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.

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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 universe through the measurement of electromagnetic radiation in the radio spectrum. We give an interpretation of the above deﬁnition by explaining its keywords: observation 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 aperture. 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, between 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 collecting 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 wavelength (see Section 2.4). The angular resolution (ρ) of a radio telescope is approximated 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 resolution and sensitivity, the size of single-dish paraboloids was enlarged (see formulae (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 cables 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 rectangular 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 telescopes constructed in the twentieth century. 2.3. Types of Radio Telescopes 15 Wavelengths 1.87 m 3, 6, 18, 21 cm 18 cm - 25 cm 21 cm 3 cm - 6 m 0.35 mm - 15 m 1 cm - 50 cm 3 mm - 3 m Steerability partially steerable fully steerable fully steerable partially steerable non-steerable fully steerable fixed location fully steerable Location Jodrell Bank, United Kingdom Dwingeloo, The Netherlands Jodrell Bank, United Kingdom Delaware, Ohio USA Arecibo, Puerto Rico Effelsberg, Germany North Caucasus, Russia Green Bank, West Virginia USA 1 Large individual twentieth-century radio telescopes Table 2.1. Name 66.5-meter Transit Telescope 25-meter Dwingeloo Antenna 76.25-meter Lovell Telescope Kraus 2-Reflector Telescope 305-meter Arecibo Telescope 100-meter Effelsberg Telescope 576-meter RATAN-600 Telescope 100-meter Green Bank Telescope Year 1949 1956 1957 1960 1963 1972 1977 2000 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 ofThe the new 100-meter radio telescope telescope resulted hascompleted in in roughly the the 2000. GBT same Source: size project, Barrett as a (2002). the project original concerned telescope; with the the dimensions construction of of the world’s dish largest antenna fully are steerable 100 radio by telescope. 110 meters. The GBT project has been 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. The technique of using the sea as reflector of radio signals is known as sea interferometry. Martin Ryle and Derek Vonberg provided a method of combining two or more radio telescopes electronically to simulate one large telescope (Ryle and Vonberg, 1946). The technique is known as radio interferometry. It combines the radio signals from a pair of antenna systems by performing corrections for the time delay resulting from the corresponding separation. Radio interferometry enabled further improvements in the angular resolution. The improvements in angular resolution were realized by radio arrays (or interferometer arrays). A radio array is a radio telescope consisting of two or more separate antenna systems which observe radio waves from the universe. The received radio signals are sent to a base station where they are combined and processed. The first radio array was built by Martin Ryle and Derek Vonberg in Cambridge, England (Ryle and Vonberg, 1946). In a radio array each pair of antenna systems defines a baseline (or interferometer). The baseline has a length (the distance between the two antenna systems) and an orientation (the angle between the line through the two antenna systems and a ref- erence axis). The length of a baseline indicates the ability to resolve nearby objects in the sky (i.e., baseline resolution). The orientation of a baseline gives a directional dimension to the resolution of the object. Figure 2.4 gives an illustration of the baseline induced by two antenna systems A and B in the Euclidean plane. The length and orientation of the baseline are indicated by rAB and θAB, respectively. Note that the baseline orientation is measured relative to the Y-axis.

Figure 2.4. The baseline induced by two antenna systems A and B 2.3. Types of Radio Telescopes 17

The imaging performance of a radio array strongly depends on the baselines of the array. It indicates the quality of the images that can be produced by the system. An array should have many baselines of different lengths and orientations in order to have a high-quality resolution standard. The distribution of the baseline lengths across the radio array determines the sensitivity proﬁle of the system. The ideal distribution for the baseline lengths should be determined on the basis of the scientiﬁc purposes of the instrument. The size of a radio array is measured by the baseline of maximum length. It determines the angular resolution of the instrument. The angular resolution of a radio array is approximated by replacing the diameter (D) in formulae (2.1) by the length of the maximum baseline (Lmax).

