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Telescope at the Orangery Kassel
Astronomical telescope with triple lens , star diagonal and viewfinder

A telescope , also called a lens telescope or refractor , is an optical instrument which, when used, makes distant objects appear many times closer or larger. This is achieved by increasing the angle of vision with the help of lenses . Prisms and mirrors can be used to straighten the image or to reduce the overall length of the telescope.

The development of powerful telescopes played an important role in the history of astronomy . Telescopes together with reflector telescopes form the class of optical telescopes .

Word origin

The word telescope is a literal Germanization of the Latin tube telescopius "television tube", from tubus "tube, tube", from ancient Greek τῆλε tele "far" and σκοπεῖν skopein "look, observe" (see also -skop ). Maximilian Hell paid tribute to Wilhelm Herschel's discovery of Uranus in 1789 by naming two constellations as Tubus Herschelii Maior and Tubus Herschelii Minor , referring to the telescopes built by Herschel. Johann Elert Bode then combined the two constellations into one in 1801 and coined the expression Telescopium Herschelii for it. The German word already existed at this time, but the initially synonymous words telescope and telescope diverged. Today telescope is the generic term. Telescope stands for an optical telescope made up of lenses. And tube designates the technical component of the shell in which the lens, mirror and prism construction is enclosed.

Structure and functionality

Telescopes generally consist of a combination of lenses held in place by a mechanical structure. Depending on the beam path of the light through the lenses, a distinction is made between Galilei telescope and Kepler telescope . Additional optical elements can align the image when looking into the telescope in the same way as the original. The beam path in the telescope can be folded using mirrors in order to obtain a short design despite the long focal length.

Galileo telescope

Beam path in a Galileo telescope

The Galilei telescope , also known as the Dutch telescope , was invented by the Dutch eyewear maker Hans Lipperhey around 1608 (and at about the same time by Jacob Metius and Zacharias Janssen and his father) and was further developed by the physicist and mathematician Galileo Galilei . It has a convex converging lens as an objective and a diverging lens with a smaller focal length as an eyepiece . Since the eyepiece has a negative focal length, it must be within the focal length of the lens so that the focal points of the lens and the eyepiece coincide on the side of the observer. The result is a virtual, upright and right-sided image, but with a small field of view . The Galilei telescope is used today for opera glasses and telescope glasses . The principle is also used in teleconverters .

Kepler telescope

Beam path in the Kepler telescope. The objective (1) generates an inverted, real intermediate image (5) of the object (4) , which can be viewed with the eyepiece (2). The eye (3) sees an enlarged, virtual image (6) at a seemingly short distance (dashed lines).

A Kepler telescope ( astronomical telescope) is a telescope that follows a design described by Johannes Kepler in 1611. According to this, the eyepiece is also a convex converging lens (with a smaller focal length). The eyepiece and lens are set up at the distance of their added focal lengths. The field of view is more extensive than with the Galileo telescope. Whether really Johannes Kepler telescope this type - the exception of the Astronomy z. B. is also used in geodetic theodolites - invented is uncertain. The first surviving Kepler telescope was built by the Jesuit Christoph Scheiner around 1613.

Since the beam path crosses in the telescope, the lens creates an upside-down and reversed (i.e. rotated 180 degrees) real image of the object being viewed, which is magnified using the eyepiece (principle of the magnifying glass ).

Swap sides

Kepler telescopes produce an image rotated by 180 ° for the observer. It is upside down compared to the original and is reversed. When the telescope is swiveled, the image moves in the opposite direction than when looking through an empty tube. The same applies to pans up and down. This can be remedied with additional lenses or with prisms:

In order to align the image in the same way as the original, there are the following "reverse optics" options:

  • two inclined mirrors placed in the beam path (usually at 45 °)
  • two prisms whose rear surfaces act like mirrors through total reflection
  • a third converging lens to reverse the image again (so-called terrestrial reversal sentences, etc.)

