How do telescopes work? Why do we send telescopes into space? More about our best tools for looking at the universe.
How the first telescopes worked
The first telescopes were developed by a Dutch spectacle maker called Hans Lippershey around 1608. The idea was seized upon by Galileo Galilei, who built his own telescope to watch the skies and make his ground-breaking observations.
The earliest telescopes used refraction or the bending of light to magnify images. Two convex lenses, bulging pieces of glass, are placed inside a tube. Light enters the telescope in parallel lines. As it passes through the first lens – known as the objective lens – and moves from air into the thick, dense glass, it bends.
The rays start converging together, to a focal point, where the rays cross each other and start to move apart again. After the rays diverge, a second lens is put in their path to straighten them out again.
This second lens is known as an eyepiece lens and creates an image on your retina. Your eye is tricked into thinking that you’re looking at something much closer than it really is. Modern telescopes follow similar principles, but most now use mirrors instead of lenses to collect and redirect light.
Telescopes: refracting versus reflecting
Essentially, all telescopes have the same job – to make things easier to see. They increase the detail we see in fuzzy, far away objects and make the invisible visible. To achieve this, telescopes gather as much light as possible and focus it into an image.
The earliest refracting telescopes used lenses to do this. A combination of 2 lenses – the objective lens and the eyepiece – first gathered light and then straightened it close to the viewer to produce a magnified image. Larger objective lenses produced larger magnification, but it is difficult to produce large lenses without imperfections, and these telescopes were subject to light loss.
Later, reflector telescopes used mirrors to gather light. It’s much easier to produce a large mirror than a large lens. Most modern telescopes are reflector telescopes, and many of them are huge. Plans for the Extremely Large Telescope, currently under construction in Chile, include a mirror with a diameter of 130 feet (39.3 meters).
The electromagnetic spectrum
Electromagnetic energy travels in waves at different wavelengths and frequencies. The electromagnetic spectrum shows how these waves are distributed. It ranges from radio waves with long wavelengths through visible light to ultraviolet light, x-rays, and gamma-rays at the highest frequencies.
Everything on the electromagnetic spectrum is fundamentally the same type of energy. But we need to take different approaches to detect the different parts of the electromagnetic spectrum.
Visible light can be detected using optical telescopes with a lens or a mirror. Radio telescopes use large metal discs to collect radio waves. Infrared telescopes are similar to optical telescopes but need to be carefully placed at high altitudes, as water vapor absorbs infrared light easily, and the atmosphere holds more water vapor close to sea level.
The Earth’s atmosphere blocks a large chunk of the electromagnetic radiation from space. This includes most ultraviolet light and all x-rays and gamma-rays. To measure these, we need to send telescopes beyond our atmosphere into space.
Limitations of ground-based telescopes
One of the biggest challenges for ground-based telescopes is light pollution. Excessive light created by humans causes light pollution. Sky glow – the brightening of the night sky – reduces the contrast between the sky and objects such as stars or galaxies, making things particularly tricky. Light from nearby stars reflecting off satellites in low orbit is also a growing issue.
Ground-based telescopes also need to contend with the blurring effects of the atmosphere. Turbulence and particles in the atmosphere distort the picture of the sky we get from Earth. You can see this effect with the naked eye while looking up at the night sky. This distortion makes the stars twinkle.
Because parts of the electromagnetic spectrum are blocked by our atmosphere, there are certain things that we just can’t see from Earth, no matter how good our telescopes get. We need space telescopes to observe x-rays, gamma-rays, or ultraviolet radiation.
Overcoming challenges
Light pollution can be minimized by building telescopes well away from major population centers. When choosing where to build telescopes, light pollution is a major consideration. Remote mountaintops are often an excellent choice as the high elevation makes for a thinner atmosphere, reducing the distorting effects of the atmosphere.
Suitable locations for ground-based telescopes aren’t easy to find: the Atacama Desert in Chile and the Andes are among some of the best candidates. And protecting dark skies from artificial light remains a huge and growing challenge.
Ground-based telescopes use adaptive optics to compensate for the blurring of the atmosphere. They use deformable mirrors to correct images. Adaptive optics can improve the resolution of ground-based telescopes, thus enhancing the level of detail and clarity of the images produced.
Interferometry
Astronomical interferometers take observations from separate telescopes and combine them into 1 image, providing the clarity and angular resolution of 1 huge telescope. These collections of telescopes are known as arrays.
Combining data from multiple telescopes needs to be matched up very precisely. This is easier with the longer wavelengths of radio signals, so the first telescope arrays were designed to detect radio waves. This technique has also been used with optical telescopes, but it does have drawbacks. Each smaller telescope has a smaller opening for light to pass through – a smaller aperture.
Because of this, the telescopes collect less light, and the images produced are dimmer than those from similarly large single aperture telescopes, which have much larger openings for light to pass through.
The Event Horizon Telescope used very long baseline interferometry (VLBI). This involved telescopes at huge distances from each other – stationed on 5 different continents. Combining data from these telescopes enabled astronomers to create the first ever image of a black hole in 2019.
Space-based telescopes
Sending telescopes into space gives us a whole new perspective on the universe. With no turbulent atmosphere between them and their subjects, they don’t have the same distorting effects to deal with. Plus, they can measure radiation levels that will never reach Earth.
NASA launched the first successful space telescope in 1968 – the Orbiting Astronomical Observatory (OAO-2). It was used to detect ultraviolet radiation and, among other things, confirmed that massive clouds of hydrogen surround comets. There have since been more than 90 telescopes sent into space.
