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University of Groningen the Logistic Design of the LOFAR Radio Telescope Schakel, L.P University of Groningen The logistic design of the LOFAR radio telescope Schakel, L.P. IMPORTANT NOTE: You are advised to consult the publisher's version (publisher's PDF) if you wish to cite from it. Please check the document version below. Document Version Publisher's PDF, also known as Version of record Publication date: 2009 Link to publication in University of Groningen/UMCG research database Citation for published version (APA): Schakel, L. P. (2009). The logistic design of the LOFAR radio telescope: an operations Research Approach to optimize imaging performance and construction costs. PrintPartners Ipskamp B.V., Enschede, The Netherlands. Copyright Other than for strictly personal use, it is not permitted to download or to forward/distribute the text or part of it without the consent of the author(s) and/or copyright holder(s), unless the work is under an open content license (like Creative Commons). Take-down policy If you believe that this document breaches copyright please contact us providing details, and we will remove access to the work immediately and investigate your claim. Downloaded from the University of Groningen/UMCG research database (Pure): http://www.rug.nl/research/portal. For technical reasons the number of authors shown on this cover page is limited to 10 maximum. Download date: 26-09-2021 Chapter 2 Radio Telescopes 2.1 Introduction This chapter explains the basics of radio telescopes, the types of radio telescopes that exist, and what they can observe in the universe. It is included to provide the reader background information on radio telescopes and to introduce concepts which will be used in later chapters. We start our discussion with the notion of radio telescope. A radio telescope is an observation instrument consisting of one or more antenna systems to study the uni- verse through the measurement of electromagnetic radiation in the radio spectrum. We give an interpretation of the above definition by explaining its keywords: obser- vation instrument, antenna system, electromagnetic radiation, and radio spectrum. The outline of this chapter is as follows. Section 2.2 describes the basic system of a radio telescope and its properties. Section 2.3 explains the instrumentation that has been developed in the last century. Section 2.4 discusses radio wave detection. 2.2 Basics of Radio Telescopes The basic system of a radio telescope consists of three components: (1) an antenna system (i.e., simple receptor, dish antenna, or multi-sensor station), (2) a receiving system (i.e., a system being composed of a mechanism for noise reduction and a mechanism for amplifying and measuring the signal), and (3) a device for recording, monitoring, and displaying the output from the system (i.e., a computer system). Conventional radio telescopes consist of a base with a single fully steerable parabolic dish antenna. The dish antenna has a small antenna placed on its aper- ture. This antenna is called the feed and can be described as a horn-shaped antenna. The dish antenna acts as a radio reflector focusing the incoming radiation onto the feed. The reflected radiation is then transferred from the feed to the receiving system. Figure 2.1 shows the components and functioning of a conventional radio telescope. 11 12 Chapter 2. Radio Telescopes Figure 2.1. A conventional radio telescope A radio telescope is usually indicated by its size and name, the latter depending on the type of the instrumentation or its location. A radio telescope can also be described by a list of properties, which are listed below. • Frequency range. The frequency range is the range of frequencies, expressed by a lower and upper frequency, within which radio waves are observed by the instrument. • Angular resolution. The angular resolution is the distance, in angular units, be- tween two close objects that can be separated by instrument. It is also called spatial resolution or resolving power. • Sensitivity. The sensitivity is the ability to observe radio sources emitting low level radio signals. • Collecting area. The collecting area is the area of an instrument capable of col- lecting electromagnetic radiation. The collecting area is positively related with the instrument’s sensitivity. There is a close relationship between the angular resolution of an instrument and the observing frequency, which is inversely proportional to the observing wave- length (see Section 2.4). The angular resolution (ρ) of a radio telescope is approx- imated by dividing the wavelength of the observed radiation (λ) by the diameter of the telescope (D). Equation (2.1) gives this relationship. In this equation, λ and D are measured in the same length units and the angular resolution ρ is measured in arcseconds (see glossary, p. 259). λ 180 ρ ≈ 3600 (2.1) D π 2.3. Types of Radio Telescopes 13 2.3 Types of Radio Telescopes In this section we discuss the types of radio telescopes that have been developed in the twentieth century. The contents of this section is mainly based on Malphrus (1996), Kellermann and Moran (2001), Thompson et al. (2001), and Burke and Graham-Smith (2002). 