With prism binoculars (binoculars) and spotting scopes , the inverted image of the Kepler telescope is rotated by 180 ° using various prism systems . Depending on the design, there is also a shorter construction. The image can also be inverted using an inverting lens . That finds z. B. for observation telescopes and some riflescopes, but also for telescopic telescopes or terrestrial telescopes for on the go or at sea. Despite enlargements of around 20x to 60x, it is small, collapsible and inexpensive. Disadvantages are the lower light intensity and the access of outside air when pulling apart, whereby dirt and water can penetrate. Newer types and spotting scopes therefore have a fixed tube and shorten the overall length with a straight Porro prism or a slightly bent inverting prism . This is also possible with a (negative, diffusing) focusing lens - for example in newer theodolites and electronic total stations .

The wrong picture is accepted with the larger telescopes used in astronomy , since the orientation of the objects to be observed in the sky is usually irrelevant. To improve the view into the eyepiece, 90 ° or 45 ° deflecting prisms and mirrors are often used, the image of which is then at least upright or laterally correct ( star diagonal ).

A fourth possibility is to use a diverging lens as an eyepiece, which turns the astronomical telescope into a Galileo telescope. It is visually less favorable, but because of the extremely short construction z. B. very common for theater glasses ( commonly known as " opera gazers "). However, the Galileo construction type does not allow the attachment of a crosshair or micrometer.

Objective and eyepiece made up of more than one lens

Every optical lens has more or less pronounced longitudinal and lateral color defects. Different wavelengths are refracted to different degrees . Long-wave red light is less refracted than short-wave blue light. Thus there is a separate focal point for each wavelength range . In practical observation, this leads to annoying color fringes.

In the past, attempts were made to reduce longitudinal chromatic aberration by constructing telescopes with the longest possible focal length. The Danzig scholar Johannes Hevelius used meter-long air telescopes .

Another possibility of minimization is the combination of glass lenses with different Abbe numbers . A group of two lenses placed a short distance apart is called an achromatic lens . With three or more lenses one speaks of apochromats . Chester Moor Hall and Joseph von Fraunhofer were the pioneers of this technology .

In the case of the eyepiece , several lenses also have the task of enlarging the field of view . With the increasing size of the telescope and demands on the quality of the image, the design and construction of such lens systems become very complex.

Folding refractors

Schaer refractor, beam path
Folding refractors with 230 mm lens diameter and 2058 mm focal length

The folding refractors are a special type of telescope. The beam path is mostly deflected by one or two plane mirrors, so it is quasi folded. The various folding variants are often named after their designer or the external appearance of the telescope. The bassoon refractor (simple folding) is reminiscent of the kinked design of the musical instrument of the same name and the Newton refractor (double folding) is reminiscent of the Newtonian reflecting telescope because of its eyepiece . The Schaer refractor is folded twice and named after its designer.

Ocular center prisms or mirrors are not included in the classification of these types of construction. They are considered accessories for all types of telescopes.

Lenticular lenses have the disadvantage that they form color fringes in the image due to the refraction of light. This so-called chromatic aberration was previously only acceptable for simple two-lens lenses (" achromatic lenses ") from an aperture ratio of less than approx. 1:15. This made the telescopes very long and unwieldy for larger openings.

Various bi-fold refractors were designed by E. Schaer, Ainslie and G. Nemec, among others. It is often difficult to differentiate between the Ainslie and the Nemec types, as they are very similar, apart from minor modifications in the beam guidance. So Ainslie led the beam path of his Newton variant after the 2nd reflection to the side of the incident beam path.

The amateur astronomers Nemec, Sorgenfrey, Treutner and Unkel became known in the 1960s to the end of the 1970s for high-quality astrophotos with their folding refractors. This fame also brought these types of refractors a certain popularity.

Today, folding refractors are mainly used as do-it-yourself devices by amateur astronomers and some public observatories. The Wachter company offered a Schaer refractor from industrial series production in the 1970s and 1980s. It was a FH 75/1200 mm from the Japanese manufacturer Unitron.

