designs may also use specially designed additional lenses or mirrors to improve image quality over a larger field of view.
Design specifications relate to the characteristics of the telescope and how it performs optically. Several properties of the specifications may change with the equipment or accessories used with the telescope; such as Barlow lenses, star diagonals and eyepieces. These interchangeable accessories don't alter the specifications of the telescope, however they alter the way the telescopes properties function, typically magnification, apparent field of view and FOV.

The smallest resolvable surface area of an object, as seen through an optical telescope, is the limited physical area that can be resolved. It is analogous to angular resolution, but differs in definition: instead of separation ability between point-light sources it refers to the physical area that can be resolved. A familiar way to express the characteristic is the resolvable ability of features such as Moon craters or Sun spots. Expression using the formula is given by the sum of twice the resolving power formula_1 over aperture diameter formula_2 multiplied by the objects diameter formula_3 multiplied by the constant formula_4 all divided by the objects apparent diameter formula_5.

"Resolving power formula_1 is derived from the wavelength formula_7 using the same unit as aperture; where 550 nm to mm is given by: formula_8."
"The constant formula_4 is derived from radians to the same unit as the objects apparent diameter; where the Moons apparent diameter of formula_10 radians to arcsecs is given by: formula_11."

An example using a telescope with an aperture of 130 mm observing the Moon in a 550 nm wavelength, is given by: formula_12

The unit used in the object diameter results in the smallest resolvable features at that unit. In the above example they are approximated in kilometers resulting in the smallest resolvable Moon craters being 3.22 km in diameter. The Hubble Space Telescope has a primary mirror aperture of 2400 mm that provides a surface resolvability of Moon craters being 174.9 meters in diameter, or sunspots of 7365.2 km in diameter.

Ignoring blurring of the image by turbulence in the atmosphere (atmospheric seeing) and optical imperfections of the telescope, the angular resolution of an optical telescope is determined by the diameter of the primary mirror or lens gathering the light (also termed its "aperture").

The Rayleigh criterion for the resolution limit formula_13 (in radians) is given by
where formula_15 is the wavelength and formula_2 is the aperture. For visible light (formula_15 = 550 nm) in the small-angle approximation, this equation can be rewritten:
Here, formula_13 denotes the resolution limit in arcseconds and formula_2 is in millimeters.
In the ideal case, the two components of a double star system can be discerned even if separated by slightly less than formula_13. This is taken into account by the Dawes limit
The equation shows that, all else being equal, the larger the aperture, the better the angular resolution. The resolution is not given by the maximum magnification (or "power") of a telescope. Telescopes marketed by giving high values of the maximum power often deliver poor images.

For large ground-based telescopes, the resolution is limited by atmospheric seeing. This limit can be overcome by placing the telescopes above the atmosphere, e.g., on the summits of high mountains, on balloon and high-flying airplanes, or in space. Resolution limits can also be overcome by adaptive optics, speckle imaging or lucky imaging for ground-based telescopes.

Recently, it has become practical to perform aperture synthesis with arrays of optical telescopes. Very high resolution images can be obtained with groups of widely spaced smaller telescopes, linked together by carefully controlled optical paths, but these interferometers can only be used for imaging bright objects such as stars or measuring the bright cores of active galaxies.

The focal length of an optical system is a measure of how strongly the system converges or diverges light. For an optical system in air, it is the distance over which initially collimated rays are brought to a focus. A system with a shorter focal length has greater optical power than one with a long focal length; that is, it bends the rays more strongly, bringing them to a focus in a shorter distance. In astronomy, the f-number is commonly referred to as the "focal ratio" notated as formula_23. The focal ratio of a telescope is defined as the focal length formula_24 of an objective divided by its diameter formula_2 or by the diameter of an aperture stop in the system. The focal length controls the field of view of the instrument and the scale of the image that is presented at the focal plane to an eyepiece, film plate, or CCD.

