Large Diameter Telescopes

11 Key excerpts on "Large Diameter Telescopes"

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  An Introduction to Modern Astrophysics

  • Bradley W. Carroll, Dale A. Ostlie(Authors)
  • 2017(Publication Date)
  • Cambridge University Press

    (Publisher)

  e Mounted with a fixed altitude angle of 37 ◦ . Large-Aperture Telescopes In addition to long integration times, large aperture sizes play an important role in obtaining a sufficient number of photons to study a faint source (recall that the illumination is pro-portional to the diameter of the primary mirror of the telescope, Eq. 6.8). With tremendous improvements in telescope design, and aided by the development of high-speed computers, it has become possible to build very large-aperture telescopes. Table 6.2 contains a list of optical and/or near-infrared telescopes with apertures of greater than 8 m that are currently in operation. A number of much larger-aperture ground-based telescopes are also currently being considered, with effective mirror diameters ranging from 20 m to 100 m. Adaptive Optics While large-aperture ground-based telescopes are able to gather many more photons than smaller telescopes over the same time interval, they are generally unable to resolve the object any more effectively without significant effort. In fact, even a ground-based 10-m telescope located at a site with exceptional seeing (e.g. the Keck telescopes at Mauna Kea) cannot resolve a source any better than an amateur’s 20-cm backyard telescope can without the aid of active optics to correct distortions in the telescope’s mirrors (page 156) and adaptive optics to compensate for atmospheric turbulence. In the la ter case, a small, deformable (“rubber”) mirror is employed that has tens or perhaps hundreds of piezoelectric crystals attached to the back that act like tiny actuators. In order to counteract changes in the shape of the wavefronts coming from the source due to 160 Chapter 6 Telescopes crystals make micrometer-size adjustments to the shape of the mirror several hundred of times per second. In order to determine the changes that need to be applied, the telescope automatically monitors a guide star that is very near the target object.

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  Vision

  • Andrew Fabian, Janet Gibson, Mike Sheppard, Simone Weyand, Andrew Fabian, Janet Gibson, Mike Sheppard, Simone Weyand(Authors)
  • 2021(Publication Date)
  • Cambridge University Press

    (Publisher)

  Also, in any population the faint objects are far more numerous than the bright ones. If we want to make statistical comparisons within a population at any epoch, we need to study a representative population rather than only catch the light from the brightest, and potentially more exceptional, examples. Large-Aperture Telescopes The basic premise of improving a telescope’s ability to collect as much light as it can is that bigger is better. The wider the collecting area of the telescope (i.e. the diameter of the main mirror/lens), the more photons it will catch, and the brighter the source will appear. The power of a telescope is routinely described by its aperture, as this determines how faint the objects it can observe can be. The very largest telescopes in operation today use mirrors 8–10 m across, enabling them to collect 4 million times more light than a human eye: examples include the Very Large Telescope, the Gemini Telescopes, and Subaru, all 8 m in diameter; and the two Keck telescopes, each 10 m across. The next generation of ground-based telescopes will have apertures of up to 40 m across – accompanied by suitably astronomical budgets (over $1,000 million). The larger mirrors are constructed from tiled hexagonal seg- ments, each a sizeable mirror in its own right. Currently there are three ‘extremely large’ telescopes in the offing. The Giant Magellan Telescope (GMT) will have a 24.5-m aperture comprised of an arrangement of seven 8.4-m mirrors; this is currently under construction in Chile, and expected to be ready for use in 2025. The Thirty Meter Telescope (TMT) in Hawaii will have a 30-m mirror constructed from 492 hexagons, each 1.4 m across; construction has been approved, with expected completion Vision of the Cosmos 133 in about 2027. Finally, the Extremely Large Telescope (ELT) will be 39.3 m across, composed of 798 1.4-m-diameter segments.

