RADIO TELESCOPE

an instrument used to detect radio emissions from the sky, whether from natural celestial objects or from artificial satellites.

What Are Radio Telescopes?

We use radio telescopes to study naturally-occurring radio light from stars, galaxies, black holes, and other astronomical objects. We can also use them to transmit and reflect radio light off of planetary bodies in our solar system. These specially-designed telescopes observe the longest wavelengths of light, ranging from 1 millimeter to over 10 meters long. For comparison, visible light waves are only a few hundred nanometers long, and a nanometer is only 1/10,000th the thickness of a piece of paper!

Naturally-occurring radio waves are extremely weak by the time they reach us from space. A cell phone signal is a billion billion times more powerful than the cosmic waves our telescopes detect.

Parts of a Radio Telescope

Radio telescopes are built in all shapes and sizes based on the kind of radio waves they pick up. However, every radio telescope has an antenna on a mount and at least one piece of receiver equipment to detect the signals.

Because cosmic radio sources are extremely weak, radio telescopes are the largest telescopes in the world, and only the most sensitive radio receivers are used inside them. Unfortunately, these huge antennas also pick up radio interference from modern electronics, and great effort is taken to protect radio telescopes from radio frequency interference.

How does a Radio Telescope Work?

The Parkes radio telescope.

In the PULSE@Parkes scheme you will use the Parkes radio telescope to make your observations. In this section you will learn the basics of how a single-dish radio telescope such as Parkes works.
A radio telescope is simply a telescope that is designed to receive radio waves from space. In its simplest form it has three components:

1. One or more antennas to collect the incoming radio waves. Most antennas are parabolic dishes that reflect the radio waves to a receiver, in the same way as a curved mirror can focus visible light to a point.
2. A receiver and amplifier to boost the very weak radio signal to a measurable level. These days the amplifiers are extremely sensitive and are normally cooled to very low temperatures to minimise interference due to the noise generated by the movement of the atoms in the metal (called thermal noise).
3. A recorder to keep a record of the signal. Most radio telescopes nowadays record directly to some form of computer memory disk as astronomers use sophisticated software to process and analyse the data.

Let us see how these components work on the Parkes radio telescope.

The support struts on the underside of the Parkes antenna

The Antenna

Parkes has a parabolic dish antenna, 64 m in diameter with a collecting area of 3,216 m². The dish is made up of aluminium panels supported by a lattice-work of supporting struts. To incoming radio waves from space, the dish surface acts in the same manner as a smooth mirror. The waves are reflected and focused into a feedhorn in the base of the telescope's focus cabin. The dish has a mass of 300 tonnes and distorts under its own weight as it points to different parts of the sky. Due to clever engineering design, however, this distortion is accounted for so that the radiowaves are always reflected to the focus cabin.

The focus cabin houses the receivers.

The telescope operates at frequencies from 440 MHz to 23 GHz which corresponds to radiowaves of 75 cm to 7 mm. For any radiowave to be reflected form the dish it must be smoother than a fraction of the wavelength. For the Parkes telescope the dish surface is accurate to within 1-2 mm of the best-fit parabola, allowing 7 mm radiowaves to be reflected.
Why is the dish so big?
The size of a dish determines the amount of incoming radiation that can be collected. The larger the collecting area, the fainter the source that can be detected. Parkes is a 64 m antenna, the second-largest single dish in the southern hemisphere.
For a single-dish radio telescope the size of the dish also determines the field-of-view of the telescope. When a single receiver is used the Parkes telescope has a beamwidth of about 15 arc minutes, half the size of the Moon in the sky.

Receivers

The Parkes multibeam receiver, shown here in the workshop without its insulating cover. It has 13 feedhorns, seen here as the bronze tubes.

The weak radio signals are channeled by the feedhorn into a receiver located in the focus cabin located at the top of the telescope. Radio receivers amplifies the incoming signal about a million times. Parkes has a suite of receivers that are optimised for different frequency ranges and applications. The receivers are cryogenically cooled, typically with helium gas refrigerators that cool them to about 10 Kelvin (-260° C) to minimise the thermal noise in the electronics that would otherwise swamp the incoming signal.
For pulsar observations at Parkes observers typically use either the central beam of the Parkes Multibeam receiver, the HOH receiver, both of which detect 21 cm (1420 MHz) radiation or the Dual-Band receiver that can observe at 10 cm and 50 cm simultaneously.

Recorders

The amplified signals are carried by fibre optic cable from the recievers in the focus cabin down into the tower where they are stored on computer disks. Depending on the type of observation some processing of the data is performed on-site using computers in the tower. For pulsar observations the rate at which data is received can be extremely high.

Parkes Radio Telescope Statistics.