2.3.1 Individual Radio Telescopes The first radio telescope built was the 175-meter long wire antenna of Charles Nord- mann in 1901. The design of the telescope turned out to be effective, however, Nord- mann failed to observe extraterrestrial radio signals due to a sunspot minimum (see glossary, p. 259). Another early radio telescope was constructed by Karl Guthe Jansky in 1930; see Figure 2.2. He built a rotate-able, horizontal and vertical wire skeleton to observe radio signals of 20.5 MHz. Jansky was the first person to discover cosmic radiation. Figure 2.2. The early radio telescope of Karl Jansky c NRAO The first parabolic dish antenna was developed by Grote Reber in 1937; see Figure 2.3. His 9.45-meter paraboloid became the prototype for a whole range of paraboloids radio telescopes. The radio telescope of Reber permitted observations at different frequencies. Reber operated the radio telescope at a wavelength of one centimeter, and later also at wavelengths of 33 centimeters and 1.87 meters. To see deeper and deeper into the universe, and with enhanced angular reso- lution and sensitivity, the size of single-dish paraboloids was enlarged (see formu- lae (2.1)). The postwar years initiated the race to build the world’s largest radio 14 Chapter 2. Radio Telescopes Figure 2.3. Grote Reber and his 9.45-meter paraboloid c NRAO telescope. The first large paraboloid was the 66.5-meter transit telescope, a fixed paraboloid built at Jodrell Bank in the United Kingdom circa 1949. The size limitation of paraboloids with a fully steerable base was already en- countered in mid-1960. It turned out that dish sizes larger than 100 meters were not possible from an engineering point of view. Solutions were found in the base of radio telescopes and the shape of the reflectors. Examples of radio telescopes to which these solutions have been applied are the 305-meter Arecibo telescope (Puerto Rico, USA) and the 2-reflector Kraus telescope (Ohio, USA). The Arecibo telescope consists of a spherical dish built inside a karst depression and hangs on eighteen ca- bles strung from three solid poles. The Kraus telescope consists of a non-steerable parabolic rectangular reflector (109.8 x 21.35 meters) and a semi-steerable flat rectan- gular reflector (103.7 x 91.5 meters). The flat reflector of the Kraus telescope reflects the incoming radio signals towards the parabolic reflector where it is focused on the feed. Currently, the largest individual radio telescope is the RATAN-600 telescope. It consists of 895 rectangular reflectors (2 x 7.4 meters) which are arranged along the boundary of a circle with a diameter of 576 meters. RATAN-600 is located in the North Caucasus, Russia. Table 2.1 gives the properties of large individual radio tele- scopes constructed in the twentieth century. 2.3. Types of Radio Telescopes 15 Table 2.1. Large individual twentieth-century radio telescopes Year Name Location Steerability Wavelengths 1949 66.5-meter Transit Telescope Jodrell Bank, United Kingdom partially steerable 1.87 m 1956 25-meter Dwingeloo Antenna Dwingeloo, The Netherlands fully steerable 3, 6, 18, 21 cm 1957 76.25-meter Lovell Telescope Jodrell Bank, United Kingdom fully steerable 18 cm - 25 cm 1960 Kraus 2-Reflector Telescope Delaware, Ohio USA partially steerable 21 cm 1963 305-meter Arecibo Telescope Arecibo, Puerto Rico non-steerable 3 cm - 6 m 1972 100-meter Effelsberg Telescope Effelsberg, Germany fully steerable 0.35 mm - 15 m 1977 576-meter RATAN-600 Telescope North Caucasus, Russia fixed location 1 cm - 50 cm 2000 100-meter Green Bank Telescope1 Green Bank, West Virginia USA fully steerable 3 mm - 3 m The original Green Bank Telescope (GBT) was built in 1962. However, due to a lack of maintenance, the telescope collapsed on Tuesday the 15th of November 1988. The loss of the 100-meter telescope resulted in the GBT project, a project concerned with the construction of world’s largest fully steerable radio telescope. The new radio telescope has roughly the same size as the original telescope; the dimensions of the dish antenna are 100 by 110 meters. The GBT project has been completed in 2000. Source: Barrett (2002). 16 Chapter 2. Radio Telescopes 2.3.2 Radio Arrays In order to improve the angular resolution beyond the size of the instrument the so- called sea interferometer was developed (Pawsey et al., 1946). A sea interferometer is a radio telescope consisting of one antenna system located along the coast side that observes radio signals directly from the sky as well as radio signals that are reflected by the sea surface.
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