Radio Telescopes

12 Key excerpts on "Radio Telescopes"

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  Astronomy

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

    (Publisher)

  Thus, the astronomical radio receiver operates much like a spectrometer on a visible-light or infrared telescope, providing information about how much radiation we receive at each wavelength or frequency. After computer processing, the radio signals are recorded on magnetic disks for further analysis. Radio waves are reflected by conducting surfaces, just as light is reflected from a shiny metallic surface, and according to the same laws of optics. A radio-reflecting telescope consists of a concave metal reflector (called a dish), analogous to a telescope mirror. The radio waves collected by the dish are reflected to a focus, where they can then be directed to a receiver and analyzed. Because humans are such visual creatures, radio astronomers often construct a pictorial representation of the radio sources they observe. Figure 6.18 shows such a radio image of a distant galaxy, where Radio Telescopes reveal vast jets and complicated regions of radio emissions that are completely invisible in photographs taken with light. Figure 6.18 Radio Image. This image has been constructed of radio observations at the Very Large Array of a galaxy called Cygnus A. Colors have been added to help the eye sort out regions of different radio intensities. Red regions are the most intense, blue the least. The visible galaxy would be a small dot in the center of the image. The radio image reveals jets of expelled material (more than 160,000 light-years long) on either side of the galaxy. (credit: NRAO/AUI) Radio astronomy is a young field compared with visible-light astronomy, but it has experienced tremendous growth in recent decades. The world’s largest radio reflectors that can be pointed to any direction in the sky have apertures of 100 meters. One of these has been built at the US National Radio Astronomy Observatory in West Virginia (Figure 6.19). Table 6.2 lists some of the major Radio Telescopes of the world. Chapter 6 Astronomical Instruments 211

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  Fundamentals of Radio Astronomy

  Observational Methods

  • Jonathan M. Marr, Ronald L. Snell, Stanley E. Kurtz(Authors)
  • 2015(Publication Date)
  • CRC Press

    (Publisher)

  75 C H A P T E R 3 Radio Telescopes T his chapter provides an overview of the principal components of a radio telescope and describes what role each of these components play in detecting astronomical radio signals. A typical radio telescope consists of a primary reflector (or dish), feed, transmission line, and receiver; these components are shown schematically in Figure 1.15. We note that Radio Telescopes operating at very long wavelengths (typically 1 m or longer) can take a very different form as discussed in Section 3.6. Most Radio Telescopes are fully steerable, mounted on Alt–Az, (also called Az–El) mounts (described in Section 1.4.2), and can point to any direction in the sky. A computer controls the motion of the telescope and continu-ously translates the sky coordinates (see Section 1.3.1) of an astronomical object into cur-rent altitude and azimuth positions. We will not discuss further the mechanics of how a radio telescope is mounted or how it moves. Some aspects of a radio telescope are common to telescopes used at ultraviolet, visible, or infrared wavelengths, such as the use of a primary reflector (and often a secondary reflec-tor) to collect and focus the light. We discuss the optics of Radio Telescopes in Section 3.1. However, there are also aspects that are quite different from telescopes at other wavelengths. One of the biggest differences is the method by which light is detected. At shorter wave-lengths, one detects the particle nature of light, meaning the individual photons. The energy of a visible-wavelength photon ( E = h ν = hc / λ ) is of order a few electron volts (an electron volt is 1.6 × 10 − 19 J), which is sufficient to either excite a valence electron to a conduction band electron in a semiconductor (as occurs in photo-conductive devices) or to produce electron-hole pairs in a semiconductor (in photovoltaic devices). Charge-coupled devices (CCDs), which are at the heart of digital cameras, for example, work by these principles.

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  Astronomical Observations: Astronomy and the Study of Deep Space

  • Britannica Educational Publishing, Erik Gregersen(Authors)
  • 2009(Publication Date)
  • Britannica Educational Publishing

    (Publisher)

  Using powerful radar systems, it is possible to detect radio signals reflected from nearby astronomical bodies such as the Moon, the nearby planets, some asteroids and comets, and the larger moons of Jupiter. Precise measurements of the time delay between the transmitted and reflected signal and the spectrum of the returned signal are used to precisely measure the distance to solar system objects and to image their surface features with a resolution of a few metres. The first successful detection of radar signals from the Moon occurred in 1946. This was quickly followed by experiments in the United States and the Soviet Union using powerful radar systems built for military and commercial applications. Both radio and radar studies of the Moon revealed the sandlike nature of its surface even before the Apollo landings were made. Radar echoes from Venus have penetrated its dense cloud cover surrounding the surface and have uncovered valleys and enormous mountains on the planet’s surface. The first evidence for the correct rotation periods of Venus and of Mercury also came from radar studies

  .