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  A Question and Answer Guide to Astronomy

  • Carol Christian, Jean-René Roy(Authors)
  • 2017(Publication Date)
  • Cambridge University Press

    (Publisher)

  This condition, which is largely accepted, is called the Rayleigh criterion. For perfect optics unaffected by atmospheric turbulence, this criterion corresponds to an angular separation of 1.22λ/D, where λ is the wavelength and D the diameter. The Hubble Space Telescope, with a primary mirror 2.40 m in diameter, has an angular resolution of 0.06 arcsecond. That is about 10 times better than that of ground tele- scopes, since their resolution is generally limited by the atmosphere regardless of their diameter. That gain in resolution explains Hubble’s fantastic scientific harvest. Just how The Ultraviolet and Visual Echelle Spectrograph (UVES) installed on one of the VLT telescopes for visible light spectroscopy. Some optical elements are seen, including the large dispersive grating showing different colors. Credit: ESO. 288 Telescopes extraordinary is its resolving power? Well, if the Earth flat and without an atmosphere and the Hubble telescope sitting in New York City, it could distinguish the two headlights of an automobile . . . in Mexico City! Adaptive optics instruments that correct for atmospheric turbulence in real time are now used on several large ground-based telescopes, but they are effective only at infrared wavelengths and for relatively small fields of view (Q. 229). 221 Do celestial objects look bigger through a large telescope? This seems an obvious question to ask, but how much a telescope magnifies an image is not one of its fundamental characteristics. If we are observing visually, simply choosing an eyepiece with stronger magnification makes the image look “bigger.” And if we are 1.22l / D (a) (b) (c) When two stars are close together, their combined images resemble that of a single star (a). As their separation increases, the peak of their combined image flattens out (b), then dips in the center (c). (a) (b) (a) Hubble Space Telescope image of Jupiter taken in the visible on April 21, 2014 with Hubble’s Wide Field Camera.

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  The Astronomy Revolution

  400 Years of Exploring the Cosmos

  • Donald G. York, Owen Gingerich, Shuang-Nan Zhang(Authors)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  237 14 Scientific Opportunities for 30-Meter-Class Optical Telescopes Richard S. Ellis INTRODUCTION: THE LEGACY OF LARGE ASTRONOMICAL TELESCOPES The ground-based optical telescope is arguably one of the most important scientific inventions as it was the first to extend the range of our natural senses in exploring the Universe, thereby heralding a new era in scientific method aided by technological advances. A modern-day large telescope is also an excellent example of what our civilization does well. In the 400 years since its invention, the col-lecting area of the optical telescope has increased 100,000 times, and we are now poised to witness a further increase of a factor of 10 (Figure 14.1). Progressively larger and more powerful telescopes have led to numerous discoveries that have shaped our view of the Universe. Although much of the early progress was driven by the natural curiosity and technical ingenu-ity of insightful and wealthy individuals who also served as the principal observers (e.g., William Herschel, 1738–1822), by the dawn of the 20th century, the concept of a large telescope servicing a community of astronomers had been established. Through national and international partnerships, the telescope has since played a vital role in the development and productivity of our astronomical communities. Two criteria in particular have been the focus of telescope development. CONTENTS Introduction: The Legacy of Large Astronomical Telescopes ....................................................... 237 The Quest for Larger Aperture .................................................................................................. 238 The Importance of Angular Resolution ..................................................................................... 238 Two Revolutions in Realizing Large Telescopes ...........................................................................

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  Astronomy

  • Andrew Fraknoi, David Morrison, Sidney C. Wolff(Authors)
  • 2016(Publication Date)
  • Openstax

    (Publisher)

