Eyes in the Sky: Space Telescopes from Hubble to Webb

Telescopes and space: the two are virtually inseparable. Most of what we know about the universe beyond our own planet is down to telescopes. To say there are craters on the Moon, or that the planet Jupiter – no more than a bright star to the naked eye – is a giant world with moons of its own, would have sounded unbelievable before these facts were revealed by the first telescopes in the 17th century. Yet today, they have been so thoroughly absorbed into our culture that everyone takes them for granted.

The advent of space travel in the 20th century gave us a new perspective on outer space – or at least the nearby part of it represented by our own Solar System. Humans themselves have only ventured as far as the Moon, but robotic probes have travelled much further. Several have visited Jupiter and its moons, including the Juno spacecraft currently in orbit there, while New Horizons is flying through the Kuiper Belt far beyond the orbit of Neptune. Everyone has marvelled at the high-definition images sent back by these missions, such as Juno’s panoramic views of Jupiter’s swirling, multicoloured clouds and the intriguing glimpses of the 4.5-billion-year-old ice world Arrokoth captured by New Horizons. What’s rarely mentioned is the fact that the probes obtained these images using – you guessed it – telescopes.

A composite image of Arrokoth produced by New Horizons using its LORRI (Long Range Reconnaissance Imager) telescope in 2019.

NASA

As for the vast universe beyond the Solar System, no human-built spacecraft is going to travel there for a very long time. Yet, if you’ve ever been blown away by breath-taking photographs of distant nebulae or galaxies – and who hasn’t? – they were almost certainly taken by a spacecraft. Not an outward-travelling space probe in this case, but one orbiting our own planet at an altitude of just 540 kilometres. This, of course, is the Hubble Space Telescope – probably the most famous juxtaposition of the words ‘space’ and ‘telescope’ of all.

Hubble and its successor the James Webb Space Telescope (JWST) are operated by NASA, the same organisation responsible for the Apollo lunar landings and interplanetary probes like Juno and New Horizons. An American government agency, NASA stands for National Aeronautics and Space Administration – and strictly speaking, the word ‘space’ in its name refers to space travel. The wider use of the word to encompass the whole of astronomy and cosmology wasn’t originally part of the agency’s remit – but thanks to Hubble and other space-based telescopes, NASA has become a world authority in those areas too. In fact, the two uses of the word – to mean space travel and the study of the cosmos – have become intertwined. Arrokoth, for example, was discovered by the Hubble telescope in 2014, while it was searching for suitable destinations for the already-launched New Horizons spacecraft.

Yet unlike New Horizons, Hubble has never been anywhere near Arrokoth, which is around 6.6 billion kilometres from Earth. Hubble, at the best of times, is a mere 540 kilometres closer to Arrokoth. And when you turn to the majestic, star-studded galaxies that Hubble is most famous for photographing, the distances become unimaginably big – a trillion times a trillion kilometres or more – so it’s not immediately obvious that Hubble has any advantage over an Earthbound telescope at all. Understanding why it does, and why the same is true of JWST and other space-based telescopes like the planet-hunters Kepler and TESS (Transiting Exoplanet Survey Satellite), is one of the aims of this book. The other is to take a closer look at some of the many amazing discoveries made with these telescopes.

But before that, it’s worth taking a moment to consider a couple of even more basic questions. Just what is a telescope, and why is it such a useful tool for astronomers?

Telescope basics

Telescopes have been associated with astronomical observation ever since the time of Galileo Galilei, one of the great pioneers of experimental science, in the early 17th century. But the basic idea of the telescope wasn’t his. It started out as a kind of toy called a ‘spyglass’, made by placing two spectacle lenses at either end of a long tube. Looking through the tube made distant objects appear closer than they really were. In 1608, an enterprising Dutchman named Hans Lipperhey applied for a patent on just such a device, only to have the application turned down on the grounds that the idea was already common knowledge. In fact, similar toys were available for purchase in several European countries by that time.

Lipperhey-style spyglasses only really had novelty value, as opposed to any practical use, since they could only magnify an image by a factor of three or so. The amount of magnification was set by the strength ratio of the two lenses used, and since people were using ready-made spectacle lenses for the purpose, three was about the best they could do. One of Galileo’s first innovations was to produce a much stronger, custom-built lens for the eyepiece end of his telescope, allowing him to achieve a much higher magnification.

Galileo’s other great breakthrough was to use his telescope to look at astronomical objects that, up to that time, had only ever been seen with the naked eye. He used a telescope with a magnification of 30× to observe the Moon, for example, and described the results in a short treatise called Sidereus Nuncius (Latin for ‘Starry Messenger’) that was published in 1610:

It is a most beautiful and a very pleasing sight to look at the body of the Moon, which is removed from us by almost 60 terrestrial radii, and to see it as if it were only two radii away. This means that the Moon’s diameter looks almost 30 times larger … Anyone can grasp for himself that the Moon’s surface is not smooth and polished but rough and uneven. Like the face of the Earth, it is covered all over with huge bumps, deep holes and chasms.

