Introduction to Telescopes
Almost everything we know about the universe beyond our own atmosphere has arrived as electromagnetic radiation, crossing unimaginable distances before landing, faintly, on a mirror or a lens. A telescope's entire job is to catch as much of that radiation as possible and turn it into something an eye or a detector can actually use.
Two separate jobs: gathering light and magnifying it
It's tempting to think a telescope's main purpose is to make distant objects look bigger, but for astronomy that's almost the wrong way round. Stars are so far away that no reasonable magnification will ever turn them into resolved discs — even the nearest stars remain, for all practical purposes, points of light. What actually limits what a telescope can show you is how much radiation it can collect, and how finely it can separate two close sources from one another.
- Light-gathering power is about collecting as many photons as possible from a faint, distant source. A bigger objective lens or primary mirror intercepts more of the radiation spreading out from the source, producing a brighter image and allowing fainter objects to be detected at all.
- Magnification is about the angular size of the image formed, i.e. how large an object appears to the observer. This matters for extended objects like planets, nebulae, or galaxies, where there is genuine detail to be enlarged — but it is meaningless if the image is too dim or too blurred to make out that detail in the first place.
A telescope is, at heart, a device for collecting radiation over a wide area (the aperture) and bringing it to a focus, where it can be viewed directly, magnified by an eyepiece, or recorded electronically. Every design choice covered in this topic — the size of the aperture, whether it uses a lens or a mirror, which part of the electromagnetic spectrum it's built to observe — comes back to this same basic trade-off between how much radiation is gathered and how sharply it can be resolved.
Observing across the electromagnetic spectrum
Astronomical sources emit radiation right across the electromagnetic spectrum, not just as visible light, and different wavelengths carry different information about what's happening in a source — its temperature, its composition, violent high-energy processes near black holes or supernovae, or cool gas clouds invisible to the eye. A complete picture of an object often needs observations at several wavelengths, made with quite different types of telescope.
Optical telescopes
Optical telescopes collect visible light and are the most familiar type, built either from lenses (refracting telescopes) or curved mirrors (reflecting telescopes) — both covered in detail in the next two lessons. Visible light passes through Earth's atmosphere relatively freely, which is why optical astronomy has been possible from the ground for centuries, though the atmosphere still blurs and distorts images to some extent.
Radio telescopes
Radio telescopes detect radiation at much longer wavelengths than visible light — typically millimetres to metres, rather than hundreds of nanometres. In principle a single-dish radio telescope works the same way as a reflecting optical telescope: a large curved (usually parabolic) dish collects incoming radio waves and reflects them to a focus, where a receiver converts the signal into something that can be recorded and analysed. Structurally the two are similar in concept, differing mainly in scale and in the reflecting surface, which for radio wavelengths can be a coarser metal mesh rather than a precisely polished mirror.
The key difference in practice is size. A telescope's ability to resolve fine angular detail depends on the ratio of wavelength to aperture diameter, $\lambda/D$ — so for the same angular resolution as an optical telescope, a radio telescope observing at a wavelength many thousands of times longer needs a correspondingly larger dish diameter $D$. This is why radio telescopes are often enormous single dishes, or arrays of many dishes linked together electronically to synthesise the resolving power of one much larger instrument.
Infrared, ultraviolet and X-ray telescopes
Infrared, ultraviolet and X-ray radiation from space is largely absorbed or scattered by gases in Earth's atmosphere before it ever reaches the ground. Water vapour and carbon dioxide absorb strongly across much of the infrared; ozone and other atmospheric gases absorb ultraviolet; and X-rays are absorbed high in the atmosphere by the same processes that protect us from harmful radiation. Only a few narrow atmospheric "windows" let some infrared through to ground level, and essentially none of the ultraviolet or X-ray radiation survives the trip down.
- Infrared telescopes are sometimes sited at very high, dry altitudes, where the amount of atmospheric water vapour above the telescope is much reduced, but the clearest infrared observations still come from telescopes carried above the atmosphere entirely on satellites.
- Ultraviolet telescopes must be positioned in space, since the atmosphere blocks ultraviolet radiation almost completely at ground level.
- X-ray telescopes also have to be placed in orbit, well above the atmosphere, since X-rays are absorbed long before reaching even high-altitude ground-based sites.
Placing a telescope on a satellite avoids atmospheric absorption entirely and also removes the blurring and distortion that Earth's turbulent atmosphere causes even for wavelengths that do get through, such as visible light — which is one reason space-based optical telescopes can achieve sharper images than equivalent ground-based instruments.
Across the spectrum, the underlying physics of collecting and focusing radiation stays the same, but the practical demands change enormously. Radio telescopes need very large dishes simply to match the resolving power of a modest optical telescope, because resolution depends on $\lambda/D$ and radio wavelengths are so much longer. Infrared, ultraviolet and X-ray telescopes face the opposite problem: the atmosphere itself is the obstacle, absorbing or distorting the radiation before it reaches the ground, so these instruments are pushed toward high-altitude sites or, more commonly, into orbit above the atmosphere altogether.
A telescope's performance is governed by two largely independent factors: how much radiation it collects (set by the aperture's area, so by $D^2$) and how finely it can resolve detail (set by the ratio $\lambda/D$). Every telescope design across the electromagnetic spectrum — whether a giant single dish, an orbiting X-ray observatory, or a classic optical reflector — is ultimately a set of engineering compromises aimed at maximising one or both of these, for the wavelength range that particular telescope is built to observe.
Key takeaways
- A telescope's two main jobs are gathering as much radiation as possible from a faint source and resolving fine angular detail — magnification alone doesn't make an image better.
- Optical telescopes use lenses or mirrors to collect visible light, which passes through the atmosphere relatively freely.
- Radio telescopes work on the same basic principle as reflecting optical telescopes but need much larger dish diameters to achieve comparable angular resolution, since resolution depends on $\lambda/D$ and radio wavelengths are far longer than optical ones.
- Infrared, ultraviolet and X-ray radiation is largely absorbed or distorted by Earth's atmosphere before reaching the ground.
- Infrared telescopes benefit from high, dry sites; ultraviolet and X-ray telescopes must be carried above the atmosphere on satellites to observe at all.
- Space-based telescopes also avoid the image-blurring effects of atmospheric turbulence, even at wavelengths like visible light that do reach the ground.
- Every telescope design, across the whole spectrum, balances the same two factors: collecting power (radiation gathered) and resolving power (angular detail).