Download Radio Telescopes

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 Enschede: 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: 14-06-2017
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
2.3
13
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 Nordmann in 1901. The design of the telescope turned out to be effective, however, Nordmann 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.
c
Figure 2.2. The early radio telescope of Karl Jansky 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
c
Figure 2.3. Grote Reber and his 9.45-meter paraboloid 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 encountered 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.
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
Steerability
partially steerable
fully steerable
fully steerable
partially steerable
non-steerable
fully steerable
fixed location
fully steerable
Table 2.1. Large individual twentieth-century radio telescopes
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 Telescope1
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
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).
Year
1949
1956
1957
1960
1963
1972
1977
2000
2.3. Types of Radio Telescopes
15
16
2.3.2
Chapter 2. Radio Telescopes
Radio Arrays
In order to improve the angular resolution beyond the size of the instrument the socalled 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 reference 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 Jodrell 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
images. A radio array with NA antenna systems gives at most N2A 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., At Bt is the location of baseline AB at time
t and At+1 Bt+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 At Bt and At+1 Bt+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 wellknown 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 electric 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
speed of light (c) and Planck’s constant (h; see glossary, p. 259): f = λc 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.
Chapter 2. Radio Telescopes
20
Name
WSRT
VLA
MERLIN
EVN
ATCA
VLBA
GMRT
Year
1970
1980
1980
1980
1988
1993
1999
# Systems
14
27
6
18
6
10
30
Size
3 km
36 km
217 km
9169 km
6 km
8000 km
25 km
Data transmission
Coaxial cables
Fiber optic cables
Radio links
VLBI
Coaxial cables
VLBI
Fiber optic cables
Table 2.2. Characteristics of well-known radio arrays
Location
The Netherlands
New Mexico (USA)
United Kingdom
Intercontinental
Australia
United States
India
Synthesis
AS/ERS
AS/ERS
AS/ERS
AS/ERS
AS/ERS
AS/ERS
AS/ERS
Freq. range
117-1,200 MHz
74-50,000 MHz
151-24,000 MHz
327-43,214 MHz
1,250-9,200 MHz
312-90,000 MHz
50-1,500 MHz
2.4. Electromagnetic Radiation
2.4.1
21
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 hydrogen 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
Extremely low frequency
Super low frequency
Ultra low frequency
Very low frequency
Low frequency
Medium frequency
High frequency
Very high frequency
Ultra high frequency
Super high frequency
Extremely high frequency
Abbrev.
ELF
SLF
ULF
VLF
LF
MF
HF
VHF
UHF
SHF
EHF
Frequencies
3-30 Hz
30-300 Hz
300-3000 Hz
3-30 kHz
30-300 kHz
300-3000 kHz
3-30 MHz
30-300 MHz
300-3000 MHz
3-30 GHz
30-300 GHz
Wavelengths
10,000-100,000 km
1,000-10,000 km
100-1000 km
10-100 km
1-10 km
100-1,000 m
10-100 m
1-10 m
100-1,000 mm
10-100 mm
1-10 mm