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15 Radio Astronomy
length of the wavelength we wish to receive. The dipole antenna has a doughnutshaped beam pattern.
The Yagi-Uda antenna, now more commonly known as just a Yagi, is a multidipole antenna, again normally half wavelength. Yagi antennas can be optimised to
balance the competing forces of gain versus bandwidth. Hence in terrestrial applications, such as receiving television, which requires wide bandwidth, Yagi antennas
tend to have reflectors to improve gain. As anybody who has tried to mount a new
television antenna will know, a Yagi tends to have a small beam pattern, which is
good for astronomical applications. Hence, the Yagi antenna is often used in low-cost
systems.
At the more professional end of radio astronomy, we come to the parabolic reflector, called simply a “dish.” These are like satellite receiving dishes only scaled up.
In fact, domestic Sky Television dishes have become popular amateur radio astronomy antennas. Traditional large-dish radio telescopes such as the 4.5 m R.W. Forrest
teaching telescope at the Bayfordbury Observatory (Fig. 15.1), consist of a large dish
designed to focus radio waves to a point. At this point is located a simple, quarterwavelength waveguide (just a conductive tube, in effect) designed to direct the signal
to the receiver. The waveguide is surrounded by the feed horn (or can), which is a
metal can open at one end that guides the incoming radiation onto the waveguide
and protects it from stray radiation.
The receiver attached to your radio telescope will have an operating frequency
range and a centre frequency. Your receiver, if designed for radio astronomy, should
be able to perform two functions. The first is a continuum plot. In this mode, it will
just display the variation in source intensity over time. It should also be capable of
spectrography. In this mode, it moves the frequency that it is listening to in small
steps, plotting the intensity of the signal against wavelength or frequency. In many
cases, it is possible to adjust the integration time, i.e., how long is spent at each
wavelength, and the step size, the range over which it will be listening to each point
along with the scan. In effect, increasing the step size is like binning with a CCD.
Your receiver will most likely report the signal strength in volts. With some difficulty,
this can be turned into instrument flux, and then standard flux. However, just as with
counts with CCDs, this conversion is not always required.
Radio sources can be divided into two basic classes: low and high energy. Lowerenergy sources are quantised sources with low energy levels. Typical of this is the
21 cm line. This is a strong emission line for monatomic hydrogen caused by the
change in the quantum spin state of the proton and electron. Monatomic hydrogen
(HI) is the most common form of hydrogen and hence is a vital tool for the identification and mapping of galactic structure. Other examples of radio spectra lines
are ammonia, water, methanol, and carbon monoxide. All these lines occur at low
temperatures and low energies. High-energy sources include the Sun, Jupiter, black
holes, active galaxies, pulsars, and supernova remnants. In these cases, strong magnetic fields accelerate charged particles in a spiral, causing the emission of braking
radiation with a characteristic spectrum related to the Larmor radius. Such sources are
often hot and are also X-ray emitters, whence the odd effect that X-ray astronomers
also tend to be radio astronomers.
15 Radio Astronomy
length of the wavelength we wish to receive. The dipole antenna has a doughnutshaped beam pattern.
The Yagi-Uda antenna, now more commonly known as just a Yagi, is a multidipole antenna, again normally half wavelength. Yagi antennas can be optimised to
balance the competing forces of gain versus bandwidth. Hence in terrestrial applications, such as receiving television, which requires wide bandwidth, Yagi antennas
tend to have reflectors to improve gain. As anybody who has tried to mount a new
television antenna will know, a Yagi tends to have a small beam pattern, which is
good for astronomical applications. Hence, the Yagi antenna is often used in low-cost
systems.
At the more professional end of radio astronomy, we come to the parabolic reflector, called simply a “dish.” These are like satellite receiving dishes only scaled up.
In fact, domestic Sky Television dishes have become popular amateur radio astronomy antennas. Traditional large-dish radio telescopes such as the 4.5 m R.W. Forrest
teaching telescope at the Bayfordbury Observatory (Fig. 15.1), consist of a large dish
designed to focus radio waves to a point. At this point is located a simple, quarterwavelength waveguide (just a conductive tube, in effect) designed to direct the signal
to the receiver. The waveguide is surrounded by the feed horn (or can), which is a
metal can open at one end that guides the incoming radiation onto the waveguide
and protects it from stray radiation.
The receiver attached to your radio telescope will have an operating frequency
range and a centre frequency. Your receiver, if designed for radio astronomy, should
be able to perform two functions. The first is a continuum plot. In this mode, it will
just display the variation in source intensity over time. It should also be capable of
spectrography. In this mode, it moves the frequency that it is listening to in small
steps, plotting the intensity of the signal against wavelength or frequency. In many
cases, it is possible to adjust the integration time, i.e., how long is spent at each
wavelength, and the step size, the range over which it will be listening to each point
along with the scan. In effect, increasing the step size is like binning with a CCD.
Your receiver will most likely report the signal strength in volts. With some difficulty,
this can be turned into instrument flux, and then standard flux. However, just as with
counts with CCDs, this conversion is not always required.
Radio sources can be divided into two basic classes: low and high energy. Lowerenergy sources are quantised sources with low energy levels. Typical of this is the
21 cm line. This is a strong emission line for monatomic hydrogen caused by the
change in the quantum spin state of the proton and electron. Monatomic hydrogen
(HI) is the most common form of hydrogen and hence is a vital tool for the identification and mapping of galactic structure. Other examples of radio spectra lines
are ammonia, water, methanol, and carbon monoxide. All these lines occur at low
temperatures and low energies. High-energy sources include the Sun, Jupiter, black
holes, active galaxies, pulsars, and supernova remnants. In these cases, strong magnetic fields accelerate charged particles in a spiral, causing the emission of braking
radiation with a characteristic spectrum related to the Larmor radius. Such sources are
often hot and are also X-ray emitters, whence the odd effect that X-ray astronomers
also tend to be radio astronomers.