Astrophysics Tutorial 3 -  Radio Telescopes 

Contents

The Radio Telescope

Resolving Power

Radio Sources in the Universe

Other kinds of Telescope

Using Telescopes

Problems of Observing Stars

 

The Radio Telescope

The radio telescope was first devised during the 1930's.  During the Second World War, considerable advances were made with all sorts of techniques used for radio for communication, detection, and navigation.  Sophisticated apparatus was made for eavesdropping on enemy radio traffic to gain intelligence.  (The Germans were excellent soldiers but never appreciated the need for good intelligence; the Allied Powers were very good at intelligence and deception.)

 

The initial driving force for RADAR (Radio detection and ranging) was originally to make a radio death-ray to make bombers fall out of the sky in flames!  The initial driving force for radio telescopy was to listen out for radio signals from other extra-terrestrial civilisations (little green men).  The first pulsar was discovered by Jocelyn Bell Burnell in 1967.  It is was called LGM-1 (Little Green Men 1)

 

In Britain, the first work to be done with radio telescopes was carried out by Professor Sir Bernard Lovell (1913 - 2011) and a team from the University of Manchester.  They set up war-surplus radio equipment bought from the army, in a field in the middle of Cheshire.  The work grew and in the middle nineteen fifties the team undertook the construction of a massive radio telescope, which is shown below.

 

Photo by Mike Peel, University of Manchester

 

This massive instrument at Jodrell Bank is 75 m across.  Its story is major work in itself, but it eventually started exploration in the late nineteen fifties, almost exactly at the same time as electric trains powered by 25 000 V overhead power lines started running on the main line that passes 200 m from the instrument.

 

The largest radio telescope in the world is the Arecibo telescope in Puerto Rico.  It is built between some small hills that had a roughly parabolic valley.  It is 300 m across.  The Puerto Rico Instrument has the valley floor paved in metal sheeting to act as the mirror.

 

 

Question 1

How much more powerful is this instrument than the one at Jodrell Bank?

Answer

Question 2

What disadvantage does the Puerto Rico instrument have over the one at Jodrell Bank?

Answer

 

 

Recent developments have concentrated on producing arrays of radio telescopes.  Instead of one huge dish, there are many smaller instruments that move in unison (at the same time) and point at the same area of the sky.  The effect of this is the same as having one huge dish, without all the difficulties of making one.  The picture shows the idea:

 

Computer generated picture by SKA Project Development Office and Swinburne Astronomy Productions, Wikimedia Commons

 

This is a current project called the Square Kilometre Array.  As its name suggests, the area of each array will be 1 km2.  There will eventually be several of these.

 

 

Resolving Power

High resolution enables us to see things in good detail.

 


The girl is low resolution; she is fuzzy and pixellated. You can see more detail on the boy.  The higher the resolution, the smaller the angle of separation that can be observed.  The angular separation of two stars, A and B, is shown below:

 

 

If the two stars are viewed through a telescope, diffraction will occur. This could blur the image.
 

The resolving power of a radio telescope is governed by the same kind of factors as a light microscope.  It is given by Rayleigh's Criterion:

 

 

Question 3

What is the angular resolution of 10 metre wavelength radio waves by the Jodrell Bank Telescope whose diameter is 75 m?

Answer

 

From your answer above you can see that the resolution is not very good. To resolve between radio sources, the telescope has to scan across to detect the precise origin of each source. 

 

The dish does not have to be as perfect as mirror for a light telescope.  As long as the surface is within about 1/20 wavelength, then the focusing will be unaffected by imperfections.  Also the reflector does not have to solid.  Fine wire mesh will do, since radio waves will not pass through a gap less than one wavelength.

 

Radio Sources in the Universe

Radio astronomy has:

The picture below shows a radio source compared with the sky seen with visible light:

 

Picture from NASA

 

 

Radio waves can penetrate dust, so we can look at the centre of our galaxy.  However radio waves of wavelength less than about 1 cm are blocked out by carbon dioxide and water.  Radio waves of wavelength 20 m and above are absorbed by the atmosphere.  Also radio signals from Earth can cause interference, just like light pollution for light telescopes.  Passing satellites can also obscure the field of view.