Coude refractor

Refractor of the Volkssternwarte Aachen

With the Coudé refractor , too , the beam path is folded by two plane mirrors or prisms. These direct the light through the mount to a fixed focus . The advantage of this design is the observation from a fixed place, which can easily be equipped with seating, aids and a work table, while the generally relatively long telescope moves independently of this. The disadvantage is the rotation of the image caused when the telescope is swiveled or simply adjusted, so that astronomical photography is only possible with short shutter speeds or complex rotation tracking has to be installed. Since the beam path is usually guided through an axis of the mount, mostly only relatively large instruments from an opening of approx. 8 inches upwards are designed as Coudé refractors.

The Coudé system is also used with reflector telescopes.

Designs for special applications

Old military telescope

For terrestrial observations one uses

  • Binoculars (binoculars): compact telescopes with a short focal length with prism systems that deliver an upright and laterally correct image. The binoculars usually have 6 to 10 times magnification and a separate beam path for each eye ( objective , prism system and eyepiece). They are called monoculars with one eye
  • Opera viewer : very short double glass (type Galilei ) with only 2 to 3 times magnification
  • Spotting scopes , relatively compact and robust refractors for single-eye (monocular) observation; Objective diameter up to 100 mm, magnification usually 20 to 60 times
  • stationary observation telescopes for nature and landscape observation , e.g. B. at striking vantage points
  • Rifle scopes with low magnification and high light intensity.

For astronomical observations:


The magnification is defined by the ratio of the focal lengths of the objective and the eyepiece. This means that a telescope with interchangeable eyepieces, as is customary in astronomy, does not have a fixed magnification; the longer the focal length of the eyepiece used, the lower the resulting magnification. Due to various factors (see disturbance variables ), an excessive magnification is pointless.


Small telescopes and binoculars are characterized by two numbers, e.g. B. 6 × 20 mm (pocket device) or (20 to 40) × 50 ( spotting scope ). The first specification relates to the magnification , the second to the opening ( aperture ) of the objective in mm. Variable magnifications (e.g. 20 to 40) are made possible by zoom eyepieces . The use of a binocular gives the impression of spatial vision , which improves perception.

In telescopes for astronomical observations, the ratio of the aperture to the focal length (the focal ratio ) is used as a parameter for the performance of the instrument. The magnification depends on the eyepiece used, which can usually be changed. A refractor 100/1000 has an aperture of 100 mm and a focal length of 1000 mm and thus an aperture ratio of 1:10 (usually written as F / 10).

The magnification of a refractor results from the ratio of the focal lengths of the objective and the eyepiece. A device with a 1000 mm objective focal length and 5 mm eyepiece focal length thus has a 200-fold magnification. Because of the limited resolution due to diffraction , such a magnification is only useful if the opening of the objective is large enough. As a guideline, the so-called useful magnification has twice the numerical value as the aperture diameter of the lens in millimeters. In the example given, the telescope should have an opening of 100 mm.

The size of the exit pupil (AP) is another interesting parameter. It is calculated as the product of the focal length of the eyepiece and the aperture ratio or as the quotient of aperture and magnification. In the above examples, the exit pupil would be 20 mm / 6 = 3.3 mm or 5 mm * 100/1000 = 0.5 mm. The construction of the eyepiece determines the position of the AP. It should be accessible to the eye. The pupil of the eye limits the amount of light that can enter the eye. When the AP is smaller than that of the eye, the image will be darker than when viewed with the naked eye. If it is larger, the image appears at most equally bright. A night lens therefore has an exit pupil of more than 5 mm.

As telescope performance is further referred to the useful power of a telescope on the sighting or the detail resolution of an object, based on the power of the naked eye.

Visual and photographic use

Coin operated telescope on the North Sea island of Juist

When using the telescope visually, the eye serves as the receiver . To do this, the optical system must be afocal , that is, the telescope must generate parallel light beams that can be received by the relaxed eye on the retina. This is achieved with the help of an eyepiece.