  PRINCIPLES OF OPERATION

  Radio Telescopes vary widely, but they all have two basic components: (1) a large radio antenna and (2) a sensitive radiometer, or radio receiver. The sensitivity of a radio telescope—i.e., the ability to measure weak sources of radio emission—depends both on the area and efficiency of the antenna and on the sensitivity of the radio receiver used to amplify and to detect the signals. For broadband continuum emission over a range of wavelengths, the sensitivity also depends on the bandwidth of the receiver. Because cosmic radio sources are extremely weak, Radio Telescopes are usually very large, up to hundreds of metres across, and use the most sensitive radio receivers available. Moreover, weak cosmic signals can be easily masked by terrestrial radio interference, and great effort is taken to protect Radio Telescopes from man-made emissions.

  The most familiar type of radio telescope is the radio reflector consisting of a parabolic antenna, which operates in the same manner as a television satellite dish to focus the incoming radiation onto a small antenna called the feed, a term that originated with antennas used for radar transmissions. This type of telescope is also known as the dish, or filled-aperture, telescope. In a radio telescope the feed is typically a waveguide horn and transfers the incoming signal to the sensitive radio receiver. Solid-state amplifiers that are cooled to very low temperatures to reduce significantly their internal noise are used to obtain the best possible sensitivity.

  In some Radio Telescopes the parabolic surface is equatorially mounted, with one axis parallel to the rotation axis of Earth. Equatorial mounts are attractive because they allow the telescope to follow a position in the sky as Earth rotates by moving the antenna about a single axis parallel to Earth’s axis of rotation. But equatorially mounted Radio Telescopes are difficult and expensive to build. In most modern Radio Telescopes, a digital computer is used to drive the telescope about the azimuth and elevation axes to follow the motion of a radio source across the sky.

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  Pulsar Astronomy

  • Andrew Lyne, Francis Graham-Smith(Authors)
  • 2012(Publication Date)
  • Cambridge University Press

    (Publisher)

  3 Telescopes and techniques Observations of neutron stars and pulsars extend over more than 19 decades of the elec- tromagnetic spectrum, from low radio frequencies (around 30 MHz) to high gamma-ray energies (above 200 GeV). The techniques used in telescopes between these extremes range from the coherent detection of radio waves to photon detection techniques more usually associated with nuclear physics. There are nevertheless elements in common over the whole range, which we will refer to in this brief survey. (1) The signal is weak, requiring large collecting areas and long integration times. (2) Identification of objects requires accurate positions and discrimination from adjacent sources. (3) Pulsed sources require high timing accuracies, often around 1 microsecond. (4) Measurements must discriminate against unwanted backgrounds, either of astronomical origin, such as radio emission or cosmic rays from the Milky Way Galaxy, or from terrestrial sources, especially man-made radio signals. The terrestrial atmosphere is transparent to radio waves (except at short millimetric wavelengths where molecular absorption occurs, and at long metric wavelengths where ionospheric refraction and reflection occur). Radio Telescopes can therefore be built at ground level, and can extend in size almost indefinitely, giving both high sensitivity and high angular resolution. X-rays and gamma-rays are absorbed in the atmosphere, and direct detection of such high-energy photons can only be achieved using space-based telescopes, where telescope apertures are limited by the capabilities of launch vehicles to a few metres in diameter. At the highest gamma-ray energies, however, individual photons can be detected at ground level through their creation of showers of energetic particles in the atmosphere; this is the basis of the Cerenkov air-shower arrays, which extend observations of pulsar wind nebulae up to 300 GeV and possibly beyond.

