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Optical light is a thin strip of the electromagnetic spectrum — about one octave between 400 nm and 700 nm. Astronomical objects emit (and absorb) across the entire spectrum, from kilometre-wavelength radio waves to picometre-wavelength gamma rays. To understand how the universe really works, astronomers must observe in every waveband, and that requires telescope architectures that look nothing like an amateur Newtonian. This lesson surveys radio, infrared, ultraviolet and X-ray telescopes, the atmospheric windows that constrain where they have to be placed, and the principle of interferometry that lets multiple radio dishes act as a single instrument the size of the Earth.
Spec mapping. This lesson covers AQA 7408 section 3.9.1, specifically: the structure and operation of single-dish radio telescopes, the comparison of radio and optical telescopes, IR / UV / X-ray space telescopes, the role of atmospheric absorption, and qualitative coverage of interferometry. (Refer to the official AQA specification document for exact wording.)
Synoptic links. Diffraction at a circular aperture (AQA 3.3) drives the resolution comparison, and the Rayleigh criterion of the previous lesson applies unchanged. Wien's displacement law (later in this course) tells us which waveband to use for which physical temperature. Thermal background and Stefan's law motivate the cooling of IR detectors. X-ray detection connects synoptically to the photoelectric effect (AQA 3.2.2).
Different physical processes emit in different wavebands:
| Waveband | Wavelength | Typical sources | Information gained |
|---|---|---|---|
| Radio | 1 mm – 30 m | Synchrotron from relativistic electrons, neutral H 21 cm line, AGN jets, pulsars | Cold gas distribution, magnetic fields, jets |
| Microwave | 0.1 – 1 mm | Cosmic microwave background, cold dust | Big Bang remnant, star-forming clouds |
| Infrared | 1 – 100 µm | Cool stars, dust-shrouded star formation, redshifted galaxies | Hidden star birth, early universe |
| Visible | 400 – 700 nm | Main-sequence stars, planetary surfaces | Stellar spectra, classifications |
| Ultraviolet | 10 – 400 nm | Young, hot, massive stars; hot interstellar gas | Hot stellar atmospheres, ISM diagnostics |
| X-ray | 0.01 – 10 nm | Accretion discs around neutron stars / black holes, hot gas in galaxy clusters | Compact objects, plasma at 10⁶–10⁸ K |
| Gamma | < 0.01 nm | Supernova decays, gamma-ray bursts, AGN | Highest-energy processes |
A single optical image of, say, the Crab Nebula shows the outer shell of the supernova remnant; a radio image shows the synchrotron emission from energetic electrons spiralling in magnetic fields; an X-ray image shows the pulsar wind nebula at the core. The same object, three completely different pictures.
Most electromagnetic radiation is absorbed by Earth's atmosphere — only two broad windows open to ground-based observation.
The figure is schematic but the headline is right:
Astronomers therefore site optical telescopes on high mountains (Mauna Kea, La Palma), radio dishes on low desert sites (Atacama, Australian outback), and put everything else in orbit. The Hubble Space Telescope was launched in 1990 partly to escape atmospheric seeing in the optical and partly to operate in the near-UV. The James Webb Space Telescope (launched 2021) operates from 0.6 µm to 28 µm, well into the infrared.
The first non-optical band to be opened to astronomy was the radio band (Karl Jansky, 1933). The basic instrument is a large parabolic dish with a feed antenna at the focus.
The dish surface need only be smooth on scales of about λ/20. For a 21 cm wavelength survey (the neutral hydrogen line) that means surface accuracy of about 1 cm — coarse by optical standards, achievable with wire mesh for the longest wavelengths. As wavelengths shorten into the millimetre and sub-millimetre bands, surface accuracy becomes much more demanding (sub-millimetre dishes require panel adjustment to better than 25 µm).
The Lovell Telescope at Jodrell Bank has a diameter of 76 m. Calculate its theoretical angular resolution at the 21 cm hydrogen line.
Solution. θ ≈ 1.22 λ / D = 1.22 × 0.21 / 76 = 3.37 × 10⁻³ rad ≈ 0.19°. In arcseconds, 695 arcsec — about 12 arcmin. That is roughly the angular size of the full Moon. Compared with Hubble's 0.06 arcsec in the visible, single-dish radio is vastly less sharp unless dishes are combined.
This calculation illustrates the central tension in radio astronomy: λ is enormous (4–5 orders of magnitude bigger than visible light), so D has to be enormous to achieve comparable resolution. A single dish that large is mechanically impossible; the answer is interferometry.
In a radio interferometer, two or more separated dishes observe the same source simultaneously. The signals are combined digitally (in a correlator), and the resulting interference pattern depends on the projection of the source brightness onto the baseline between the dishes. The angular resolution achievable is
θ ≈ λ / B
where B is the maximum baseline — the separation between the most distant pair of dishes. The whole array behaves, for resolution purposes, like a telescope of aperture B.
An interferometer of N dishes each of diameter D has:
That is why arrays still want individual dishes to be as large as feasible. SKA-Mid will achieve both: thousands of medium dishes with maximum baselines of 150 km.
| Property | Optical (e.g. 2.4 m Hubble) | Radio (e.g. 76 m Lovell) |
|---|---|---|
| Wavelength | 500 nm | 0.21 m |
| Aperture | 2.4 m | 76 m |
| λ / D | 2 × 10⁻⁷ rad | 2.8 × 10⁻³ rad |
| Resolution | 0.06 arcsec | 12 arcmin (single dish) |
| Surface accuracy required | ≤ 25 nm | ≤ 1 cm |
| Detector | CCD | Cooled radio receiver + correlator |
| Atmospheric site | Space or high mountain | Sea-level or desert |
| Day / night | Night only | Day or night (sky is dark at radio) |
The take-home message: radio telescopes need to be enormous because λ is enormous, but they can be built large because surface accuracy is forgiving. Optical telescopes need superb surface accuracy but can be physically smaller because λ is short.
The detector and the telescope itself emit thermal radiation in the IR; the telescope is, in effect, looking at itself. To suppress this background:
UV telescopes use fused-silica optics with aluminium coatings; ordinary glass absorbs UV below 350 nm and is useless. Below about 120 nm, even aluminium becomes inefficient and grazing-incidence mirrors (used for X-rays) start to be needed.
X-rays cannot be focused with normal mirrors. At their tiny wavelengths (0.01–10 nm), an X-ray photon striking glass at normal incidence simply passes through or is absorbed. The trick is to use grazing-incidence optics: nested cylindrical-paraboloidal-hyperboloidal Wolter mirrors at angles of only ~1° from the incoming rays. At those shallow angles X-rays reflect from a gold or iridium surface much like a stone skipping on water.
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