An example of a telescope with a focal length of 1200 mm and aperture diameter of 254 mm is given by:
formula_26

Numerically large Focal ratios are said to be "long" or "slow". Small numbers are "short" or "fast". There are no sharp lines for determining when to use these terms, and an individual may consider their own standards of determination. Among contemporary astronomical telescopes, any telescope with a focal ratio slower (bigger number) than f/12 is generally considered slow, and any telescope with a focal ratio faster (smaller number) than f/6, is considered fast. Faster systems often have more optical aberrations away from the center of the field of view and are generally more demanding of eyepiece designs than slower ones. A fast system is often desired for practical purposes in astrophotography with the purpose of gathering more photons in a given time period than a slower system, allowing time lapsed photography to process the result faster.

Wide-field telescopes (such as astrographs), are used to track satellites and asteroids, for cosmic-ray research, and for astronomical surveys of the sky. It is more difficult to reduce optical aberrations in telescopes with low f-ratio than in telescopes with larger f-ratio.

The light-gathering power of an optical telescope, also referred to as light grasp or aperture gain, is the ability of a telescope to collect a lot more light than the human eye. Its light-gathering power is probably its most important feature. The telescope acts as a "light bucket", collecting all of the photons that come down on it from a far away object, where a larger bucket catches more photons resulting in more received light in a given time period, effectively brightening the image. This is why the pupils of your eyes enlarge at night so that more light reaches the retinas. The gathering power formula_27 compared against a human eye is the squared result of the division of the aperture formula_2 over the observer's pupil diameter formula_29, with an average adult having a pupil diameter of 7mm. Younger persons host larger diameters, typically said to be 9mm, as the diameter of the pupil decreases with age.

An example gathering power of an aperture with 254 mm compared to an adult pupil diameter being 7 mm is given by: formula_30

Light-gathering power can be compared between telescopes by comparing the areas formula_31 of the two different apertures.

As an example, the light-gathering power of a 10 meter telescope is 25x that of a 2 meter telescope: formula_32

For a survey of a given area, the field of view is just as important as raw light gathering power. Survey telescopes such as the Large Synoptic Survey Telescope try to maximize the product of mirror area and field of view (or etendue) rather than raw light gathering ability alone.

The magnification through a telescope magnifies a viewing object while limiting the FOV. Magnification is often misleading as the optical power of the telescope, its characteristic is the most misunderstood term used to describe the observable world. At higher magnifications the image quality significantly reduces, usage of a Barlow lens—which increases the effective focal length of an optical system—multiplies image quality reduction.

Similar minor effects may be present when using star diagonals, as light travels through a multitude of lenses that increase or decrease effective focal length. The quality of the image generally depends on the quality of the optics (lenses) and viewing conditions—not on magnification.

Magnification itself is limited by optical characteristics. With any telescope or microscope, beyond a practical maximum magnification, the image looks bigger but shows no more detail. It occurs when the finest detail the instrument can resolve is magnified to match the finest detail the eye can see. Magnification beyond this maximum is sometimes called "empty magnification".

To get the most detail out of a telescope, it is critical to choose the right magnification for the object being observed. Some objects appear best at low power, some at high power, and many at a moderate magnification. There are two values for magnification, a minimum and maximum. A wider field of view eyepiece may be used to keep the same eyepiece focal length whilst providing the same magnification through the telescope. For a good quality telescope operating in good atmospheric conditions, the maximum usable magnification is limited by diffraction.

The visual magnification formula_33 of the field of view through a telescope can be determined by the telescopes focal length formula_24 divided by the eyepiece focal length formula_35 (or diameter). The maximum is limited by the focal length of the eyepiece.

An example of visual magnification using a telescope with a 1200 mm focal length and 3 mm eyepiece is given by: formula_36

There is a lowest usable magnification on a telescope. The increase in brightness with reduced magnification has a limit related to something called the exit pupil. The exit pupil is the cylinder of light coming out of the eyepiece, hence the lower the magnification, the larger the exit pupil. The minimum formula_37 can be calculated by dividing the telescope aperture formula_2 over the exit pupil diameter formula_39. Decreasing the magnification past this limit cannot increase brightness, at this limit there is no benefit for decreased magnification. Likewise calculating the exit pupil formula_39 is a division of the aperture diameter formula_2 and the visual magnification formula_33 used. The minimum often may not be reachable with some telescopes, a telescope with a very long focal length may require a "longer-focal-length" eyepiece than is possible.