Telescopes that only have one objective do not produce a stereoscopic image. In addition, the objects being observed are usually so far away that the light rays are almost parallel. However, binocular approaches for binocular vision are used. These should enable more relaxed vision. To do this, the beam path is split, which reduces the brightness of the image.

When observing distant objects, the incident rays are almost parallel. In this case, the telescope converts incident, almost parallel rays into exiting parallel rays, but changes the angle and density of these rays beforehand. Changing the angle causes the enlargement. The greater density of the rays increases the brightness of the image. In the case of two-dimensional observation objects, however, the brightness of the image cannot be greater than the brightness of the object.

In photographic use, the telescope has the function of a very long focal length lens . Because of their long focal length and their weight, large telescopes are held and moved by mounts .

Disturbance variables


Because of the diffraction of light, the resolution of the telescope is limited by the diameter of the objective. The magnification that optimally adapts the resolving power of the telescope to that of the human eye is known as useful magnification . In terms of numbers, this is roughly the same as the aperture (opening) of the telescope objective in millimeters. At a higher magnification, stars do not appear as points, but rather as discs surrounded by concentric circles (diffraction rings).

Air turbulence

Warmed air rising from the floor , insufficiently tempered observatories - domes or observation at the open window cause annoying streaks .

Especially in winter and in certain weather conditions , a sparkle of stars called scintillation can be clearly seen. This is caused by rotating convection cells that are created by the transfer of heat between colder and warmer layers of air. Often the stars and planets appear in small telescopes as "wobbling spots"; they become blurred when taking photographs. Usually the situation improves as the night progresses.

Astronomers call this factor seeing . The position of a star can vary by 1 "to 3" due to poor seeing. A good telescope with a resolution of 1 ", which has to have an aperture of around 150 mm, is therefore seldom fully exploited. Seeing is less important when observing two-dimensional objects such as nebulae or comets .

Stability of the telescope setup

The mount with which the telescope is held and moved determines which magnifications can be used sensibly with a telescope. Any excessive vibration in the mount (e.g. due to wind) is noticeable as a tremor of the observed object in the field of view of the eyepiece. The mount should therefore be as stiff as possible, with little vibration and not be overwhelmed by the weight of the telescope used.

In binoculars that are often only hand-held , eyepieces are usually permanently installed, which only allow relatively low magnifications. With these instruments, greater emphasis is placed on light intensity . A fixed tripod is also an advantage here.

Precautions when observing the sun

When observing the sun through a telescope, a suitable solar filter must be used, which is attached in front of the lens. Filters that are screwed in front of the eyepiece receive the increased intensity and can burst as a result of the development of heat and, in the worst case, lead to the observer becoming blind. Light-reducing alternatives are the Herschel wedge , pentaprism and Bauernfeind prism , both of which may and must (visually) be used with gray attenuation filters in the eyepiece. The sun projection method , which is suitable for simultaneous observation by several people, can be used without reducing the light .

Field of view in the telescope

When using a telescope, the field of view is on the one hand noticeably restricted, on the other hand it is presented more clearly. The eyepiece essentially determines the quality of the image and the ergonomics of the observation, especially the size of the apparent field of view. Modern eyepieces show a field of view of around 45 °, with wide-angle eyepieces 55 to 75 °, depending on the price.

The true field of view, the visible section of the object space, is about the magnification factor of the instrument smaller than the apparent field of view. Has an eyepiece z. B. an apparent field of view of 50 °, then a telescope with 50x magnification has a true field of 1 °. Typical astronomical telescopes are 0.5 ° ( moon diameter), common binoculars are around 7 ° (5 ° to 10 °), observation telescopes a few degrees.