Space telescopes do have their drawbacks. They’re expensive and difficult to build and launch. And once they’re up in space, it’s very tricky to make repairs if things go wrong. Space shuttles have flown crews to the Hubble Space Telescope to make fixes and carry out servicing, but this is very unusual. Most space telescopes aren’t serviced – even the most remote telescope on Earth would be easier to fix.
Compton Gamma-ray and Spitzer
To truly understand the universe, we need to ‘see’ using the whole electromagnetic spectrum. NASA’s Great Observatories were designed to detect radiation from across the electromagnetic spectrum.
The 4 telescopes involved were the Hubble Space Telescope, the Compton Gamma-ray Telescope, the Chandra X-ray Observatory, and the Spitzer Space Telescope. Crucially, the missions were designed to overlap so that the telescopes could observe the same objects at the same time but using different wavelengths.
The Compton Gamma-ray Telescope operated from 1991 to 2000 and detected x-rays and gamma-rays.
The Spitzer Space Telescope operated from 2003 to 2020 and detected infrared radiation. This allowed us to discover more about relatively cooler objects in space, such as failed stars and planets outside our solar system. It also allowed us to see into normally hidden regions, such as the centers of galaxies.
Hubble and the Chandra X-ray Observatory
Two of the Great Observatories are still in operation – the Hubble Space Telescope and the Chandra X-ray Observatory.
Arguably the most famous of the four, the Hubble Space Telescope was launched in 1990 and is still operating more than 30 years later.
In fact, it’s grown more powerful in space thanks to five servicing missions carried out by space shuttles to repair and upgrade its equipment. It observes a range of electromagnetic radiation from near-infrared, through visible light, and into ultraviolet. It has made more than 1.5 million observations, and, among other things, helped to detect the universe’s accelerating expansion.
The Chandra X-ray Observatory launched in 1999 and is still operating to detect x-rays from sources billions of light years away. X-rays are released when matter is heated to extremely high temperatures – millions of degrees.
These temperatures might be because of extreme gravity or explosive forces, among other things. The Chandra X-ray Observatory examines very hot, turbulent regions of space to answer fundamental questions about the universe’s origin.
Other significant missions
Our exploration of the universe with space telescopes goes beyond the great observatories: more than 90 telescopes have been launched by NASA and the ESA since 1970.
The Kepler Space Telescope searched our galaxy for possibly habitable planets. Launched in 2009, the reaction wheels meant to keep the telescope straight failed between 2012 and 2013. A rescue plan was conceived to stabilize its view using the pressure of the sun. Reborn as K2, it continued to look for planets until its retirement in 2018. It discovered more than 2600 planets while it operated.
The Wilkinson Microwave Anisotropy probe (WMAP) mapped slight irregularities in the background microwave radiation of the universe, giving us a better understanding of the universe just after the Big Bang. It operated between 2001–2010 and was succeeded by the Planck Observatory of the ESA, which ran between 2009–2013.
Planned missions include NASA’s Nancy Grace Roman Space telescopes, which are scheduled for launch in 2027, hoping to measure the effects of dark energy, among other things.
Space telescopes like these have deepened our understanding of the universe and continue to do so.
The James Webb Space Telescope
One hundred times more powerful than the Hubble Space Telescope, the James Webb Space Telescope (JWST) has been designed to detect infrared radiation more precisely than ever before. To do this, the telescope needs to be shielded from the heat emitted by the Earth, the moon, and the sun. It also needs a huge mirror – 21 feet (6.5 meters) in diameter.
The mirror and sun shield of the JWST are so large that they could not fit into a rocket for launch. To get around this, engineers designed JWST to unfold in space once it reached its orbit around 932,000 miles (1.5 million kilometers) away from Earth. With 300 possible points for things to go wrong, this unfolding must have been a nerve-shredding process for the scientists involved.
What the JWST can tell us
The JWST was launched on the 25th of December, 2021. The first images from the telescope were released in mid-2022, showing a stunning level of detail from events throughout the universe.
The JWST is looking at the formation of the very first galaxies and stars in the universe. As it captures data from enormous distances, it is also peering back in time due to the speed at which light travels. Light from far-distant parts of the universe is stretched to redder wavelengths, thanks to the expansion of the universe. Using infrared radiation reveals more about these far-distant events than other wavelengths can.
One of the first images released is the deepest and clearest image we have ever had of the distant universe. Packed with thousands of distant galaxies, this deep-field image by the telescope covers a tiny patch of sky – about the size of a grain of sand held at arm’s length. It’s a breathtaking reminder of the scale of the universe.
As the JWST continues to operate, we will receive more data and images that reveal the extraordinary secrets of the universe.
Dealing with the data: citizen science and machine learning
As telescopes have evolved, the way we collect data has changed. We’ve progressed from making observations by eye, to capturing data with photography, to where we are now: using charged coupled devices (CCDs) in modern telescopes to collect and store data. These CCDs are also found in digital cameras.
The amount of data produced by telescopes have increased as our technology has improved. Modern telescopes give us a torrent of data about the universe. For example, the Hubble Space Telescope has produced more than 153 terabytes of data during its mission so far. The question is: how do we learn as much as we possibly can from this data?
Citizen science projects such as Galaxy Zoo recruit members of the public to help examine data from telescopes – crowdsourcing astronomy and helping us learn more. As artificial intelligence (AI) advances, both citizen scientists and astronomers have also trained machine learning algorithms to classify data, so we can use the huge amounts of data that we’re gathering from the universe to start to answer our biggest questions.