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  Apertures

  • R. C. Hansen(Author)
  • 2014(Publication Date)
  • Academic Press

    (Publisher)

  CHAPTER Radio-Telescope Antennas HSIEN CHING KO I. Introduction A . DEFINITION AND FUNCTIONS OF A RADIO TELESCOPE The radio telescope is the basic instrument used by radio astronomers for the observation and study of radio frequency waves of extraterrestrial origin. The extraterrestrial radio waves were first discovered by K. G. Jansky in 1932, who opened the way for a new means of exploring the universe. A radio telescope consists of an antenna system, a highly sensitive radio receiving system, and output recording equipment. The radio telescope is analogous to an optical telescope. Its antenna, like the optical objective lens or mirror, serves to collect celestial radio signals and to concentrate them on the receiver. The receiver-recorder amplifies, detects, and records the signals, and thus acts like the photographic plate or the eye. The arrangement of a sunple radio telescope is illustrated in Fig. 1 together with an optical telescope for comparison. Radio waves from a radio source are focused by a parabolic dish antenna to a focal point for detection. As the antenna beam sweeps across the sky, the radiation it intercepts is detected and amplified by the receiver, and pen-recorded on a moving paper chart. The presence of a radio source is noted as a hump on the recorded profile as shown in Fig. 1. If the radio source is a point source, the signature on the record is a direct measure of the antenna pattern. Figure 2 is a sample record taken in the Cygnus region using the Ohio State University 96-helix radio telescope (Ko, 1958). 263 4 264 Hsien Ching Ko Incoming radio waves — > » ^ R a c e i ^ Receiver R0Cordtr|».../''*VJi^ Parabolic reflector antenna Optical reflecting telescope Hoto plate holder at prime focus Fig. 1. Analogy between the radio telescope and the optical telescope. It is seen that, from a simple radio telescope only one piece of informa-tion about the image is obtained at a time.

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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)

  20 Space Telescopes—An In-Depth View 20.1 Introduction The space telescopes are categorized by the task that they were designed for. In most cases, these tasks are for the research of gamma rays, X-rays, ultraviolet light, visible light, infrared, microwave emission, and radio waves. Space telescopes which collect particles, such as cosmic ray nuclei, electrons, and instruments used to detect gravitational waves, are also among the categories listed. Two values are given for the dimensions of the initial orbit. For telescopes in Earth orbit, the minimum and maximum altitude are given in kilometers. For telescopes in solar orbit, the minimum distance (periapsis) and the maximum distance (apoapsis) between the telescope and the center of mass of the Sun are given in astronomical units (AU). 20.2 Gamma Ray Space Telescopes Gamma ray telescopes (Table 20.1) are used to collect and measure individual, high-energy gamma rays from different astrophysical sources. These are absorbed by the atmosphere, requiring that observations are made by high-altitude balloons or space missions. Gamma rays are generated by supernovae, pulsars, neutron stars, and black holes. Gamma ray bursts, with sufficiently high energies, have also been detected but have yet to be identified [ 1 ]. 20.3 X-Ray Telescopes The purpose of X-ray telescopes (Table 20.2) are to measure high-energy photons termed as X-rays. X-rays cannot travel long distances through the atmosphere. They can only be seen in the upper altitudes of our atmosphere or in space. There are a variety of astrophysical objects that emit X-rays, from galaxy clusters, through black holes, supernova remnants, stars, and binary stars having white dwarf and neutron stars. Some solar system bodies emit X-rays through the process of reflection

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  Stars and Galaxies

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

    (Publisher)

  Cengage Learning reserves the right to remove additional content at any time if subsequent rights restrictions require it. 107 Chapter 6 Light and Telescopes reasons, all large astronomical telescopes built since the start of the 20th century have been reflecting telescopes. Telescopes intended for the study of visible light are called optical telescopes ( Figure 6-7a ). As you learned previously, radio waves as well as visible light from celestial objects can penetrate Earth’s atmosphere and reach the ground. Astronomers gather radio waves using Radio Telescopes such as the one in Figure 6-7b that resemble giant TV satellite dishes. It is technically extremely difficult to make a lens that can focus radio waves, so all Radio Telescopes, including small ones, are reflecting telescopes; the dish is the primary mirror. 6-2b The Powers and Limitations of Telescopes A telescope’s capabilities are described in three mathematical relationships that are called the three powers of a telescope. The two most important of these powers depend on the diameter of the telescope. Light-Gathering Power Nearly all of the interesting objects in the sky are faint sources of light, so astronomers need telescopes that can collect large amounts of light to be able to study those objects. Light-gathering power refers to the ability of a telescope to collect light. Catching light in a telescope is like catching rain in a bucket— the bigger the bucket, the more rain it can catch ( Figure 6-8 ). Light-gathering power is proportional to the area of the telescope primary lens or mirror; a lens or mirror with a large area gathers a large amount of light. The area of a circular lens or mirror written in terms of its diameter D is  D 2 /4.