An example of the lowest usable magnification using a 254 mm aperture and 7 mm exit pupil is given by: formula_43, whilst the exit pupil diameter using a 254 mm aperture and 36x magnification is given by: formula_44

A useful reference is:
Only personal experience determines the best optimum magnifications for objects, relying on observational skills and seeing conditions.

Field of view is the extent of the observable world seen at any given moment, through an instrument (e.g., telescope or binoculars), or by naked eye. There are various expressions of field of view, being a specification of an eyepiece or a characteristic determined from and eyepiece and telescope combination. A physical limit derives from the combination where the FOV cannot be viewed larger than a defined maximum, due to diffraction of the optics.

Apparent FOV is the observable world observed through an ocular eyepiece without insertion into a telescope. It is limited by the barrel size used in a telescope, generally with modern telescopes that being either 1.25 or 2 inches in diameter. A wider FOV may be used to achieve a more vast observable world given the same magnification compared with a smaller FOV without compromise to magnification. Note that increasing the FOV lowers surface brightness of an observed object, as the gathered light is spread over more area, in relative terms increasing the observing area proportionally lowers surface brightness dimming the observed object. Wide FOV eyepieces work best at low magnifications with large apertures, where the relative size of an object is viewed at higher comparative standards with minimal magnification giving an overall brighter image to begin with.

True FOV is the observable world observed though an ocular eyepiece inserted into a telescope. Knowing the true FOV of eyepieces is very useful since it can be used to compare what is seen through the eyepiece to printed or computerized star charts that help identify what is observed. True FOV formula_45 is the division of apparent FOV formula_46 over magnification formula_33.

An example of true FOV using an eyepiece with 52° apparent FOV used at 81.25x magnification is given by: formula_48

Max FOV is a term used to describe the maximum useful true FOV limited by the optics of the telescope, it is a physical limitation where increases beyond the maximum remain at maximum. Max FOV formula_49 is the barrel size formula_50 over the telescopes focal length formula_24 converted from radian to degrees.

An example of max FOV using a telescope with a barrel size of 31.75 mm (1.25 inches) and focal length of 1200 mm is given by: formula_52

There are many properties of optical telescopes and the complexity of observation using one can be a daunting task; experience and experimentation are the major contributors to understanding how to maximize one's observations. In practice, only two main properties of a telescope determine how observation differs: the focal length and aperture. These relate as to how the optical system views an object or range and how much light is gathered through an ocular eyepiece. Eyepieces further determine how the field of view and magnification of the observable world change.

"Observable world" describes what can be seen using a telescope, when viewing an object or range the observer may use many different techniques. Understanding what can be viewed and how to view it depends on the field of view. Viewing an object at a size that fits entirely in the field of view is measured using the two telescope properties—focal length and aperture, with the inclusion of an ocular eyepiece with suitable focal length (or diameter). Comparing the observable world and the angular diameter of an object shows how much of the object we see. However, the relationship with the optical system may not result in high surface brightness. Celestial objects are often dim because of their vast distance, and detail may be limited by diffraction or unsuitable optical properties.

Finding what can be seen through the optical system begins with the eyepiece providing the field of view and magnification; the magnification is given by the division of the telescope and eyepiece focal lengths. Using an example of an amateur telescope such as a Newtonian telescope with an aperture formula_2 of 130 mm (5") and focal length formula_24 of 650 mm (25.5 inches), one uses an eyepiece with a focal length formula_55 of 8 mm and apparent field of view formula_46 of 52°. The magnification at which the observable world is viewed is given by: formula_57. The true field of view formula_45 requires the magnification, which is formulated by its division over the apparent field of view: formula_59. The resulting true field of view is 0.64°, allowing an object such as the Orion nebula, which appears elliptical with an angular diameter of 65 × 60 arcminutes, to be viewable through the telescope in its entirety, where the whole of the nebula is within the observable world. Using methods such as this can greatly increase one's viewing potential ensuring the observable world can contain the entire object, or whether to increase or decrease magnification viewing the object in a different aspect.

