Non-Optical Telescopes

Why non-optical telescopes matter

• An optical telescope detects wavelengths of light from the visible part of the electromagnetic spectrum.
• Telescopes that observe other parts of the electromagnetic spectrum are known as non-optical telescopes, including radio, infrared (IR), ultraviolet (UV), and X-ray telescopes.
• Being able to collect radiation from all parts of the electromagnetic spectrum provides a wealth of new information for astronomers. For example, different regions of a supernova remnant (the Crab Nebula) emit strongly at different wavelengths.
• Fundamentally, images of astronomical objects are often given "false colour" to help us visualise wavelengths the human eye cannot see.

Ground-based vs space-based telescopes

• The operating wavelength range of a telescope is greatly limited by the absorption of certain wavelengths by the Earth's atmosphere.
• The graph of atmospheric opacityA measure of how much of the incoming electromagnetic radiation is absorbed by the atmosphere. 100% opacity means the radiation is completely blocked; 0% opacity means it passes through freely. against wavelength shows that large ranges of wavelengths are partially or completely absorbed by our atmosphere.
• Ground-based telescopes are able to observe:
  • All visible wavelengths (although there is often some distortion).
  • Very narrow ranges of infrared wavelengths.
  • Most microwave and radio wavelengths.
• Space-based telescopes, above the atmosphere, can detect all wavelengths, making it possible to clearly observe:
  • Gamma rays, X-rays, and ultraviolet rays.
  • All infrared wavelengths (usually split into near-IR, mid-IR, and far-IR).
• 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: Both radio and optical telescopes use parabolic surfaces to reflect waves. However, a radio telescope uses a single primary reflector (a parabolic dish), whereas an optical reflector uses two mirrors. The radio dish does not need to be as smooth as an optical mirror.
• Positioning: Both can be ground-based, as the atmosphere is transparent to most radio and optical wavelengths. However, optical telescopes must be placed high up (to avoid atmospheric distortions) and away from cities (to avoid light pollution), while radio telescopes 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.
• Resolving power: Radio waves are longer than optical waves, so radio telescopes have a much lower resolving power ($\sim 10^{-3}$ 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 (IR) 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).
• Structure: Both IR and optical telescopes are constructed using a primary concave mirror and a secondary convex mirror. The key difference is that mirrors in IR telescopes must be kept very cold to avoid interference from surrounding heat.
• Positioning: Many ground-based telescopes can detect both optical and near-IR wavelengths as long as they are positioned away from cities and high above the ground. However, 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.
• 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 is similar as their diameters are similar.

Ultraviolet (UV) telescopes

• Location: space.
• Wavelength range: 10 to 400 nm.
• Typical resolution: $10^{-7}$ rad.
• Structure: Both UV and optical telescopes are constructed using a primary concave mirror and a secondary convex mirror. The key difference is that mirrors in UV telescopes must be smoother than those used in optical telescopes.
• Positioning: 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: UV telescopes can detect objects not visible at other wavelengths, such as hot gas clouds near stars, supernovae, and quasars. Both UV and optical telescopes can be used to determine the chemical composition and temperatures of objects.
• 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 is similar as their diameters are similar.

X-ray and gamma telescopes

• Location: space.
• Wavelength range: X-rays = 0.01 to 10 nm; gamma < 10 nm.
• Typical resolution: $10^{-6}$ rad.
• Structure: X-ray and optical telescopes both use parabolic mirrors to reflect and focus waves. However, X-ray telescopes are made from a combination of parabolic and hyperbolic mirrors, all of which must be extremely smooth. Gamma telescopes do not use mirrors at all; they use specialised detectors.
• Positioning: All X-ray and 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 and gamma telescopes can observe otherwise non-visible objects and energetic events, such as neutron stars, black holes, pulsars, and gamma-ray bursts (GRBs).
• Resolving power: X-ray and 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 and gamma telescopes is much lower than optical telescopes as they have smaller objective diameters. However, X-ray and gamma sources tend to be extremely bright.