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  The Froehlich/Kent Encyclopedia of Telecommunications

  Volume 15 - Radio Astronomy to Submarine Cable Systems

  • Fritz E. Froehlich, Allen Kent(Authors)
  • 2022(Publication Date)
  • CRC Press

    (Publisher)

  R Radio Astronomy

  What Is Radio Astronomy?

  Radio astronomy refers to astronomy using radio waves naturally generated by cosmic bodies. The term excludes radar astronomy, which consists of studying radar echoes from objects in the solar system.

  Today, astronomy (or, better, astrophysics) research is generally independent of wavelength or frequency. Modern astronomers combine observations from many wavelength ranges to study particular astronomical phenomena.

  For astronomers, radio is an imprecise term that today might include the electromagnetic spectrum between γ 300 μm (micrometers) and λ 30,000 m (meters), a range of 108 . Overlaps with other named bands occur. The far infrared band includes submillimeter wavelengths. The International Telecommunication Union specifies the use of radio bands from 9 kilohertz (kHz) (λ 33,333 m) to 275 GHz (gigahertz) (λ 1.1 millimeters [mm]) to minimize conflict of radio services.

  Conducting astronomy research at radio wavelengths requires understanding astrophysics in general and, specifically, the physical mechanisms that generate cosmic radio emissions. Observationally, it means understanding the specialized equipment designed to respond quantitatively to cosmic radio waves.

  Early Years

  Radio astronomy began in the first half of this century. Thomas Edison suggested observations of radio waves from the sun around 1890. Sir Oliver Lodge unsuccessfully searched for radio emissions from the sun between 1897 and 1900. Karl G. Jansky is considered the father of radio astronomy through his discovery of cosmic radio emission at a frequency of 20.5 MHz from the Milky Way (our galaxy) in 1932 (1

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  Physical Principles of Astronomical Instrumentation

  • Peter A. R. Ade, Matthew J. Griffin, Carole E. Tucker(Authors)
  • 2021(Publication Date)
  • CRC Press

    (Publisher)

  8 Radio Instrumentation

  8.1 Introduction

  In this chapter, we consider the techniques used for coherent detection in radio astronomy, which are used today for frequencies up to several THz (wavelengths down to ~ 100 μm), and review the key performance measures for Radio Telescopes and receivers.

  Considering coherent detection necessarily means adopting the wave picture of electromagnetic radiation, and radio receivers are capable of measuring both the radio-frequency amplitude and phase. An astronomical radio receiver has to select the observing frequency and bandwidth, detect and amplify the signal, and measure the power arriving from the source in that band. At low frequencies, the receiver electronics can operate at the same frequency as the signal, but electronic circuits do not perform well at high radio frequencies (>100 GHz) because components and the links between them operate inefficiently (for instance, due to stray electrical capacitances tending to short the signals to ground). Many receivers, therefore, utilise a technique known as mixing, whereby the signal is combined with a locally generated reference at a slightly different frequency to produce a beat-frequency signal at the difference frequency. The lower-frequency signal still contains the desired information, which has been “down-converted” and can now be further processed by electronic circuits.

  Radio techniques have developed enormously since their early development during and after the Second World War and have been applied to higher and higher frequencies. To achieve good angular resolution at radio wavelengths, very large telescopes are needed. Modern radio observatories include single-dish facilities and arrays of many antennas to form interferometers which can, through combining their signals, provide high angular resolution through a technique known as aperture synthesis.

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  Foundations of Astronomy, Enhanced

  • Michael Seeds, Dana Backman(Authors)
  • 2016(Publication Date)
  • Cengage Learning EMEA

    (Publisher)

  118 PART 1 EXPLORING THE SKY government in a mountain-ringed bowl-shaped valley like the one in Puerto Rico. FAST will have more than 2.5 times the collecting area of Arecibo’s dish. A radio astronomer works under two disadvantages relative to optical astronomers: poor resolution and low signal intensity. Recall that the resolv-ing power of a telescope depends on the diameter of the primary lens or mirror but also on the wavelength of the radiation. At very long wave-lengths like those of radio waves, the diffraction fringes are quite large. This means that images or maps from indi-vidual Radio Telescopes generally don’t show such fine details as are seen in optical images. The second handicap radio astronomers face is the low intensity of the radio signals. You learned previ-ously that the energy of a photon depends on its wavelength. Photons of radio energy have such long wavelengths that their individual energies are quite low. The cosmic radio signals arriving on Earth are astonishingly weak—as little as one-billionth the strength of the signal from a commer-cial radio station. To get detectable signals focused on the antenna, radio astronomers must build large collecting areas either as single large dishes or by combining arrays of smaller dishes. Even then, because the radio energy from celestial objects is so weak, it must be strongly amplified before it can be mea-sured and recorded. LSST Corporation Figure 6-17 The 305-m (1000-ft) radio telescope in Arecibo, Puerto Rico, is nestled in a naturally bowl-shaped valley. The receiver platform is suspended over the dish. A consortium led by SRI International and Universities Space Research Association (USRA) manages Arecibo Observatory for the National Science Foundation (NSF). Figure 6-16 The 8.4-m Large Synoptic Survey Telescope (LSST) will use a special three-mirror design to create an exceptionally wide field of view, with the ability to survey the entire southern sky every three nights.