The surface brightness at such a magnification significantly reduces, resulting in a far dimmer appearance. A dimmer appearance results in less visual detail of the object. Details such as matter, rings, spiral arms, and gases may be completely hidden from the observer, giving a far less "complete" view of the object or range. Physics dictates that at the theoretical minimum magnification of the telescope, the surface brightness is at 100%. Practically, however, various factors prevent 100% brightness; these include telescope limitations (focal length, eyepiece focal length, etc.) and the age of the observer.

Age plays a role in brightness, as a contributing factor is the observer's pupil. With age the pupil naturally shrinks in diameter; generally accepted a young adult may have a 7 mm diameter pupil, an older adult as little as 5 mm, and a younger person larger at 9 mm. The minimum magnification formula_60 can be expressed as the division of the aperture formula_2 and pupil formula_62 diameter given by: formula_63. A problematic instance may be apparent, achieving a theoretical surface brightness of 100%, as the required effective focal length of the optical system may require an eyepiece with too large a diameter.

Some telescopes cannot achieve the theoretical surface brightness of 100%, while some telescopes can achieve it using a very small-diameter eyepiece. To find what eyepiece is required to get minimum magnification one can rearrange the magnification formula, where it is now the division of the telescope's focal length over the minimum magnification: formula_64. An eyepiece of 35 mm is a non-standard size and would not be purchasable; in this scenario
to achieve 100% one would require a standard manufactured eyepiece size of 40 mm. As the eyepiece has a larger focal length than the minimum magnification, an abundance of wasted light is not received through the eyes.

The increase in surface brightness as one reduces magnification is limited; that limitation is what is described as the exit pupil: a cylinder of light that projects out the eyepiece to the observer. An exit pupil must match or be smaller in diameter than one's pupil to receive the full amount of projected light; a larger exit pupil results in the wasted light. The exit pupil formula_65 can be derived with from division of the telescope aperture formula_2 and the minimum magnification formula_60, derived by: formula_68. The pupil and exit pupil are almost identical in diameter, giving no wasted observable light with the optical system. A 7 mm pupil falls slightly short of 100% brightness, where the surface brightness formula_50 can be measured from the product of the constant 2, by the square of the pupil formula_62 resulting in: formula_71. The limitation here is the pupil diameter; it's an unfortunate result and degrades with age. Some observable light loss is expected and decreasing the magnification cannot increase surface brightness once the system has reached its minimum usable magnification, hence why the term is referred to as "usable".

When using a CCD to record observations, the CCD is placed in the focal plane. Image scale (sometimes called "plate scale") describes how the angular size of the object being observed is related to the physical size of the projected image in the focal plane

where formula_73 is the image scale, formula_74 is the angular size of the observed object, and formula_75 is the physical size of the projected image. In terms of focal length image scale is

where formula_73 is measured in radians per meter (rad/m), and formula_24 is measured in meters. Normally formula_73 is given in units of arcseconds per millimeter ("/mm). So if the focal length is measured in millimeters, the image scale is

The derivation of this equation is fairly straightforward and the result is the same for reflecting or refracting telescopes. However, conceptually it is easier to derive by considering a reflecting telescope. If an extended object with angular size formula_74 is observed through a telescope, then due to the Laws of reflection and Trigonometry the size of the image projected onto the focal plane will be

Thefore, the image scale (angular size of object divided by size of projected image) will be

and by using the small angle relation formula_84, when formula_85 (N.B. only valid if formula_86 is in radians), we obtain

