Most telescopes used by astronomers are on Earth. We call these ground-based telescopes.
Building a telescope on Earth is much easier and cheaper than in space. It is also much easier to fix if things go wrong.
However, there are downsides as well. A telescope on the ground has to look through the Earth's atmosphere to see into space.
This is a problem because the atmosphere can blur our images.
New telescopes can use some techniques to improve ground-based observations by dealing with the blurring caused by the atmosphere, weather, and stress on the telescope.
- Active Optics
This technology has been around since the 1980s. It uses small controls on the back of the main mirror on the telescope to change its shape.
These changes help to get rid of issues caused by high winds moving the telescope around or large temperature differences changing the size of the materials in the telescope.
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CreditThis work by NOIRLab/NSF/AURA is licensed under Creative Commons Attribution 4.0 InternationalThe back of the WIYN telescope's primary mirror showing the complex control systems for the active optics When telescopes increased in size in the 1990s to have 8+ metre mirrors, keeping their shape was also much harder. These mirrors had to be much thinner than ever before, so they didn't weigh too much to move around effectively.
The thin mirrors could not hold their shape, especially if they pointed close to the horizon where the gravitational load across the surface was at its highest. So, adding these actuators on the back allows the mirror to be corrected back into shape.
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CreditThis work by KPNO/NOIRLab/NSF/AURA/T. Slovinský is licensed under Creative Commons Attribution 4.0 InternationalA closer view of the active optics on the WIYN telescope's primary mirror. These actuators can apply forces to the mirror, making it the correct shape for the current conditions. - Adaptive Optics
Adaptive optics systems grew from learning about active optics.
It is possible to change the shape of a mirror to correct for issues facing the telescope. Adaptive optics tries to correct for the blurring in the atmosphere.
They shine a laser through the sky and measure how it changes due to turbulence in the air.
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CreditThis work by ESO/G. Hüdepohl is licensed under Creative Commons Attribution 4.0 InternationalAdaptive optics at the Very Large Telescope (VLT) at the European Southern Observatory (ESO) in Chile They can quickly react to any changes in the air above the telescope by doing this before every image. They can apply just the right changes to a small mirror in the telescope, which corrects for the atmosphere before the light moves into the instrumentation on the back of the telescope.
The corrections stop images from looking 'blurry' and allow us to look at the Universe in more detail.
The air also blocks light from parts of the electromagnetic spectrum, such as x-rays and gamma rays. So even if we have the right telescope, it can't see this type light from Earth. The air gets in the way, which is why some telescopes are in space.
We call the parts of the light spectrum that can pass through the air atmospheric windows. These are the parts of the electromagnetic spectrum where the opacity (how much light is blocked) is close to 0%. If the opacity is 100%, then no light with that wavelength can pass through the air to reach the ground.
- Optical and Near-Infrared Light
Most telescopes on Earth are designed to look at the light passing through the atmosphere in the optical and near-infrared wavelengths. Optical light is the same as what our eyes can see.
Telescopes that study this type of light are placed on high mountains, where there is less atmosphere to look through. They are also placed in places with clear nights most of the year so that clouds and rain do not stop them from observing.
These telescopes have been used for thousands of years to look at planets, stars and galaxies. The biggest telescopes used today have mirrors 8 to 10 metres in diameter. In the next 10 years, the next generation of telescopes will come online with mirrors more than 30 metres across!
Each time the size of a telescope increases, the science we can do with it also expands. These telescopes will be used to look for life on other planets, the very earliest stages of the Universe after the Big Bang, and how galaxies form and evolve.
- Radio Waves
Telescopes on Earth can also look at much of the radio part of the light spectrum. This type of light can also pass straight through clouds in the atmosphere, meaning that they don't need to worry about getting good weather all the time! Radio astronomy can also be done 24 hours a day, as the light from the Sun doesn't swamp this part of the spectrum as it does with optical light.
The best radio telescopes on Earth are built away from areas with many people to reduce interference from electronic devices like mobile phones. It's good to look at the radio to see different information about objects in space. Pulsars and shock waves caused by the Big Bang, for example, are bright in the radio. These telescopes are also used to search for signs of alien life in the Universe.
Some radio telescopes are huge single dishes used to collect light, and others are made of lots and lots of smaller dishes all working together.
- Gravitational Waves
Telescopes on Earth can also look for information from space which doesn't travel as light, but as vibrations in space itself.
These vibrations are called gravitational waves, and to find them, we need more than one detector spaced kilometres apart along long arms. Laser beams travel down the arms, bounce off a mirror, and travel back again.
Scientific tools measure the time it takes for the beam to travel to the mirror and back to work out the distance travelled. If a gravitational wave passes through the detector, it squeezes and stretches space. This changes the distance the laser beams must travel to reach the mirror and return.
However, the distance will only change by about 10-18 metres. This is less than one-thousandth of the diameter of a proton! It is very difficult to detect such a small change, which means detectors must be very sensitive to tiny changes!