Giant telescope

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The so-called giant telescope of the Berlin trade fair in 1896. It was renovated and is again in use at the Archenhold observatory .

In the 18th and 19th centuries, astronomical telescopes with unusually large dimensions were called giant telescopes . This could refer to both the length ( focal length ) of the telescope and the opening ( aperture ) of its lens. The large instruments of the 18th century were mainly metal mirror telescopes . The 19th century, on the other hand, produced long telescopes (refractors) that bundled light with large lenses .

The term “giant telescope” was used in the professional world of the two centuries, but it was not clearly defined. Some media still use the term for the large reflecting telescopes of the present, which is why they are also dealt with here.

Objective lens versus mirror

The question of which construction method is more favorable for astronomy has been controversial for centuries. Since the invention of the mirror telescope (around 1650), the purity of color and the possibility of larger diameters have been taken into account for this . Disadvantages were initially the loss of light with metal mirrors (about 50%), the stronger diffraction at the auxiliary mirror and the more difficult adjustment. In favor of the refractor (and still speaks for amateur astronomers to this day), the better resolution of fine details and the mostly lower influence of air turbulence were the main factors .

As far as the technically possible size and the usability for the respective research goals are concerned, the two types of construction have alternated several times since around 1750 and experienced many innovations in the optical system , in the mechanics, the mounting and the sensors used. Mirror telescopes dominate research today.

While a telescope mirror with a diameter of 50 cm was already considered to be extraordinarily large around 1800 - just like a 50 cm lens on a refractor around 1850 - this mark is today at mirror sizes of 6–10 m and will increase to over 30 m by 2025. In contrast, the size development of lens systems in the 1890s ended at 90-100 cm (Lick and Yerkes observatory ) because the glass body of the lens already deforms noticeably under its own weight from this size. In the case of mirrors, problems of this kind can be controlled and corrected much more easily because - unlike lenses that are only held at the edge by their mount - they can be supported from behind over their entire surface.

17th century: The problem of lens defects and light intensity

Hevels giant telescope

Soon after the invention of the telescope in 1609 by Galileo , Kepler and Stafford, efforts followed to increase their magnification and, in particular, the light intensity of the objectives, which were only about 3 cm in size . While higher magnifications could be achieved relatively quickly through longer focal lengths, higher image brightness was opposed by the color and lens errors , which increased sharply with the size of the lens . Before the development of two- lens achromatic lenses by John Dollond around 1760, large lenses with sufficient image sharpness could only be achieved with a very flat lens cut, which required extremely long focal lengths.

Particularly spectacular were the long lens telescopes belonging to Gdansk Mayor Johannes Hevelius , which he had set up on the terrace of the town hall around 1670 and later in front of the city gates. The largest aerial telescope at the town hall was about 10 meters long and was the sensation of many state receptions. Even the longest telescope of all time goes back to Hevelius. It was about 25 meters long and mounted on a pulley block on a mast about 20 meters high . Because a continuous tube would have been too heavy, the side stray light was reduced by a system of 20 apertures. Because of its extreme sensitivity to wind and the difficult-to-use mount (the pulley block had to be operated by an assistant on the top of the mast), this giant telescope was of limited value for astronomical practice.

Schroeters mirror telescopes

Giant telescope from Lord Rosse, Birr (Ireland)

Around 1670 Isaac Newton developed an improved type of reflector telescope, which was the basis for the powerful telescopes that Johann Hieronymus Schroeter (1745–1816) built for his private observatory in Lilienthal near Bremen in the 1770s and which were the largest in the world at the time. It was here that the excellent lunar observations were made (published as Selenotopographische Fragments ), and his assistant Friedrich Wilhelm Bessel , who was employed as an observer , was trained as an astronomer here.

Schroeter began his later groundbreaking moon observations in 1779 with a pure color telescope from Dollond with an aperture of only 6 cm and a focal length of 90 cm. Herschel's discovery of Uranus (1781) prompted the well-off official to order a reflecting telescope 12/122 cm from him. A 17/214 cm Newtonian telescope soon followed, for which he built a two-story observatory in the garden. His program included systematic observations of the moon and the sun , the large planets as well as variables , binary stars and nebulae.