No telescope can form a perfect image. Even if a reflecting telescope could have a perfect mirror, or a refracting telescope could have a perfect lens, the effects of aperture diffraction are unavoidable. In reality, perfect mirrors and perfect lenses do not exist, so image aberrations in addition to aperture diffraction must be taken into account. Image aberrations can be broken down into two main classes, monochromatic, and polychromatic. In 1857, Philipp Ludwig von Seidel (1821–1896) decomposed the first order monochromatic aberrations into five constituent aberrations. They are now commonly referred to as the five Seidel Aberrations.Optical defects are always listed in the above order, since this expresses their interdependence as first order aberrations via moves of the exit/entrance pupils. The first Seidel aberration, Spherical Aberration, is independent of the position of the exit pupil (as it is the same for axial and extra-axial pencils). The second, coma, changes as a function of pupil distance and spherical aberration, hence the well-known result that it is impossible to correct the coma in a lens free of spherical aberration by simply moving the pupil. Similar dependencies affect the remaining aberrations in the list.

Optical telescopes have been used in astronomical research since the time of their invention in the early 17th century. Many types have been constructed over the years depending on the optical technology, such as refracting and reflecting, the nature of the light or object being imaged, and even where they are placed, such as space telescopes. Some are classified by the task they perform such as Solar telescopes.

Nearly all large research-grade astronomical telescopes are reflectors. Some reasons are:

Most large research reflectors operate at different focal planes, depending on the type and size of the instrument being used. These including the prime focus of the main mirror, the cassegrain focus (light bounced back down behind the primary mirror), and even external to the telescope all together (such as the Nasmyth and coudé focus).

A new era of telescope making was inaugurated by the Multiple Mirror Telescope (MMT), with a mirror composed of six segments synthesizing a mirror of 4.5 meters diameter. This has now been replaced by a single 6.5 m mirror. Its example was followed by the Keck telescopes with 10 m segmented mirrors.

The largest current ground-based telescopes have a primary mirror of between 6 and 11 meters in diameter. In this generation of telescopes, the mirror is usually very thin, and is kept in an optimal shape by an array of actuators (see active optics). This technology has driven new designs for future telescopes with diameters of 30, 50 and even 100 meters.

Relatively cheap, mass-produced ~2 meter telescopes have recently been developed and have made a significant impact on astronomy research. These allow many astronomical targets to be monitored continuously, and for large areas of sky to be surveyed. Many are robotic telescopes, computer controlled over the internet (see "e.g." the Liverpool Telescope and the Faulkes Telescope North and South), allowing automated follow-up of astronomical events.

Initially the detector used in telescopes was the human eye. Later, the sensitized photographic plate took its place, and the spectrograph was introduced, allowing the gathering of spectral information. After the photographic plate, successive generations of electronic detectors, such as the charge-coupled device (CCDs), have been perfected, each with more sensitivity and resolution, and often with a wider wavelength coverage.

Current research telescopes have several instruments to choose from such as:

The phenomenon of optical diffraction sets a limit to the resolution and image quality that a telescope can achieve, which is the effective area of the Airy disc, which limits how close two such discs can be placed. This absolute limit is called the diffraction limit (and may be approximated by the Rayleigh criterion, Dawes limit or Sparrow's resolution limit). This limit depends on the wavelength of the studied light (so that the limit for red light comes much earlier than the limit for blue light) and on the diameter of the telescope mirror. This means that a telescope with a certain mirror diameter can theoretically resolve up to a certain limit at a certain wavelength. For conventional telescopes on Earth, the diffraction limit is not relevant for telescopes bigger than about 10 cm. Instead, the seeing, or blur caused by the atmosphere, sets the resolution limit. But in space, or if adaptive optics are used, then reaching the diffraction limit is sometimes possible. At this point, if greater resolution is needed at that wavelength, a wider mirror has to be built or aperture synthesis performed using an array of nearby telescopes.

In recent years, a number of technologies to overcome the distortions caused by atmosphere on ground-based telescopes have been developed, with good results. See adaptive optics, speckle imaging and optical interferometry.