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  Space Antenna Handbook

  • William A. Imbriale, Steven Shichang Gao, Luigi Boccia(Authors)
  • 2012(Publication Date)
  • Wiley

    (Publisher)

  Chapter 16 Space Antennas for Radio Astronomy Paul F. Goldsmith Jet Propulsion Laboratory, California Institute of Technology, USA

  16.1 Introduction

  This chapter discusses antennas used for radio astronomy observations carried out from spacecraft. The considerations for space radio astronomy antennas differ somewhat from those applied to satellite communications and remote sensing of the Earth. Radio astronomy observations from space cover an enormous range in frequency from below 1 MHz to over 1000 GHz. The upper frequency limit is certainly arbitrary as the terminology as well as the technology change gradually as one moves from the submillimeter to the far infrared. The critical antenna parameters also vary considerably depending on the type of observations to be carried out. This necessarily incomplete discussion includes four areas in which space observations have played a major role: cosmic microwave background observations, submillimeter/far-infrared astronomy, low-frequency radio astronomy, and space very long baseline interferometry. For each area, we discuss key aspects of antenna performance and give a summary of the space missions with some detailed information about the antennas that have been employed.

  16.2 Overview of Radio Astronomy and the Role of Space Antennas

  Radio astronomy deals with the collection and analysis of electromagnetic signals from celestial sources in what is quite imprecisely defined as the radio wavelength or frequency range. The first observations of radiation from the center of the Milky Way by Karl Jansky in the early 1930s were carried out at a frequency of 20.6 MHz. Subsequent observations by Grote Reber were at a frequency of 160 MHz, and he soon moved up to 480 MHz. Technological improvements driven by the development of radar during World War II soon allowed observations of the Sun at centimeter wavelengths, and the first spectral line, the 21 cm hyperfine transition of atomic hydrogen at 1420 MHz, was detected in 1951. By the mid 1960s observations of Solar System and extragalactic objects were being carried out at wavelengths of 3.4 mm and even 1 mm.

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  Remote and Robotic Investigations of the Solar System

  • C.R. Kitchin(Author)
  • 2017(Publication Date)
  • CRC Press

    (Publisher)

  Most investigations in astronomy and astrophysics are pure remote sensing rather than the experimental approaches used in other sciences. That is to say, all the astronomer can do is look to the best of his/her ability – changing the experimental conditions in the way that, for example a chemist might alter the concentration of a solution or its temperature in order to clarify a result, simply cannot be done. The use of radar (or lidar) to investigate an object, however, enables the astronomer to act in some ways as an experimental scientist for once. The power, frequency, pulse repetition rate, distance from the target, etc. of the radar apparatus may all be controllable to a greater or lesser degree in order to improve the observations.

  Radar studies of solar system objects take two different forms:

  • Earth-based investigations using high power transmitters and sensitive receivers combined with the largest available radio telescope dishes
  • Spacecraft-based instruments that have lower powers and smaller dishes, etc. but nevertheless much higher angular and/or spatial resolutions

  2.4.2  Physical Principles of Radar Systems

  The performance of a radar system depends upon many factors and these are combined within the radar equation. For targets that are angularly unresolved and where the transmitting dish and receiving radio telescope are the same, the equation takes the form

  F =

  P α

  A e 2

  ν 2

  4 π

  c 2

  R 4

  (2.5)

  where F is the returned signal strength, P is the power broadcast by the transmitter, α is the radar cross section of the target (defined as the cross–sectional area of a perfectly isotropically scattering sphere which would return the same amount of energy to the receiver as does the target), A e is the effective area of the transmitting and receiving antenna/dish, ν is the operating frequency and R is the separation between the radar and its target.

  When the target is angularly resolved (and perhaps also resolved in depth) the target’s radar cross section is replaced by an appropriate integral. Thus, a spherical target would have

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