Schroeter completed his actual “giant telescope” in 1794 - it had an aperture of 51 cm and a focal length of 825 cm. In addition to the previous measurement program, he was also able to examine the night side of the moon , discover numerous star clusters and nebulae and make daytime observations of the bright planets. The beginning amateur astronomer soon achieved fame and received visits from astronomers, politicians and the military. With Franz Xaver von Zach and Wilhelm Olbers , he founded the Astronomical Society in 1800 - which soon got involved in cutting-edge research - and the so-called " Sky Police " for the targeted search for the presumed minor planet between the orbits of Mars and Jupiter. In fact, 3 of the 4 largest asteroids were discovered on Schröter's large reflecting telescope.

The next giant telescope was built by Lord Rosse in 1845 at a cost of £ 12,000 (around £ 1,300,000 in today's purchasing power). The primary mirror was 72 inches (1.83 m) in diameter and had a focal length of 16 m. This telescope was characterized by an unbelievably high light intensity for its time and soon got the nickname " Leviathan of Parsonstown ".

Fraunhofer achromats

The problem of lens defects was in principle solved by the British John and Peter Dollond as early as 1760/70 , but it was only Josef Fraunhofer around 1820 that made the breakthrough to larger lens diameters. For example , the Dorpater refractor built by Fraunhofer had a 245 mm lens.

In a report to the Bavarian Academy of Sciences in 1824, Fraunhofer spoke about the advantages of the Dorpater instrument:

“It is the largest of its kind, and also new with regard to the more important parts of the set-up … The greatest visual tools that have existed so far are the telescopes made of metal mirrors. But since the most perfect metal mirror reflects only a small part of the incident light ... the mirror telescopes must be extremely large ... [Furthermore] the deviation of the light rays cannot be corrected because of the spherical shape of the reflecting surfaces, and ... B. never use for meridian instruments etc. "

With the last remark, Fraunhofer refers to the astronomical research priorities of his time - celestial mechanics and their basis, the speed and position determination of celestial bodies ( astrometry ). Bessel made the first distance measurements of fixed stars in 1838 with the heliometer designed by Fraunhofer for the finest angle measurements (see 61 Cygni ).

Although the research focus changed with the advent of astrophysics and photography in the second half of the 19th century, Fraunhofer’s developments remained a model for all instrument makers in Europe until the early 20th century. It was not until the great Hooker telescope on Mount Wilson (USA) that the turn to new mirror systems in the construction of large telescopes began around 1920.

The "great refractors" of the late 19th century

Dome of the refractor in Nice

In the last quarter of the 19th century, when astrophysics and research on small planets became more important, the observation of newly constructed large refractors came to the fore for a few decades. One of the main reasons for this was that they enabled precise spectral analyzes as well as positional astronomy and the observation of the finest details on planets.

It started with the Nice Observatory in Europe (see picture) and in America with the US Naval Observatory in Washington with a 10 meter long lens telescope. It had a 26-inch lens (approx. 65 cm diameter) weighing approx. 100 kg. Under the direction of Simon Newcomb , the naval observatory worked primarily on the precise determination of the fundamental astronomical constants .

The large refractor of the new Vienna University Observatory (which at that time was still far outside the built-up area) was a little bigger , a 27-inch (68 cm aperture ), which was constructed in 1875 by the then top producer Grubbs in Ireland. The telescope, which was the world's largest for 5 years and is still in use after a renovation, has a focal length of over 10 meters and was mounted in a dome with a diameter of 14 meters. Initially, two assistants were required to operate the system, and operation was later partially automated.

The refractor, the dome of which is 27–36 m above the site, rests on a 35 m high pillar that is 5 m deep in the rock. At its base it is almost 10 m in diameter. The brick construction, ventilated in the basement, guarantees an even temperature without any rotation of the pillars . The pillars of the even larger Lick and Yerkes telescopes (USA around 1895), on the other hand, are hollow at the top .

The focus of observations in Vienna was on position measurements and the spectral analysis of binary stars and the bodies of the solar system. It emerged u. a. the most accurate recordings of the gas planet Jupiter at the time . The large instrument prompted the Trieste observatory director Johann Palisa to move to Vienna as an observer . In total, he discovered 123 minor planets and thus holds the record for their visual discovery. He organized exact orbit determinations and published the photographic Palisa-Wolf-Sternatlas with Max Wolf (Heidelberg) around 1900 .