The most accurate way to determine the field of view is to go through a star : We look for a star close to the equator - preferably in the south , at about 40 ° altitude (more precisely 90 ° minus latitude ) - and measure how long it takes to wander through the field of view. The (decimal) (time) minutes are to be divided by four. If the passage takes 2.4 minutes, the telescope has a field of view of φ = 0.60 °. If you know this value, distances can be estimated. E.g. a standing person of 1.70 m who fills the 0.60 ° straight, is 1.70 / sin (φ) = 162 m away from us. Hunters , seafarers and the military use telescopes or binoculars with scales for this purpose - but there are useful rules of thumb . Anyone who wants to perfect the process described could first try it out on binoculars. Better devices indicate the degrees (or the meters over a distance of 1 kilometer). For example, normal 7x50 binoculars have a field of view of around 7.2 ° or 125 m over 1 km.

Connection of a camera to a telescope

LM digital adapter with Canon EOS 5D

A mechanical and optical adjustment is necessary for connecting a camera . An adapter connects either the camera housing with the focuser or the camera and lens with the eyepiece. A firm mechanical connection is particularly important as the smallest movements (vibrations) of the camera greatly reduce the image quality. A wireless remote control should be used to trigger the camera. Furthermore, an optical adjustment of the beam path is necessary so that a fully illuminated and sharp image is projected onto the camera's sensor ( CCD / CMOS ) or the film.

If you have a very steady hand, you can take photos without an adapter. However, the photo lens must firstly have the correct focal length and secondly be exactly centered (and at the correct distance) behind the eyepiece so that parts of the field of view are not cut off. This is more of a danger than possible blurring .

In the case of terrestrial recordings, it should be noted that the exposure meter through the telescope does not have to be accurate. In astrophotography , tracking is necessary due to the rotation of the earth, which can only be omitted for bright objects ( sun , moon, Venus to Jupiter).


Before the invention of the telescope with lens looking through a simple tube (a so-called served periscope ) for suppression of stray light , so that individual celestial objects could be seen clearly. The effect has been known since ancient times, although claims such as B. Aristotle and Pliny that one can see the stars even during the day from the bottom of a deep well have not yet been confirmed beyond doubt.

Only with the advent of spectacle lenses in the 13th century was it even possible to build a telescope. The principle of lenses was known from eyeglass lenses. However, the glasses used were too imprecise in the beginning to be able to build a usable telescope with them. Precisely machined lenses were required for the telescope lenses, which were not available in this way.

Before the invention of the telescope, researchers all over the world thought about how to better observe the stars with optical aids. In the Codex Atlanticus by Leonardo da Vinci , for example, there is a note that proves his intention to point an optically magnifying device at the moon: Fa ochiali davedere / la luna grande […] . (German translation: "Put on glasses to see / the big moon"). Heinz Herbert Mann comments on this entry as follows: “In his analogy-based way of thinking, Leonardo may have asked himself: Which lens enlarges the moon? With that he thinks about which lens would enlarge over a long distance. This was just an idea that was not yet based on a feasible technical concept. "

It is interesting why the lenses suddenly became useful in the late 15th century. This had a lot to do with the emerging book printing , for which Gutenberg had given the impetus. As the number of books increased, so did the number of people who could read in the bourgeoisie. Inevitably, the demand for visual aids for reading rose rapidly, which led to the previous Venetian (Italian) monopoly in the field of the production of lenses and glasses being broken. For example, eyewear makers have now also settled in Nuremberg. The increased demand not only led to an expansion of eyeglass grinding, but also to the development of new techniques. The quality of the lenses improved and ensured that at the end of the 16th century it was possible to use the material that was now available to build devices with which one could see very far into the distance.

The invention and development of the telescope at the beginning of the 17th century

The first telescope was finally constructed in 1608 by the glasses grinder Hans Lipperhey . He presented it in the context of the confrontation between the ailing great power Spain and the unified Netherlands that was being formed. Moritz von Nassau , Governor General of the Northern Provinces, had Lipperhey's novel discovery demonstrated in front of the Spanish ambassador Spinola near The Hague , probably on September 29, 1608. At this meeting and the demonstration of the device, it became clear that this new optical aid was being used could bring far-reaching advantages, especially in the military sector. This demonstration by Moritz von Nassau was therefore not a simple demonstration of a curiosity in a noble environment, but rather a demonstration of his own technical superiority over the Spaniards. In addition to Moritz von Nassau's intention, it was also pointed out at this meeting and the demonstration that this instrument could be used to carry out more precise observations of the sky, since small celestial bodies could now be recognized that would otherwise hardly or not at all. It can be said, however, that the main focus during this demonstration and in the centuries to come was the military use of the telescope.