  6.2 TELESCOPES TODAY Learning Objectives By the end of this section, you will be able to: Recognize the largest visible-light and infrared telescopes in operation today • What types of objects will you be observing? Are you interested primarily in comets, planets, star clusters, or galaxies, or do you want to observe all kinds of celestial sights? You may not know the answers to some of these questions yet. For this reason, you may want to “test- drive” some telescopes first. Most communities have amateur astronomy clubs that sponsor star parties open to the public. The members of those clubs often know a lot about telescopes and can share their ideas with you. Your instructor may know where the nearest amateur astronomy club meets; or, to find a club near you, use the websites suggested in Appendix B. Furthermore, you may already have an instrument like a telescope at home (or have access to one through a relative or friend). Many amateur astronomers recommend starting your survey of the sky with a good pair of binoculars. These are easily carried around and can show you many objects not visible (or clear) to the unaided eye. When you are ready to purchase a telescope, you might find the following ideas useful: • The key characteristic of a telescope is the aperture of the main mirror or lens; when someone says they have a 6-inch or 8-inch telescope, they mean the diameter of the collecting surface. The larger the aperture, the more light you can gather, and the fainter the objects you can see or photograph. • Telescopes of a given aperture that use lenses (refractors) are typically more expensive than those using mirrors (reflectors) because both sides of a lens must be polished to great accuracy. And, because the light passes through it, the lens must be made of high-quality glass throughout. In contrast, only the front surface of a mirror must be accurately polished. • Magnification is not one of the criteria on which to base your choice of a telescope.

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  A Journey through the Universe

  Gresham Lectures on Astronomy

  • Ian Morison(Author)
  • 2014(Publication Date)
  • Cambridge University Press

    (Publisher)

  Such a doublet lens was first patented by John Dolland in 1758 but it is believed that the first achromatic lenses were made by Chester Moore Hall in about 1733. This allowed refractors to be made with far larger aperture objective lenses. These not only collected more light and so were able to detect fainter objects, but also improved the image quality or ‘resolution’ of the telescope and so began the era of the giant refractors, which culminated in the construction of the 40-inch aperture Yerkes Telescope. Though larger aperture telescopes will theoretically give higher resolution, in practice the resolution is usually limited by what is called the ‘seeing’ – a function of turbulence in the atmosphere. The atmosphere contains cells of gas with slightly differing refractive indices, which are carried high above the tele- scope by the wind and act rather like the glass used for screens that blur what is seen beyond. A star is effectively a point source and should theoretically give an image (a central disc surrounded by some faint rings, called the Airy pattern) determined by the telescope aperture. In practice, a stellar image as seen from the UK will probably be of order 2 to 3 arcseconds across and will be highly unsteady. This is one reason why professional telescopes are located on high mountains on islands, such as La Palma in the Atlantic Ocean and Hawaii in the Pacific Ocean, or high in the Chilean Andes. There is far less atmosphere above the telescope and the air tends to be less turbulent as it has been flowing over 120 A Journey through the Universe the sea. Under the best conditions the seeing might limit the resolution to half an arcsecond, so larger aperture telescopes will see more detail but not signifi- cantly more than a telescope whose aperture is ~400 cm across.

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  Stars and Galaxies

  • Michael Seeds(Author)
  • 2018(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  Editorial review has deemed that any suppressed content does not materially affect the overall learning experience. Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. PART 1 Exploring the Sky 114 in diameter comprised of 492 hexagonal segments and was intended for a site on Mauna Kea in Hawai’i. In 2016, TMT construction was halted by the Hawai’i Supreme Court because of a petition filed by native Hawaiian activists and allied groups opposed to the project. At the time of this writing in early 2017, the TMT directors were beginning a search for alternate sites. Another international team is designing the European Extremely Large Telescope (E-ELT) to carry 906 segments, mak-ing up a mirror 39 meters in diameter . The E-ELT will be built on Cerro Armazones, a mountain in the Atacama Desert of Chile. Other very large telescopes have been proposed with esti-mated completion dates of 2020 or later. A ground-based telescope is normally operated by astrono-mers and technicians working in a control room in the same build-ing, but some telescopes are now used by astronomers many miles, even thousands of miles, from the observatory. Other telescopes are fully automated and operate without direct human supervi-sion. That, plus continuous improvement in computer speed and storage capacity, has made possible huge surveys of the sky in which millions of objects have been observed or are planned for observation. For example, the Sloan Digital Sky Survey (SDSS) Southern Observatory built the Very Large Telescope (VLT) in the foothills of the Andes Mountains in northern Chile. The VLT actu-ally consists of four telescopes, each with a mirror 8.2 m (323 in., about 27 ft) in diameter and only 17.5 cm (6.9 in.) thick. U.S. and Italian astronomers have built the Large Binocular Telescope (LBT) on Mount Graham in Arizona. The LBT carries a pair of 8.4-m (331-in.) mirrors on a single mount.

