# Astrophysics 6 - Non-Optical Telescopes

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```					Non-Optical Telescopes

Astrophysics Lesson 6
Learning Objectives
Single dish radio telescopes, I-R, U-V and X-ray
telescopes
Similarities and differences compared to optical
telescopes including:-
Structure
Positioning and use

Compare the resolving and collecting power of
telescopes operating at different wavelengths.
Homework
Marking last homework.

For this Friday:-
 Describe and explain the structure, positioning,
and uses of I-R, U-V and X-ray telescopes.

Reference correctly.
Visible Light blocked by Dust
Astronomy across the Spectrum
Same patch of sky viewed at different wavelengths:-
Atmospheric Opacity
In Britain, the first work to
Telescopes was carried out
by Professor Sir Bernard
Lovell and a team from the
University of Manchester.

This massive instrument at
Jodrell Bank is 75 m across.
started work in the late
1950s.
Arecibo Observatory
telescope in the world is
in Puerto Rico. It is built
between some small hills
parabolic valley. It is
300m across.

• The valley floor is paved
in metal sheeting to act
as the mirror.
 The dish is parabolic, reflecting radio waves
onto an antenna at the principal focus. The radio
waves are very weak, and the focusing by the
reflector makes them much more intense.
 The receiver has to be tuned in, just like any
 The signal is passed down to very high quality
amplifiers, and the signals are analysed by a
computer.
Gathering Power
• The energy/photons collected per second.
• The gathering power is proportional to the area:-

• So a bigger dish collects more photons and
energy from an object in a given time  can
detect fainter objects.
Questions
1) Calculate the resolution of the Lovell (75m)
and Arecibo (300m) telescope observing at a
wavelength of 21cm in radians and degrees.
2) Compare their light gathering power.
3) What disadvantage does the Puerto Rico
instrument have over the one at Jodrell Bank?
1) θ = 0.21/75 = 0.0028 rad (= 0.16º)
θ = 0.21/300 = 0.0007 rad (= 0.04º)
2) The diameter of the Puerto Rico instrument is
four times that of Jodrell Bank. Therefore its
gathering power is 16 times larger.
3) The Puerto Rico instrument is static, so it
cannot track objects. It sweeps across the sky
as a result of the Earth's rotation.
Similarities with Optical
Parabolic dish  objective mirror in telescopes.
Antenna  used as detector at principal focus
Tracking of Source  telescope moves with the
source’s position in the sky as the Earth rotates.
Differences from Optical
 Resolving power is worse because of large λ.
^ linking lots of telescopes together.
 Need to scan across the radio source to build
up the image.
 Precision of about λ/20 needed in shape of dish
required to avoid spherical aberration.
 Fine wire mesh will do, since radio waves will
not pass through a gap less than one wavelength.
^ last two points mean cheaper and easier to build.
The picture below shows a radio
source called M82
Quasars        Pulsars
Analyse Chemical Elements in
Objects
Cosmic Microwave Background
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.

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