9.1.7 Radio, IR, UV & X-Ray Telescopes

Non-Optical Telescopes

An optical telescope is one which detects wavelengths of light from the visible part of the electromagnetic spectrum
Telescopes that look at other parts of the electromagnetic spectrum are known as non-optical telescopes, such as
- Radio telescopes
- Infrared (IR) telescopes
- Ultraviolet (UV) telescopes
- X-ray telescopes

Being able to collect radiation from all parts of the electromagnetic spectrum opens up a whole world of new information for astronomers
- For example, different areas of a supernova remnant (the Crab Nebula) are found to emit strongly at all wavelengths
- In particular, radio waves, X-rays and gamma rays all appear to originate from the neutron star at its centre, whilst the infrared, visible and ultraviolet wavelengths appear to come from the nebula that surrounds it

Note: images of astronomical objects are often given 'false colour' to help us visualise wavelengths the human eye cannot see

Crab Nebula at Different Wavelengths

9-1-7-crab-nebula-in-different-wavelengths

The different wavelengths detected in the Crab Nebula tell astronomers a lot about the final stages of a massive star's life cycle

Ground-Based Telescopes

Many telescopes are designed to detect a range of wavelengths that span multiple regions of the electromagnetic spectrum
However, the operating wavelength range of a telescope is greatly limited by the absorption of certain wavelengths by the Earth's atmosphere

Absorption of Wavelengths by the Atmosphere

9-1-7-atmospheric-absorption-em-spectrum

Gamma-rays, X-rays, ultraviolet and infrared wavelengths are best observed from space

The graph of atmospheric opacity against wavelength shows that large ranges of wavelengths are partially, or completely, absorbed by our atmosphere
This means that ground-based telescopes are able to observe:
- All visible wavelengths (although there is often some distortion)
- Very narrow ranges of infrared wavelengths
- Most microwave & radio wavelengths

Space-Based Telescopes

Above the atmosphere, space-based telescopes can detect all wavelengths, making it possible to clearly observe:
- Gamma rays, X-rays & ultraviolet rays
- All infrared wavelengths (usually split into near-IR, mid-IR and far-IR)

Ground & Space-Based Telescopes

9-1-7-ground-and-space-based-telescopes

Some of the ground-based and space-based telescopes currently in operation

The main advantages of putting telescopes into space are:
- There is no absorption of electromagnetic waves by the atmosphere
- No light pollution or other sources of interference at ground level
- No atmospheric effects, such as scattering or scintillation (i.e. twinkling) of light

Radio Telescopes

Location: ground-based
Wavelength range: 1 mm to 10 m
Typical resolution: 10⁻³ rad

Structure of a Radio Telescope

9-1-7-radio-telescope-structure

The radio telescope is made up of a detector and parabolic dish

The following table shows the comparison of radio and optical telescopes in terms of their structure, positioning and uses:

	Similarities with optical telescopes	Differences with optical telescopes
Structure	Both use parabolic surfaces to reflect waves	Radio uses a single primary reflector, optical uses two mirrors Radio dish does not need to be as smooth as optical mirrors
Positioning	Both can be ground-based as the atmosphere is transparent to most radio and optical wavelengths	Optical must be placed high up (to avoid atmospheric distortions) and away from cities (to avoid light pollution) Radio must be located remotely (away from radio sources)
Uses	Both are used to detect hydrogen emission lines (radio at 21 cm, visible at 410 nm, 434 nm, 486 nm and 656 nm)	Radio waves are not absorbed by dust, whereas optical waves are, so radio telescopes are used to map the Milky Way

The following table shows the comparison of radio and optical telescopes in terms of resolving and collecting power:

	Comparison with optical telescopes
Resolving power	Radio waves are longer than optical waves, so radio telescopes have a much lower resolving power (~10⁻³ rad) than optical telescopes Optical telescopes are more likely to produce detailed images
Collecting power	Radio telescopes are larger in diameter, so they have a greater collecting power than optical telescopes Therefore, radio telescopes are more likely to produce brighter images (although many radio sources are weak)

Infrared Telescopes

Location: predominantly space-based, but some ground-based observatories exist
Wavelength range: 700 nm to 1 mm
Typical resolution: 10⁻⁶ rad (ground) to 10⁻⁷ rad (space)

