Telescopes Across the Spectrum and in Space

Beyond Visible Light

Optical telescopes gather visible light — but the universe radiates across the entire electromagnetic spectrum. Hot gas emits X-rays; cold dust clouds glow in infrared; accelerating electrons in magnetic fields produce radio waves. To build a complete picture of the cosmos, astronomers have built telescopes sensitive to radio waves, infrared, ultraviolet, X-rays and gamma rays. Each wavelength window reveals phenomena invisible to all others.

CCD Detectors and Digital Imaging

CCD (charge-coupled device) detectors have replaced photographic film in modern telescopes. CCDs convert incoming photons into electrical charge and store the charge in a grid of pixels. They are far more efficient than film: a typical CCD detects about 90% of incoming photons (quantum efficiency of ~90%), compared with only a few percent for film. The recorded data are image files that can be processed digitally — brightening faint features, subtracting sky background, stacking multiple exposures to reduce noise. CCDs work across a wide wavelength range, from near-ultraviolet to near-infrared.

Why the Atmosphere Limits Telescopes

Earth’s atmosphere absorbs or scatters most of the electromagnetic spectrum. Only two main windows allow radiation to reach ground level:

The optical window — visible light and small parts of the near-infrared and near-ultraviolet. This is why conventional ground-based telescopes work.
The radio window — radio waves from about 1 mm to 10 m wavelength pass through relatively freely. This allows ground-based radio astronomy.

Ultraviolet, X-ray, gamma-ray and most infrared radiation is absorbed before reaching sea level. Additionally, the atmosphere causes atmospheric seeing — turbulent air cells refract starlight, making stars twinkle and blurring fine detail in optical images. This limits the resolution of ground-based telescopes regardless of their size.

Key Fact

Earth’s atmosphere transmits only the optical window (visible light) and the radio window. All other wavebands (UV, X-ray, gamma ray, most IR) are absorbed by the atmosphere and require telescopes at high altitude or in space.

Radio Telescopes

A simple radio telescope consists of a large parabolic dish that focuses radio waves onto a receiver at the focal point, similar in principle to a reflecting optical telescope. Radio wavelengths (centimetres to metres) are much longer than visible light, which creates two challenges:

Resolution — angular resolution depends on wavelength divided by aperture. Longer wavelengths produce poorer (larger) resolution for a given dish size. A single dish must be enormous to achieve decent resolution. Jodrell Bank’s Lovell Telescope (76 m diameter) achieves only modest angular resolution at typical radio wavelengths.
Aperture synthesis — instead of a single enormous dish, astronomers link many smaller dishes spread over long baselines using a technique called aperture synthesis. The Very Large Array (VLA) in New Mexico uses 27 dishes spread over up to 36 km, achieving the resolution equivalent to a single dish 36 km across. This is the principle behind very long baseline interferometry (VLBI), where dishes on different continents are combined.

Radio observations have revealed features invisible in optical wavelengths: the large-scale structure of the Milky Way’s spiral arms (mapped using neutral hydrogen’s 21 cm emission line); jets of particles emitted by active galaxies and quasars; protoplanetary discs of gas and dust around young stars; and the cold molecular clouds where star formation begins. Pulsars — rapidly rotating neutron stars — were first detected at radio wavelengths. The cosmic microwave background (CMB) radiation peaks at millimetre wavelengths and was first detected with radio receivers.

Infrared Telescopes

Infrared radiation is absorbed mainly by water vapour in the lower atmosphere. To observe at infrared wavelengths, telescopes must be built at high-altitude dry sites — such as Mauna Kea in Hawaii (altitude 4,200 m) or the Atacama Desert in Chile — where the atmosphere above is thin and dry. Even better, space-based infrared telescopes avoid all atmospheric absorption. Infrared astronomy reveals: molecular clouds and protostars hidden behind dust (dust absorbs visible light but is largely transparent in infrared); galaxies that emit most of their energy in infrared (starburst galaxies); and the thermal glow of cool objects such as brown dwarfs and exoplanets.

Space Telescopes

Space telescopes operate above the atmosphere entirely, solving both the absorption problem and the seeing problem. Key examples:

Hubble Space Telescope (HST) — optical and near-UV/near-IR. Its position above the atmosphere gives it resolution far exceeding ground-based telescopes of similar size. Images from HST of the Hubble Deep Field showed thousands of galaxies in a tiny patch of sky, demonstrating the depth of the observable universe.
James Webb Space Telescope (JWST) — primarily infrared. Its gold-coated beryllium mirror (6.5 m) and cool operating temperature (~40 K, shielded from the Sun) allow it to detect extremely faint infrared signals from the earliest galaxies and from exoplanet atmospheres during transits.

X-rays and gamma rays require space-based detectors. X-ray telescopes (such as the ESA XMM-Newton and NASA Chandra) use grazing-incidence mirrors — X-rays bounce off mirrors only at very shallow angles. X-ray observations reveal hot gas in galaxy clusters, accretion discs around black holes, neutron star binary systems, and the hot corona of the Sun. Gamma-ray observatories (such as NASA’s Fermi Gamma-ray Space Telescope) detect gamma-ray bursts — the most energetic explosions in the universe — and jets from active galaxies. Because X-ray and gamma-ray photons are so energetic, they can be detected by sensors sensitive to individual photon impacts rather than traditional mirrors that focus light.

What Telescopes Reveal About Familiar Objects

Even objects visible to the naked eye look dramatically different through a telescope. Double stars (two stars close together in the sky) may be resolved into separate points. Some double stars are true binary stars — gravitationally bound pairs — while others are merely optical doubles (coincidentally aligned along the line of sight). Open clusters reveal dozens of individual stars. Globular clusters appear as a dense ball of stars. Nebulae show as glowing or dark patches of gas and dust. Distant galaxies appear as fuzzy ellipses or spiral forms at the limits of resolution.