Data Transmission

The transmission of radio signals from the antenna systems to the base station is performed by transmission lines, radio links, or by the shipment of magnetic tapes or hard disks. The type of data transmission to be used mainly depends on the separations between the antenna systems, the wavelength of the transmitted signals, and the required speed of the transmission. In case of physical connections, the transmission speed is measured in terms of data transfer rate which is the number of bits that can be sent in one second. Next, we briefly review the types of data transmission used in radio arrays. Transmission lines are coaxial cables and fiber optic cables. Coaxial cables are used for the transmission of high-frequency radio signals over short distances (i.e., distances up to 500 meters). They allow data transfer rates from 10 to 100 megabit per second. Fiber optic cables are used for the transmission of radio signals of different frequencies. Multi-mode optical fibers allow data transfer rates of 100 megabit per second for distances up to two kilometers, one gigabit per second for distances up to 500 meters, and ten gigabit per second for distances up to 300 meters. Single-mode optical fibers carry light waves with a data transfer rate of ten gigabits per second over distances up to 60 kilometers. Radio links are used for the transmission of high-frequency radio signals over long distances (i.e., distances up to 200 kilometers). The data transfer rate is limited to about 128 megabits per second. An example of a radio array using radio links is the Multi-Element Radio-Linked Interferometer Network (MERLIN) located at Jo- drell Bank, United Kingdom. Transmission lines and radio links put restrictions on the size of a radio array. In order to enable arbitrarily long, variable-length baselines the technique of very long baseline interferometry (VLBI) was developed; see Matveenko et al. (1965). VLBI is a technique that records the observations of each antenna system on magnetic tapes or hard disks with timing information. They are later shipped to a base station where the information is combined and synchronized. An example of a radio array using VLBI is the Very Long Baseline Array (VLBA) which is a radio telescope of 10 dish antennas located throughout the whole of the United States. 18 Chapter 2. Radio Telescopes

Synthesis Techniques

The sources in the universe observed by radio telescopes are usually at very remote distances. The radio waves emitted from these sources may be considered as parallel when they arrive at the earth’s surface. Since a baseline is in general not normal to the source direction, the actual spacings by which a source is observed is less than the length of the baseline. Therefore, the actual imaging performance of a radio array depends on a mapping of the baselines into a plane normal to the observing direction, which is called the UV plane. A large number of spacings in the UV plane is required to form high-resolution NA images. A radio array with NA antenna systems gives at most 2 unique baselines. Solutions to the problem have been found in synthesis techniques which are techniques to increase the number of spacings in the UV plane. Next, we discuss several of these techniques. Aperture synthesis (AS) is a technique to generate supplementary baselines by moving antenna systems in two dimensions relative to one another (Blythe, 1957). It can be applied to radio arrays with transportable antenna systems. The technique was ﬁrst decribed by O’Brien, who used a variable spacing two-element interferometer to observe the sun (O’Brien, 1953). Nowadays, the notion of aperture synthesis is also used to indicate the type of interferometry that combines signals from two or more static antenna systems to produce images having the same angular resolution as an individual instrument with the same size. Earth rotation synthesis (ERS) is a technique to generate supplementary baselines using the diurnal motion of the earth (Ryle and Hewish, 1960). A source can be studied once, but also over an elongated period when it is above the horizon. In the latter case, the observing direction of the radio array is adjusted over time to observe the same source from different directions. The earth’s rotation sweeps the baselines through three-dimensional space thereby causing new spacings with different length and orientation in the UV plane. Figure 2.5 illustrates the concept of ERS for a north-south baseline AB. This baseline has different locations over times, i.e., AtBt is the location of baseline AB at time t and At+1Bt+1 is its location at time t + 1. We assume that baseline AB receives radiation from a source that emits radio waves parallel to the earth’s equator. This radiation is depicted by the double-headed arrows. For convenience, the UV plane is shown on the other side of the earth. Since baseline AB is never perpendicular to 0 0 0 0 the source direction, the projected baselines AtBt and At+1Bt+1 have smaller spacings in the UV plane. Note that projected baselines also have different orientations. Multi-frequency synthesis (MFS) is a technique to generate supplementary spacings in the UV plane by varying the observing frequency (Conway et al., 1990). We will see, in Chapter 5, that the length of a projected baseline in the UV plane also depends on the observing frequency. The effect of varying the observing frequency is that a source is observed over a narrow set of radio frequencies instead of a single one. Therefore, MFS can be considered as a technique that multiplies the number of spacings in the UV plane by changing the lengths of the projected baselines. 2.4. Electromagnetic Radiation 19

Figure 2.5. Earth rotation synthesis for a north-south baseline

This section ends with an overview of the main characteristics of some well- known radio arrays. Table 2.2 gives this overview. Each row of Table 2.2 corresponds to one radio array with the entries indicating the name of the instrument, the year of establishment, the location, the cardinality, the size, the type of data transmission, the applied synthesis techniques, and the frequency range, respectively.