Lipperhey's achievement in the development of the device consisted in using the knowledge he already had, using two lenses to build a telescope and then adding a diaphragm to this construction, which ensured that the image was no longer blurred. He also called at the court not only to open up a market for his telescope, but also to obtain a patent for his device. He was denied this, however, on the grounds that others had already developed similar devices and that the replica was too easy. Even Jacob Metius is engaged with the invention of the telescope in conjunction, he later than Lippershey applied for three weeks by the patent. In October 1608, the States General Lippershey placed an order for telescopes, Metius received a recognition bonus, about which he was so angry that he withdrew completely from the telescope business. Zacharias Janssen , on the other hand, as the third inventor, presented the invention at the Frankfurt trade fair in 1608.

Invention of the Galileo telescope

Galileo Galilei found out about the invention of the telescope in the Netherlands by Lipperhey in April or May 1609 . For him as a scientific instrument maker, the news was a stroke of luck and so he built a small tube with two to three times magnification using commercially available spectacle lenses by trying out various distance combinations with convex objective lenses and concave ocular lenses. Shortly afterwards he was able to build better instruments with about eight times and then even thirty times magnification. He demonstrated one of these powerful telescopes in August 1609 on the tower of San Marco in Venice. Spectators were Venetian patricians . The spectators immediately recognized the value of the 60 cm long tube, the objective of which consisted of a concave lens and the eyepiece of a convex lens. With the Galileo telescope one could see the ships on the high seas two hours before arrival in port.

Above all, this had military and commercial advantages. Scientific knowledge was secondary to the Venetian statesmen. He gave an even better telescope to the Doge of Venice , Leonardo Donati, and another to the Grand Duke of Tuscany. Galileo was thanked accordingly with a salary increase of 1,000 Florentines and a professorship for life. The efforts to use the telescope for measurements are characteristic of Galileo as a physicist. To do this, he determined the diameter of the field of view in terms of angle and estimated the distance to be measured, for example the distances between the moons of Jupiter, in fractions of the field of view. Galileo did not change or even improve anything on the construction principle of the Dutch telescope or on the understanding of physics. This only happens with the invention of the astronomical telescope by Kepler .

Two telescopes of Galileo

Telescope observations by Galileo

In the same year 1609 he began to use the telescope extensively for astronomical observations. The discoveries he made in the sky with his instrument up to the beginning of March 1610, he reported in his little writing " Sidereus Nuncius " German star message. The beginning of the observation is the view of the moon. He was fascinated by the particular roughness of the moon. In addition, he describes in his work that the surface appears broken and jagged. Mountains, deep ravines and flat areas had become visible with the telescope. These observations did not correspond to the classic image of the moon, which presented it as a smooth ball. In January 1610, Jupiter dominated the night sky and Galileo aimed his telescope at the planet. He notices three stars immediately near Jupiter, two to the east of Jupiter and one to the west. As the position and number of the stars changed over the next few days, Galileo understood that it was always the same little stars that swarmed around Jupiter. This was to be understood as a confirmation of the Copernican world system, not only because of the fact that here several small celestial bodies orbit a much larger one. If Jupiter can carry its moons with it on its orbit around the sun, then the weighty objection to the heliocentric worldview fell , that the earth could not possibly carry the earth's moon along on its orbit around the sun. In the further course of time, two other important discoveries were made with the telescope. In Galileo's observations, Saturn appeared as if it were made up of three touching stars, the central large one being supported by two smaller ones and forming a line. With regard to Venus , he was able to observe that, depending on its position in relation to the sun and earth, it shows phases like the earth's moon. He was also able to prove that Venus is not just a source of light in the sky, but represents a body with sharply defined edges that rotates around the sun.