Almost at the same time, the Astrophysical Institute Potsdam was set up on Telegrafenberg not far from Berlin . It was the first of its kind and initially received a smaller telescope than the one in Vienna, but was equipped with the most modern spectroscope . When Russia (in Pulkowo near Petersburg) and the USA (Harvard, Washington) built much more powerful refractors, Potsdam ordered its large 28-inch double refractor , which could be used to observe visually, spectroscopically and photographically at the same time. It had a focal length of 12 meters and was inaugurated in 1899 under the patronage of Kaiser Wilhelm II . Companies from all over Germany worked on the 7-ton telescope. The two lenses came from Munich, the mobile observer stage (" astronomical chair ") from Berlin.

Breakthrough discoveries included a. the interstellar matter , which the scientist Johannes Franz Hartmann explored in 1910 and the model concepts of star formation revolutionized. In the 1930s the observers were occupied with the asterism of the novae . After the Second World War , the focus was on the exploration of the binary stars, whereby precise statements about the mass of these celestial bodies became possible. After that, the refractor was out of service for 30 years due to weather damage, but was renovated for the anniversary year 1999.

Vienna lost the rank of having the largest telescope around 1880 to the Pulkowo Observatory , which Russia built near the capital St. Petersburg as the main observatory of the Tsarist Empire. The more than 10-meter-long refractor had a 30-inch lens and was supposed to bring Russia back to the forefront of global astronomy. Its quality was high, but it soon suffered from the harsh weather conditions 700 km south of the Arctic Circle .

By the turn of the century, measurements at the US Naval Observatory laid the foundations of modern astrometry and the fundamental system of astronomy . The Harvard Observatory and several other large observatories also took part in this international project.

Due to the success of Edward Holden , the main observator in Washington and later director of the Lick Observatory , the Lick Observatory put an even larger lens telescope into operation with a 91 cm aperture in 1888, 12 years after the Vienna “Great Refractor”. For this so-called Lick refractor , the first observatory building was built on a higher mountain, the climatically extremely favorable Mount Hamilton ( 1327  m ). With an average of 330 clear nights per year and minimal air turbulence, it stood out among all of California's easily accessible peaks.

Edward Holden wrote about it in 1888:

“I'm gradually becoming familiar with the handling of the large telescope and learning how to best observe with it. It requires special treatment, but when all external circumstances are favorable, it does an excellent job ... I have seen the brightest planets, nebulae, the Milky Way ... as beautiful as no astronomer before me. Jupiter's moons , which appear as disks in other telescopes, show up here as full round masses like the planets ... The famous nebula in Hercules appears as a mass of isolated, independent points. "

Holden goes on to say that he was “particularly attracted” to finding familiar objects when he compared them “to drawings by Lord Rosse in his six-footed giant reflector” and found his images to be sharper.

The fact that the Lick refractor was superior to a metal mirror twice as large contributed to the decision in 1891 to build an even larger lens telescope with the Yerkes refractor . To this day it is the largest refractor in the world. His 102 cm lens, however, marks the end of this line of development, as the bending of the glass and telescope already exceeded the permissible level. From now on, all large telescopes were designed as reflector telescopes and soon no longer visual, but designed for astrophotography .

Mount Wilson, Mount Palomar and Zelenchuk

At the turn of the century, the focus of telescope development increasingly shifted to the USA, which is largely due to the generous support from private sponsors. The ever advancing astrophotography and astrophysics resulted in the necessity of larger apertures, which stimulated the construction of extraordinarily large reflector telescopes. Technically, it was now possible to manufacture mirrors from glass instead of metal mirrors .

The first of these large reflectors was the 1.5 m mirror for the newly established observatory on Mount Wilson. Barely 10 years later, the 2.5-meter mirror of the Hooker telescope , installed in 1917, followed , making it the largest mirror telescope in the world for almost 30 years. With him, groundbreaking insights into neighboring spiral nebulae (then referred to as "world islands") and the expansion of space . Here the Andromeda nebula could be resolved into single stars for the first time, so that the long discussion as to whether the spiral nebula consisted of gas or were very distant islands of the world (galaxies) was decided. For a long time, Europe had nothing to counter this technological development because of the global economic crisis .

After the Mount Palomar mirror telescope with a 5-meter aperture had been the largest telescope in the world for 30 years , this rank was transferred in 1975 to the Soviet 6-meter telescope at the Zelenchuk Observatory in the Caucasus. Both of these giant mirrors represented a technical tightrope walk in their production; Just cooling the 80 m² and 110 m² glass blanks took many months.