 

Many radio telescopes have been set up well away from cities.  Satellites with radio telescopes have been used to investigate the microwave radiation that points to the Big Bang.

 

 

Other kinds of Telescope

The diagram above shows that telescopes can be made to look at a large range of different electromagnetic radiations:

The next picture shows the same patch of sky visible to all sorts of different wavelengths.

 

Picture from NASA

 

Infra-red telescopes allow astronomers to observe objects that are too cool to give out visible light.  These include cooling dead stars, planets, and dust clouds.  A difficulty with infra-red telescopes is the fact that on the ground, the surface is irradiating the instrument with infra red all the time.  Therefore the dish has to be cooled, or the infra red would swamp what is being observed.  It's a bit like trying to observe a star behind the Sun.  Even worse is that water vapour absorbs and retransmits infra red.  The latter problem can be reduced by sending a satellite into orbit.  However the receiver has to be cooled to a few Kelvin to observe very weak sources.

 

Infra red sources give wavelengths of about 700 nm to 1 mm.

 

 

UV Telescopes

These will not work on the Earth's Surface as UV light is absorbed by the Earth's atmosphere.  So they are sent up into orbit.  Glass lenses absorb UV, so mirrors are used to focus the rays onto a charge-coupled device.  The data are transmitted digitally to be interpreted by appropriate software.  UV is emitted by materials at high temperatures.  Objects can be looked out using a variety of detectors, and the UV light shows hot-spots.  Here is such an image:

 

Picture from NASA

 

X-ray Telescopes

It is hard to reflect X-rays like light from an ordinary mirror; they are transmitted or absorbed.  They can be reflected using a technique called glancing mirrors, with the angle between the X-rays and the mirror being less than 2 degrees (angle of incidence 88 o).  The mirrors are made of ceramic or metal foil.  Modern instruments can handle X-ray photons of energy up to 78 keV.  These instruments have discovered X-ray pulsars and "bursters".

 

 

Gamma Ray Telescopes

These are carried up above the atmosphere either by balloons or satellites.  The photons are not collected with mirrors, but are detected using complex arrays of charged coupled devices which track the passage of a gamma ray photon.  Data are transmitted digitally and powerful software is used to analyse the images.  Such an image is shown below:

 

Photo from NASA

 

The picture shows two vast gamma ray bubbles appearing symmetrically from a super-massive black hole that is thought to exist in the centre of the Milky Way galaxy.  Gamma ray bursts are observed when very large supernovae (hyper-novae) collapse into black holes.

 

 

Using Telescopes

The table below shows the uses and draw-backs of using different kinds of instruments:

 

Type

Location

Wavelength

Resolution (degrees)

Advantages

Disadvantages

Radio

Ground

1 mm - 10 m

0.2

Radio waves pass through dust and the atmosphere

Steerable dish is large, complex, and expensive.

IR

Ground or in space

700 nm - 1 mm

5.7 10-5

Can detect warm but invisible objects in space.

Mirror has to be cooled

Light

Ground or in space

600 nm - 300 nm

8.5 10-6

Detailed images possible, images are easily interpreted, telescopes can be accessed easily.

Ground instruments have images distorted by refraction and turbulence of the atmosphere.

UV

Space

300 nm - 10 nm

10-7

Can detect very hot objects.

Has to be in space.

X-ray

Space

10 nm - 0.01 nm

10-7

Detect X-ray pulsars

Has to be in space.  Optics are complex.

Gamma

Space

10-11 m - 10-14 m

0.2

Detect g ray burst from supernovae.

Has to be in space.  g rays are not possible to focus, so image is limited by the CCD array.

 

 

Problems of Observing Stars

Meaningful astronomy is getting harder.  Research astronomers are generally to be found in the major universities whose observatories were often set up in major cities.  The observatories are often quite old.  Although the instruments are very capable, their situation limits them:

These problems can be solved:

If other radiations are being investigated, satellites equipped with appropriate receivers are used.