How non-optical telescopes see the universe
Non-optical telescopes allow us to expand our range of vision beyond the visible spectrum and view celestial objects in unprecedented detail. Jake Port explains how they work.
The human eye can only see a tiny band of the electromagnetic spectrum. That tiny band is enough for most day-to-day things you might want to do on Earth, but stars and other celestial objects radiate energy at wavelengths from the shortest (high-energy, high-frequency gamma rays) to the longest (low-energy, low-frequency radio waves).
Beyond the visible spectrum
To see what’s happening in the distant reaches of the spectrum, astronomers use non-optical telescopes. There are several varieties, each specialised to catch radiation of particular wavelengths.
Non-optical telescopes utilise many of the techniques found in regular telescopes, but also employ a variety of techniques to convert invisible light into spectacular imagery. In all cases, a detector is used to capture the image rather than an eyepiece, with a computer then processing the data and constructing the final image.
There are also more exotic ways of looking at the universe that don’t use electromagnetic radiation at all, like neutrino telescopes and the cutting-edge gravitational wave telescopes, but they’re a separate subject of their own.
To start off, let’s go straight to the top with the highest-energy radiation, gamma rays.
Gamma ray telescopes
Gamma radiation is generally defined as radiation of wavelengths less than 10−11 m, or a hundredth of a nanometre.
Gamma-ray telescopes focus on the highest-energy phenomena in the universe, such as black holes and exploding stars. A high-energy gamma ray may contain a billion times as much energy as a photon of visible light, which can make them difficult to study.
Unlike photons of visible light, that can be redirected using mirrors and reflectors, gamma rays simply pass through most materials. This means that gamma-ray telescopes must use sophisticated techniques that track the movement of individual gamma rays to construct an image.
One technology that does this, in use in the Fermi Gamma-ray Space Telescope among other places, is called a pair production telescope. It uses a multi-layer sandwich of converter and detector materials. When a gamma ray enters the front of the detector it hits a converter layer, made of dense material such as lead, which causes the gamma-ray to produce an electron and a positron (known as a particle-antiparticle pair).
The electron and the positron then continue to traverse the telescope, passing through layers of detector material. These layers track the movement of each particle by recording slight bursts of electrical charge along the layer. This trail of bursts allows astronomers to reconstruct the energy and direction of the original gamma ray. Tracing back along that path points to the source of the ray out in space. This data can then be used to create an image.
The video below shows how this works in the space-based Fermi Large Area Telescope.
X-rays are radiation with wavelengths between 10 nanometres and 0.01 nanometres. They are used every day to image broken bones and scan suitcases in airports and can also be used to image hot gases floating in space. Celestial gas clouds and remnants of the explosive deaths of large stars, known as supernovas, are the focus of X-ray telescopes.
Like gamma rays, X-rays are a high-energy form of radiation that can pass straight through most materials. To catch X-rays you need to use materials that are very dense.
X-ray telescopes often use highly reflective mirrors that are coated with dense metals such as gold, nickel or iridium. Unlike optical mirrors, which can bounce light in any direction, these mirrors can only slightly deflect the path of the X-ray. The mirror is orientated almost parallel to the direction of the incoming X-rays. The X-rays lightly graze the mirror before moving on, a little like a stone skipping on a pond. By using lots of mirrors, each changing the direction of the radiation by a small amount, enough X-rays can be collected at the detector to produce an image.
To maximise image quality the mirrors are loosely stacked, creating an internal structure resembling the layers of an onion.
Ultraviolet light is radiation with wavelengths just too short to be visible to human eyes, between 400 nanometres and 0.01 nanometres. It has less energy than X-rays and gamma rays, and ultraviolet telescopes are more like optical ones.
Mirrors coated in materials that reflect UV radiation, such as silicon carbide, can be used to redirect and focus incoming light. The Hopkins Ultraviolet Telescope, which flew two short missions aboard the space shuttle in the 1990s, used a parabolic mirror coated with this material.
As redirected light reaches the focal point, a central point where all light beams converge, they are detected using a spectrogram. This specialised device can separate the UV light into individual wavelength bands in a way akin to splitting visible light into a rainbow.
Analysis of this spectrogram can indicate what the observation target is made of. This allows astronomers to analyse the composition of interstellar gas clouds, galactic centres and planets in our solar system. This can be particularly useful when looking for elements essential to carbon-based life such as oxygen and carbon.
Optical telescopes are used to view the visible spectrum: wavelengths roughly between 400 and 700 nanometres. See separate article here.
Sitting just below visible light on the electromagnetic spectrum is infrared light, with wavelengths between 700 nanometres and 1 millimetre.
It’s used in night vision goggles, heaters and tracking devices as found in heat-seeking missiles. Any object or material that is hotter than absolute zero will emit some amount of infrared radiation, so the infrared band is a useful window to look at the universe through.
Much infrared radiation is absorbed by water vapour in the atmosphere, so infrared telescopes are usually at high altitudes in dry places or even in space, like the Spitzer Space Telescope.
Infrared telescopes are often very similar to optical ones. Mirrors and reflectors are used to direct the infrared light to a detector at the focal point. The detector registers the incoming radiation, which a computer then converts into a digital image.
Radio telescopesAt the far end of the electromagnetic spectrum we find the radio waves, with frequencies less than 1000 megahertz and wavelengths of a metre and more. Radio waves penetrate the atmosphere easily, unlike higher-frequency radiation, so ground-based observatories can catch them.
Radio telescopes feature three main components that each play an important role in capturing and processing incoming radio signals.
The first is the massive antenna or ‘dish’ that faces the sky. The Parkes radio telescope in New South Wales, Australia, for instance, has a dish with a diameter of 64 metres, while the Aperture Spherical Telescope in southwest China has a whopping 500-metre diameter.
The great size allows for the collection of long wavelengths and very quiet signals. The dish is parabolic, directing radio waves collected over a large area to be focused to a receiver sitting in front of the dish. The larger the antenna, the weaker the radio source that can be detected, allowing larger telescopes to see more distant and faint objects billions of light years away.
The receiver works with an amplifier to boost the very weak radio signal to make it strong enough for measurement. Receivers today are so sensitive that they use powerful coolers to minimise thermal noise generated by the movement of atoms in the metal of the structure.
Finally, a recorder stores the radio signal for later processing and analysis.
Radio telescopes are used to observe a wide array of subjects, including energetic pulsar and quasar systems, galaxies, nebulae, and of course to listen out for potential alien signals.