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  Solar Planetary Systems

  Stardust to Terrestrial and Extraterrestrial Planetary Sciences

  • Asit B. Bhattacharya, Jeffrey M. Lichtman(Authors)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  Solar telescopes can rely on magnifica-tion mainly as they do not have to look deep into space or gather much light for clearly viewing the Sun. Instrument quality : A research telescope is only as good as the cameras and other instruments which record and analyze the light that it captures. Instruments are judged by factors like the quality of their images, how effectively they spread out light, and how much light they capture. Lower resolution image Higher resolution image FIGURE 19.10 Lower and higher resolution images. 421 The Telescope—The Essential Tool 19.7 NASA’s Great Observatories The four Great Observatories are a series of space telescopes with a purpose to give the complete picture of objects at different wavelengths. Each observatory investigates a par-ticular wavelength region in a greater detail [6–8]. The telescopes, in order of launch, are (1) the Hubble Space Telescope (1990); (2) Compton Gamma Ray Observatory, popularly called CGRO (1991); (3) Chandra X-ray Observatory (1999); and (4) Spitzer Space Telescope (2003). Figure 19.11 shows each telescope including the wavelength region it was built to observe. All except for CGRO are providing informa-tion at present. Figure 19.12 exhibits the images showing the remains of an exploded star (Kepler’s supernova), as seen by three telescopes of the Great Observatories. 19.7.1 Hubble Space Telescope’s Design The Hubble Space Telescope was deployed from the space shuttle Discovery during STS-31 on April 25, 1990. Since then, there have been five servicing missions that contin-ued to upgrade the telescope’s scientific instruments and operational systems. Hubble reached a major milestone, its twentieth anniversary in orbit, on April 24, 2010.

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  The Cosmos

  Astronomy in the New Millennium

  • Jay M. Pasachoff, Alex Filippenko(Authors)
  • 2019(Publication Date)
  • Cambridge University Press

    (Publisher)

  If the signals are converted to sound, it is usually only so that the astronomers can monitor them to make sure no radio broadcasts are interfering with the celestial signals. Radio telescopes were originally limited by their very poor angu- lar resolution. The resolution of a telescope depends only on the telescope’s diameter, but we have to measure the diameter relative to the wavelength of the radiation we are studying. For a radio tele- scope studying waves 10 cm long, even a 100-m telescope is only 1000 wavelengths across. A 10-cm optical telescope studying ordinary light is 200,000 wavelengths across, so it is effectively much larger and gives much finer images (■ Fig. 3–34); see Figure It Out 3.3: Angular Resolution of a Telescope. For the human eye, it is the lens’s optical quality rather than its diameter that limits the resolution that we can perceive to about 1 minute of arc, about the size of Jupiter in the sky. A radio telescope at the National Radio Astronomy Observatory’s site at Green Bank, West Virginia, replaced one that collapsed. This Robert C. Byrd Green Bank Telescope, operated since 2016 by the independent Green Bank Observatory, has a reflecting surface 100 m in diameter in an unusual design (■ Fig. 3–35). New technology has made it possible to observe radio waves well at relatively short wavelengths, those measured to be a few millimeters. Molecules in space are especially well studied at these wavelengths. The Green Bank Telescope is useful for studying such molecules. A breakthrough in providing higher resolution has been the devel- opment of arrays of radio telescopes that operate together and give the resolution of a single telescope spanning kilometers or even continents. The Jansky Very Large Array (VLA), named after Karl G. Jansky (the founder of radio astronomy), is a set of 27 radio tel- escopes, each 26 m in diameter; it was seen in the movie Contact (■ Fig. 3–36).