Only parts of Galileo’s 30× telescope survive today, but it’s believed that it had a length of around 1.7 metres and a diameter of 40 millimetres at the main lens. These measurements effectively correspond to the two most important parameters of any telescope, its focal length and aperture. The first is the distance at which the main lens (or mirror, in a reflecting telescope) brings incoming light to a sharp focus, while the aperture is simply the diameter of that lens (or mirror). If you see a telescope characterised by just a single dimension, then it’s the aperture that’s being referred to. So an amateur astronomer boasting, as they’re wont to do, of having a ‘ten-inch reflector’ isn’t saying that it’s all of ten inches long (about 25 centimetres) but that it has a mirror of that diameter.

When I said a telescope has two important parameters, you may wonder why I didn’t mention the one that was so important to Galileo: magnification. But magnification isn’t really a property of a telescope per se, so much as the eyepiece that’s used with it – and that’s why we don’t need to worry about it in this book. Space telescopes don’t have eyepieces, they have sensors similar to the ones used in digital cameras. For that matter, the same is true of virtually all the ground-based telescopes used by professional astronomers these days, and many amateur astronomers too.

In a camera, the equivalent of increasing the magnification is ‘zooming in’. A zoom lens is simply one that has a variable focal length, and making this longer causes the object you’re looking at to fill a larger portion of the field of view. Inside the camera, the image of the object spans a greater number of sensor pixels at high zoom than it does at low zoom.

This effect is the digital counterpart to the magnification produced by Galileo’s eyepiece, and the technical term for it is ‘resolution’. In order to explain what this is, and how it differs from traditional magnification, we’re going to have to get to grips with one of the more brain-twisting concepts in this book – but one that’s absolutely fundamental to the way telescopes are used in astronomy. I’ll start by asking a rhetorical question: which is bigger, the Sun or the Moon? I’m sure everyone knows the answer: the Sun is by far the bigger of the two – but it’s also further away by roughly the same factor. This means the apparent sizes of the Sun and Moon in the sky are pretty much the same (that’s why total eclipses work out as neatly as they do). If you imagine looking at the full Moon (I don’t want you looking straight at the Sun, even in a thought experiment) and drawing straight lines to opposite edges of it, then the angle between the two lines would be around half a degree. That’s what astronomers call the ‘angular size’ of the Moon – and it’s the angular size of the Sun, too.

That’s the ‘brain-twisting concept’ I warned you about: astronomers like to measure objects in terms of angles rather than linear measurements. To complicate matters further, they’re usually talking about very small angles, so they divide a degree into 60 arcminutes, and an arcminute into 60 arcseconds, by analogy with minutes and seconds of time. An arcsecond is small, but not unimaginably small – about the same as a five-pence coin (or an American dime) viewed at a distance of four kilometres.

Now let’s go back to the question of magnification versus resolution. When Galileo described his 30× magnified view of the Moon, he used words anyone can understand. He said that his telescope made the Moon look 30 times closer than it really is, or 30 times bigger. He didn’t need to say anything about angular sizes, even though that’s what he was talking about. He meant that when viewed through the eyepiece of his telescope, the Moon appeared to be fifteen degrees across, compared to just half a degree when seen without a telescope. But he didn’t need to say that, because the magnification is just the first number divided by the second.

Unfortunately, we can’t apply such simple logic to a photograph taken with a digital camera. If you point your smartphone or digital camera at the Moon, set it to maximum zoom and take the best picture you can, what magnification is that? It might look tiny on the device’s screen, but what if you take it indoors and look at it on a 50-inch TV screen? What matters here isn’t how big the image looks, but how many of the camera sensor’s pixels the Moon spans. If the answer is, say, 1,000, that means you’ve got around half a degree, or 1,800 arcseconds, spanning those thousand pixels. This gives you a resolution of 1.8 arcseconds per pixel – and it turns out that’s the most meaningful analogue of magnification for a digital system.

It’s the same with telescopes, up to and including Hubble. At this point, if you’ve been following closely, you may be getting the germ of an idea. ‘What if I just buy a super-high-resolution sensor that’s millions of pixels across?’ you might be thinking. ‘Will that make my backyard telescope just as powerful as Hubble?’ Unfortunately, it’s not that simple. You might be able to get as many pixels across the image as a giant professional telescope, but the result isn’t going to be any clearer than it was with a fraction of those pixels. The laws of optics put a natural limit on a telescope’s resolution that’s inversely proportional to its aperture,* so a small telescope is never going to be as good as a large one. That’s one of the reasons that serious astronomical telescopes are designed with the biggest aperture that circumstances allow (the other reason being the obvious one that a large aperture can collect more light than a small one).

The other feature common to virtually all the telescopes used in professional astronomy, besides their enormous size, is the fact that light is collected and brought to a focus by a mirror rather than a lens. At first sight this may