2.4 Electromagnetic Radiation

An electromagnetic wave is a self-propagating wave in space that exists due to an elec- tric field and a magnetic field. The fields are oriented perpendicular to each other and mutually alternate causing a wave that travels in a direction normal to both the fields. An electromagnetic wave can be described by its wavelength (λ), frequency (f), and energy (E). The wavelength of an electromagnetic wave is defined as the distance between two adjacent crests of the wave. It is measured in units of length. The frequency of an electromagnetic wave is the number of oscillations that it makes per second. It is measured in Hertz (Hz). The energy of an electromagnetic wave is measured in Joules. The three physical properties are directly connected by the c speed of light (c) and Planck’s constant (h; see glossary, p. 259): f = λ and E = hf. The first relationship is known as the frequency-wavelength relationship. The collection of all possible types of electromagnetic waves is called the electromagnetic spectrum (EMS). It consists of gamma-rays, X-rays, ultraviolet light, visible light, infrared light, microwaves, and radio waves. 20 Chapter 2. Radio Telescopes GMRT VLBA ATCA EVN MERLIN VLA WSRT Name 1999 1993 1988 1980 1980 1980 1970 Year India States United Australia Intercontinental Kingdom United (USA) Mexico New Netherlands The Location al 2.2. Table # Systems hrceitc fwl-nw ai arrays radio well-known of Characteristics 30 10 18 27 14 6 6 00km 8000 km 9169 1 km 217 5km 25 km 36 km 6 km 3 Size ie pi cables optic Fiber VLBI cables Coaxial VLBI links Radio cables optic Fiber cables Coaxial transmission Data Synthesis AS/ERS AS/ERS AS/ERS AS/ERS AS/ERS AS/ERS AS/ERS ,5-,0 MHz 1,250-9,200 1-000MHz 312-90,000 MHz 327-43,214 MHz 151-24,000 45,0 MHz 74-50,000 MHz 117-1,200 0150MHz 50-1,500 rq range Freq. 2.4. Electromagnetic Radiation 21

2.4.1 Visibility of the Electromagnetic Spectrum Many sources in the universe emit electromagnetic radiation that can be detected by observation facilities. Examples of sources that emit radio waves are neutral hydro- gen and carbon monoxide which are usually found in spiral galaxies and quasars. The visibility of electromagnetic radiation at the earth’s surface strongly depends on the atmosphere and the ionosphere. That is, most of the extraterrestrial radiation is absorbed by the atmosphere (i.e., nitrogen, oxygen, ozone, vapor, and carbon dioxide) or blocked by the ionosphere. The optical and radio window are the only parts of the EMS that are completely transparent (Burke and Graham-Smith, 2002). Figure 2.6 shows the terrestrial visibility of the EMS.

Figure 2.6. The electromagnetic spectrum and its terrestrial visibility

2.4.2 Radio Spectrum Radio telescopes observe the universe through the measurement of electromagnetic radiation in the radio spectrum. The radio spectrum is the frequency range from about several hundreds of hertz to roughly one thousand of gigahertz (i.e., the wavelength varies from 1 millimeter to 100 kilometer). The bounds of the radio spectrum are fairly arbitrary since each scientiﬁc ﬁeld uses its own interpretation. Table 2.3 shows a decomposition of the radio spectrum. Radio telescopes are also limited by the opaqueness of the ionosphere. In fact, terrestrial radio telescopes cannot detect extraterrestrial radio waves when the radio frequency is below 20 MHz or above 300 GHz. These radio frequencies can only be observed by extraterrestrial radio telescopes like the Space Radio Telescope (SRT) (ASC, 2007). 22 Chapter 2. Radio Telescopes

Table 2.3. The radio spectrum (Wikipedia, 2007) Band name Abbrev. Frequencies Wavelengths Extremely low frequency ELF 3-30 Hz 10,000-100,000 km Super low frequency SLF 30-300 Hz 1,000-10,000 km Ultra low frequency ULF 300-3000 Hz 100-1000 km Very low frequency VLF 3-30 kHz 10-100 km Low frequency LF 30-300 kHz 1-10 km Medium frequency MF 300-3000 kHz 100-1,000 m High frequency HF 3-30 MHz 10-100 m Very high frequency VHF 30-300 MHz 1-10 m Ultra high frequency UHF 300-3000 MHz 100-1,000 mm Super high frequency SHF 3-30 GHz 10-100 mm Extremely high frequency EHF 30-300 GHz 1-10 mm