The large telescopes of the present

Reflector telescopes with 8 to 10 meters have been planned since the early 1980s. For example, the Very Large Telescope (“Very Large Telescope”, VLT for short ) was developed under European management . operated by the European Southern Observatory (ESO) on the Paranal in Chile ( 2635  m ). It consists of four mirror telescopes, each with an 8.2 m opening, which can be electronically interconnected to form a network. The first of these large telescopes was put into operation in 1998/99. All four telescopes are designed for spectral ranges from visible light to mid- infrared and can be used together for the finest angle measurements ( interferometry ). By adaptive optics a significant reduction is air turbulence possible, so that the VLT nearly the resolution capability of the space telescope HST achieved of about 0.1 ". The four telescopes are as Altazimute mounted, that is, their primary axis of rotation does not have the celestial pole , but the zenith This enables a noticeable reduction in weight: the tracking of the earth's rotation no longer requires inclined axes as with the large telescopes of the 19th and 20th centuries, but takes place through digital control as with a modern - only 100 times smaller - theodolite .

The list of the largest optical telescopes currently shows the following instruments as large telescopes in operation with a calculated mirror diameter of at least 8 m:

Plans for the next few decades

An end to the development of giant telescopes - as happened around 1900 with the lens telescopes after unexpected bending of the 1 m lens at the Yerkes Observatory - is not in sight for reflector telescopes. In early 2009 the following were planned:

  • Giant Magellan Telescope (GMT) with seven 8.4 m individual mirrors and an equivalent diameter of 21.4 m (area) or 24.5 m (resolution). The construction was decided in February 2009 and should be completed in 2029. The Las Campanas observatory in Chile is the preferred location .
  • Thirty Meter Telescope (TMT) with 492 hexagonal mirrors of 1.4 m. The Mauna Kea observatory was chosen as the location . Commissioning is planned for mid-2022.
  • European Extremely Large Telescope (E-ELT) . It will have a main mirror 39 meters in diameter made up of 798 hexagonal mirror elements. A mirror with a diameter of 42 meters and 906 segments was originally planned. Construction is expected to be completed in 2022. On April 26, 2010, the mountain Cerro Armazones in the Chilean Atacama Desert was selected as the location .

Concepts going beyond this are currently not actively pursued. Project studies exist, for example, for

literature

  • Wilhelm Foerster : The exploration of the universe. History of the study of nature and the utilization of natural forces in the service of peoples . In: Hans Kraemer (Ed.): Universe and humanity . Volume III, Part 1. Bong & Co., Berlin / Leipzig / Vienna 1903, p. 286 .
  • Steffi Menzenbach: The giant telescope is no longer a rust tube. To mark its 100th birthday, the “Great Refractor” on Potsdamer Telegrafenberg was restored. In: Berliner Zeitung , August 11, 1999 ( berliner-zeitung.de ).
  • Franz Xaver von Zach: Stars and Space . Spectrum of Science, 2005, ISSN  0039-1263 .
  • Günter D. Roth: Cosmos of astronomy history. Astronomers, instruments, discoveries . Franckh-Kosmos, Stuttgart 1989, ISBN 3-440-05800-X .
  • H. Karttunen, P. Kroger, H. Oja (Eds.): Fundamental Astronomy . 4th edition. Springer, Berlin / Heidelberg / New York 2004, ISBN 3-540-00179-4 (English).
  • Bernhard Mackowiak: The new super telescopes . In: The world . September 30, 2006 ( welt.de ): “In order to look deeper and deeper into space, the telescopes have to get bigger and bigger. But there are technical limits when it comes to lenses. The telescopes of the future will be composed of many smaller ones. "
  • Paul Rincon: Ground Telescope to Super Size . In: BBC News . London April 10, 2005 (English, news.bbc.co.uk ).
  • Dietrich Lemke: The future is bright - but expensive . In: Stars and Space . No. 10 , October 2008, ISSN  0039-1263 , p. 28-35 ( schattenblick.net ).

Individual evidence

  1. ↑ quoted from Roth
  2. ↑ Messier 13 globular cluster , in which Messier did not recognize any single stars
  3. ↑ cit. from GD Roth: History of Astronomy
  4. ^ E-ELT Site Chosen eso.org, European Southern Observatory; Retrieved April 19, 2011
  5. ↑ Large Aperture Mirror Array. bibcode : 2004SPIE.5382..115H