The following table shows the comparison of IR and optical telescopes in terms of their structure, positioning and uses:

	Similarities with optical telescopes	Differences with optical telescopes
Structure	Both IR and optical telescopes are constructed using a primary concave mirror and a secondary convex mirror	Mirrors in IR telescopes must be kept very cold to avoid interference from surrounding heat
Positioning	Many ground-based telescopes are able to detect both optical and near-IR wavelengths as long as they are positioned away from cities and are high above the ground	IR radiation is strongly absorbed by water vapour in the atmosphere, so telescopes must be built in dry, high-altitude locations, or above the atmosphere The atmosphere is transparent to most optical wavelengths but blocks most IR wavelengths, so space-based IR telescopes are preferable
Uses	Most objects that emit visible light also emit IR radiation, so valuable information can be obtained from both	IR telescopes can detect warm objects that do not emit visible light, such as dust in nebulae and brown dwarfs

The following table shows the comparison of IR and optical telescopes in terms of resolving and collecting power:

	Comparison with optical telescopes
Resolving power	IR telescopes have a lower resolving power than optical telescopes of the same size due to having a longer wavelength
Collecting power	The collecting power of IR and optical telescopes are similar as their diameters are similar

Ultraviolet Telescopes

Location: space
Wavelength range: 10 to 400 nm
Typical resolution: 10⁻⁷ rad

The following table shows the comparison of UV and optical telescopes in terms of their structure, positioning and uses:

	Similarities with optical telescopes	Differences with optical telescopes
Structure	Both UV and optical telescopes are constructed using a primary concave mirror and a secondary convex mirror	Mirrors in UV telescopes must be smoother than those used in optical telescopes
Positioning	Many space-based telescopes are able to detect both optical and UV wavelengths	All UV wavelengths are strongly absorbed by the atmosphere (ozone) so UV telescopes must be located in space Space-based UV telescopes can be inconvenient to maintain
Uses	Both can be used to determine the chemical composition and temperatures of objects Many objects that emit visible light also emit UV radiation, so valuable information can be obtained from both	UV telescopes can detect objects not visible at other wavelengths, such as hot gas clouds near stars, supernovae and quasars

The following table shows the comparison of UV and optical telescopes in terms of resolving and collecting power:

	Comparison with optical telescopes
Resolving power	UV telescopes have a higher resolving power than optical telescopes of the same size due to having a shorter wavelength
Collecting power	The collecting power of UV and optical telescopes are similar as their diameters are similar

X-Ray & Gamma Telescopes

Location: space
Wavelength range: X-rays = 0.01 to 10 nm, gamma < 10 nm
Typical resolution: 10⁻⁶ rad

The following table shows the comparison of X-ray, gamma and optical telescopes in terms of their structure, positioning and uses:

	Similarities with optical telescopes	Differences with optical telescopes
Structure	X-ray & optical telescopes both use parabolic mirrors to reflect and focus waves	X-ray telescopes are made from a combination of parabolic and hyperbolic mirrors, all of which must be extremely smooth Gamma telescopes don't use mirrors at all, they use specialised detectors
Positioning	X-ray, gamma and optical telescopes all perform best when positioned in space, away from the restrictions imposed by the Earth's atmosphere	All X-ray & gamma wavelengths are strongly absorbed by the atmosphere so these telescopes must be located in space Space-based telescopes can be inconvenient to maintain
Uses	X-ray & gamma can provide valuable additional information about visible objects, such as supernova remnants	X-ray & gamma telescopes can observe otherwise non-visible objects and energetic events, such as neutron stars, black holes, pulsars and gamma-ray bursts (GRBs)

The following table shows the comparison of X-ray, gamma and optical telescopes in terms of resolving and collecting power:

	Comparison with optical telescopes
Resolving power	X-ray & gamma telescopes have a much higher resolving power than optical telescopes of the same size due to their shorter wavelengths
Collecting power	The collecting power of X-ray & gamma telescopes is much lower than optical telescopes are they have smaller objective diameters However, X-ray and gamma sources tend to be extremely bright

Exam Tip

You need to learn the key points for each type of telescope, so you can back up your arguments for comparisons between them. This could be useful information for a 6 mark question.

AQA A Level Physics

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