Kepler's contribution to the improvement of the telescope

The news of Galileo's telescope observations spread within a very short time and in March 1610 Kepler learned in Prague that Galileo had discovered four moons of Jupiter with a telescope. At the beginning of April he received Galileo's work “Sidereus Nuncius” from a Tuscan envoy with a request for an opinion. Kepler was fascinated and recognized the perspectives that had opened up in the fields of optics and astronomy. He confirmed receipt of Galileo's observations and wrote:

"You have awakened a strong desire in me to see your instrument, so that I can finally enjoy the spectacle in the sky like you".

The typeface triggered a productive creative period in the field of optics for Kepler himself . In contrast to Galilei, Kepler provides accurate explanations for the optical mode of operation of the instrument and immediately suggests possible improvements, since as an experienced optician he was able to rely on his previous work and considerations in this area. He improved the function of the Galileo telescope by suggesting that the ocular lens, like the objective lens, must be convex (converging lens) instead of concave, which makes the image clearer and brighter. The image is then turned upside down and useless for earth observations, but this fact makes no difference for astronomical observations. Thus the astronomical or Kepler telescope was born. The most important achievement of Kepler in the field of optics is the production of his work “Dioptrice”, in which he systematically examines and depicts the interaction between eye and lens. Some further lines of thought by Kepler to explain the telescopes are:

  • An image observed through a converging lens is always enlarged and the image size increases as the lens is removed from the eye
  • If the eye is at the intersection of the object, it will be seen the most blurred
  • If the lens is removed so far from the eye that it is outside the intersection of rays from distant objects, an inverted image is seen

By inserting a third lens, the images are not only sharp and enlarged, but also upright. This gives the basic shape of the terrestrial or earth telescope. In summary, one can say that Kepler largely improved the Galileo telescope, especially for astronomical purposes, and was able to build a theoretical foundation for its mode of operation. Nevertheless, Kepler did not manage to build his own powerful telescope because he could not obtain sufficiently good long focal length convex lenses in Prague. It was not until August 1610 that the Archbishop and Elector Ernst of Cologne made a telescope available to him for a short time. Kepler was able to observe the moons of Jupiter himself in the period from August 30th to September 9th, 1610. Since Galileo's observations of Jupiter had meanwhile been questioned, Kepler's confirmation, which gave in the small pamphlet “Narratio de Observatis quatuor Jovis Satellitibus”, had special weight.

Title page Kepler's Dioptrice


  • Hans-Georg Pellengahr: Simon Marius - the exploration of the world of Jupiter with the Perspicillum 1609-1614. In: Gudrun Wolfschmidt (Ed.): Simon Marius, the Franconian Galilei, and the development of the astronomical worldview. (Nuncius Hamburgensis - Contributions to the History of Natural Sciences, Volume 16), Hamburg 2012.
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  • Uwe Laux: Astro optics . 2nd, updated and expanded edition. Spektrum Akademischer Verlag , Heidelberg 2002, ISBN 3-87973-928-5 (first edition by Verlag Sterne und Weltraum , Munich, ISBN 3-8274-1305-2 ).
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Web links

Commons : Telescopes  - Collection of images, videos and audio files
Wiktionary: Telescope  - explanations of meanings, word origins, synonyms, translations