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  The Cosmos

  Astronomy in the New Millennium

  • Jay M. Pasachoff, Alex Filippenko(Authors)
  • 2013(Publication Date)
  • Cambridge University Press

    (Publisher)

  It is to be erected in Chile on Cerro Pachón, already the site of Gemini South. The 2010 Astronomy and Astrophysics Decadal Survey, a scientific-community-wide effort, ranked LSST first in its category of large ground-based projects. A wide variety of universities and institu- tions across the U.S. are already members of the consortium that will run the effort, which is directed by a professor at the University of California at Davis. 3.5 AMATEURS ARE PARTICIPATING It is fortunate for astronomy as a science that so many people are interested in looking at the sky. Many are just casual observers, who may look through a telescope occasionally as part of a course or on an “open night,” when people are invited to view through telescopes at a professional observatory, but others are quite devoted “amateur astronomers” for whom astronomy is a serious hobby. Some amateur astronomers make their own equipment, ranging up to quite large telescopes perhaps 60 cm in diameter. But most amateur astronomers use one of several commercial brands of telescopes. Computer power and the techniques for CCD image process- ing have advanced so much that these days, amateur astronomers are producing pictures that professionals using the largest telescopes would have been proud of a decade ago. One interesting technique is to take thousands of photos very quickly, use a computer to throw out the blurriest ones, and combine the rest to form a sharp image. Some amateurs are even contributing significantly to joint work with professional research astronomers, obtaining high-quality complemen- tary data. Some of the amateur telescopes are Newtonian reflectors, with mirrors 15 cm in diameter being the most popular size (■ Fig. 3–21a). It is quite possible to shape your own mirror for such a telescope. The Dobsonian telescope is a variant of this type, made with very inexpensive mirrors and construction methods to provide affordable systems with relatively large mirrors.

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  Solar Planetary Systems

  Stardust to Terrestrial and Extraterrestrial Planetary Sciences

  • Asit B. Bhattacharya, Jeffrey M. Lichtman(Authors)
  • 2016(Publication Date)
  • CRC Press

    (Publisher)

  Instrument quality : A research telescope is only as good as the cameras and other instruments which record and analyze the light that it captures. Instruments are judged by factors like the quality of their images, how effectively they spread out light, and how much light they capture.

  FIGURE 19.10 Lower and higher resolution images.

     

  19.7 NASA’s Great Observatories

  The four Great Observatories are a series of space telescopes with a purpose to give the complete picture of objects at different wavelengths. Each observatory investigates a particular wavelength region in a greater detail [6 , 7, and 8 ].

  The telescopes, in order of launch, are (1) the Hubble Space Telescope (1990); (2) Compton Gamma Ray Observatory, popularly called CGRO (1991); (3) Chandra X-ray Observatory (1999); and (4) Spitzer Space Telescope (2003). Figure 19.11 shows each telescope including the wavelength region it was built to observe. All except for CGRO are providing information at present.

  Figure 19.12 exhibits the images showing the remains of an exploded star (Kepler’s supernova), as seen by three telescopes of the Great Observatories.

  19.7.1 Hubble Space Telescope’s Design

  The Hubble Space Telescope was deployed from the space shuttle Discovery during STS-31 on April 25, 1990. Since then, there have been five servicing missions that continued to upgrade the telescope’s scientific instruments and operational systems. Hubble reached a major milestone, its twentieth anniversary in orbit, on April 24, 2010.

  As seen in Figure 19.13 , when light hits the “concave” primary mirror, it is reflected to the convex secondary mirror, then returns through a hole in the center of the “primary mirror.” The light comes to the “focal point” and then passes to one of Hubble’s “instruments.” Telescopes of this design are called Cassegrain telescopes, after the person who first designed the same. Figure 19.13a shows the design of the telescope [9 , 10 and 11 ].

  Hubble is one of the largest and most versatile and is well known as both a vital research tool and a public relations boon for astronomy. The telescope (Figure 19.13b ) was built by the United States space agency NASA with contributions from the European Space Agency. This is operated by the Space Telescope Science Institute [9

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