Individual evidence

  1. ^ Arnold Hanslmeier : Introduction to Astronomy and Astrophysics. Springer-Verlag, 2013, p. 105.
    Günter D. Roth : Handbook for Sternfreunde: Guide for practical astronomical work. Springer-Verlag, 2013, p. 12.
  2. z. B. Customers' questions - experts answer: Exit pupil and Transmission @, December 26, 2016.
  3. The Observation Well ( English )
  4. ^ Il codice atlantico di Leonardo da Vinci. Ed. in facsimile dopo il restauro dell 'originale conservato nella Biblioteca Ambrosiana di Milano. Vol. 1-12. Florence 1973–1975, here: Volume 6, p. 518.
  5. ^ Heinz Herbert Mann: Optical instruments. In: Hans Holländer (Ed.): Knowledge, Invention, Construction. Studies on the visual history of natural sciences and technology from the 16th to the 19th century. Gebr. Mann, Berlin 2000, pp. 357-407, here: p. 362.
  6. Jürgen Hamel: Kepler, Galilei, the telescope and the consequences. In: Karsten Gaulke, Jürgen Hamel (eds.): Kepler, Galilei, the telescope and the consequences. (= Acta Historica Astronomica. Vol. 40). Frankfurt am Main 2010, pp. 9–35, here: pp. 10f.
    P. Del Santo, J. Morris, R. Morris, G. Strano, A. Van Helden: Galileos Telescope. In: Giorgio Strano (Ed.): Galileo's Telescope. The instrument that changes the world. Florence 2008, pp. 35-38.
  7. Dieter B. Herrmann: The Cyclops. The cultural history of the telescope. Braunschweig 2009, ISBN 978-3-14-100860-9 , pp. 56-62.
    S. Dupré: The Prehistory of the Invention of the Telescope. In: Giorgio Strano (Ed.): Galileo's Telescope. The instrument that changed the world. Florence 2008, pp. 19-32.
  8. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 21
  9. ^ Hehl, Walter, Galileo Galilei controversial: A scientist between Renaissance genius and despot, Wiesbaden 2017, p. 98
  10. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 21
  11. Jump up ↑ Chapman, Allan, Stargazers: Copernicus, Galileo, the Telescope and the Church, 2014, p. 139
  12. ^ Schmitz, Emil-Heinz, Handbuch zur Geschichte der Optik: Das Fernrohr, Wiesbaden 1982, p. 44
  13. Jump up ↑ Chapman, Allan, Stargazers: Copernicus, Galileo, the Telescope and the Church, 2014, p. 139
  14. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 24
  15. ^ Hehl, Walter, Galileo Galilei controversial: A scientist between Renaissance genius and despot, Wiesbaden 2017, p. 99
  16. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 21
  17. ^ Chapman, Allan, Stargazers: Copernicus, Galileo, the Telescope and the Church, 2014, p. 140
  18. ^ Chapman, Allan, Stargazers: Copernicus, Galileo, the Telescope and the Church, 2014, p. 140
  19. ^ Chapman, Allan, Stargazers: Copernicus, Galileo, the Telescope and the Church, 2014, p. 140
  20. ^ Hehl, Walter, Galileo Galilei controversial: A scientist between Renaissance genius and despot, Wiesbaden 2017, p. 118
  21. ^ Hehl, Walter, Galileo Galilei controversial: A scientist between renaissance genius and despot, Wiesbaden 2017, p. 119
  22. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 22
  23. ^ Chapman, Allan, Stargazers: Copernicus, Galileo, the Telescope and the Church, 2014, p. 144
  24. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 22
  25. ^ Chapman, Allan, Stargazers: Copernicus, Galileo, the Telescope and the Church, 2014, p. 144
  26. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 27
  27. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 27
  28. ^ Letter from Kepler to Galileo, August 9, 1610: KGW, vol. 16, letter 484, pp. 319 - 323, Kepler, letters, pp. 344–351
  29. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 30
  30. ^ Chapman, Allan, Stargazers: Copernicus, Galileo, the Telescope and the Church, 2014, p. 102
  31. ^ Chapman, Allan, Stargazers: Copernicus, Galilei, the Telescope and the Church, 2014, p. 102
  32. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 30
  33. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 32
  34. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 32
  35. Kepler, Johannes, Schriften zur Optik 1604 - 1611: Introduced and supplemented by historical contributions to the history of optics and telescopes by Rolf Riekher, Frankfurt 2008, p. 407
  36. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 30
  37. Riekher, Rolf, Fernrohre und ihr Meister, Berlin 1990, p. 30