The BIG problem for telescopes looking into space from Earth is that the Earth’s atmosphere ‘blurs’ the view. It’s like when you look at the stars and they twinkle. What is actually happening is that the turbulence in the Earth’s atmosphere is distorting the light that you see coming from the stars.
Adaptive optics (AO) is the cool tech that works to ‘fix’ that problem. A branch of optical engineering, AO works by measuring the distortion, and then sophisticated, deformable mirrors which are controlled by computers, in real-time, correct the distortion – enabling the world’s telescopes to bring the Universe into focus. Deformable mirrors can be large or small, and they work by physically pushing and pulling the back surface of an originally flat mirror, to change its shape.
Star HIC 59206 uncorrected and AO corrected image.
On the left the uncorrected image and on the right the corrected image.
At the Science and Technology Facilities Council’s (STFC) UK Astronomy Technology Centre in Edinburgh, optical engineer Dr. Noah Schwartz is the field expert applying AO tech to the groundbreaking instruments for the world’s biggest and most powerful telescopes.
HARMONI – a spectrograph – which is a type of instrument that splits the light emitted by astronomical objects into their different colours, to record them – will use AO tech. It is being designed and built by a European team led jointly by the University of Oxford and STFC’s UK Astronomy Technology Centre. HARMONI is a state-of-the-art ‘first light’ instrument which will go on the Extremely Large Telescope (ELT) – currently under in construction in Northern Chile. HARMONI will carry out observations of planets around nearby stars, the very first galaxies, and the very first stars ever formed. The ELT will be the largest and most powerful visible and infrared light telescope in the world when it is operational in the late 2020s. At a size of twice the length of a cricket pitch, if it were placed at Land’s End, it would be able to see a bumblebee at John O’Groats!
As part of a larger team, being led from the Laboratoire d’Astrophysique de Marseille, Dr. Schwartz and the optical engineering team at the UK Astronomy Technology Centre are developing the AO for HARMONI, ensuring the ELT a sensitivity that is many hundreds of times better than any current telescope of its kind. It is astronomical seeing – seeing the unseen in unprecedented fascinating detail.
Other groundbreaking astronomical instruments, that teams at UK Astronomy Technology Centre are developing use AO tech too. These are METIS – an imager and spectrograph also for the ELT; and ERIS – an imager and spectrograph for the Very Large Telescope (VLT).
1 - Ain’t no mountain high enough!
The only remedy is a most serene and quiet air, such as may perhaps be found on the tops of the highest mountains above the grosser clouds
Let’s say you want to observe your favourite science target. A galaxy. Light travels from the celestial objects in that galaxy in space, and then at the last 20km or so it encounters atmospheric turbulence from the Earth, and gets distorted.
Atmospheric turbulence happens when the wind mixes air of different temperatures, caused by the heating and cooling of the Earth’s surface by the Sun. Press the power switch to see how the heat from a domestic hairdryer recreates how the earth’s atmosphere affects our views of the Universe. Or observe the shimmering air above a road on a hot summer day.
This is why the world’s telescopes are often built high up in the mountains where there is less air, and therefore turbulence is weaker.
But, building a telescope on a mountain top, doesn’t completely solve the problem.
2 - It’s the mirror everybody wants in their bathroom!
All modern telescopes have the same essential design. They have large curved surfaces (or ‘mirrors’) that collect the light. This light is then focused into a camera that captures the image. The bigger the surface the more light is gathered – so we can see the Universe in more detail. Bigger is better when it comes to telescopes!
When light passes through atmospheric turbulence and gets distorted, it’s this distorted light that is collected by the telescope. The result – you get a fuzzy image – also called a ‘seeing limited’ image. All the fine details of the galaxy are lost. A deformable mirror, however, changes shape to best compensate for the optical distortion, therefore correcting it. The image is now nice and crisp – also called the ‘diffraction limited’ image. You can see a lot more detail in the image.
3 - The AO system
Adaptive optics system
A camera combined with a special set of optics and software, called a wavefront sensor, measures the distortion of the incoming light, taken from a ‘guide star’. This is a light source bright enough and close enough to what the astronomer wants to observe, that acts as a reference point.
This information is sent to a computer, and it calculates the optimal shape for the mirror, so it corrects for the optical distortion.
The deformable mirror is then changed to the required shape, to bring the light into focus.
And all of this happens in real-time. As the turbulence in the atmosphere constantly changes, so too does the mirror need to change shape to accommodate this. The computer monitors incoming light and sends adaptive signals to the mirror, constantly – to undo the turbulent effects of the atmosphere.
4 - Star Wars
This spectacular image shows the four beams emerging from the new laser system on Unit Telescope 4 of the VLT
(Credit: ESO/F. Kamphues)
Have you ever wondered why there are sometimes lasers beaming out of some of the world’s biggest telescopes? The answer is AO. With only about 1% of the night sky bright enough and close enough to act as a natural guide star, laser beams are used to create an artificial guide star.
How does it work? The colour of the laser is tuned to energise a layer of sodium atoms found in one of the upper layers of the atmosphere – about 90km above Earth. When the atoms are excited by the light from the laser, they start glowing, forming a small bright spot that can be used as an artificial reference star. The light from this artificial star travels back to the telescope and gets distorted by the same turbulent air that all the other light coming to the telescope must pass through. The AO system in the telescope corrects for this, using the laser star as its reference for the distortion.
HARMONI will use 6 lasers – so that the adaptive optics system ‘sees’ turbulence from multiple points-of-view, and can therefore measure the entire volume of turbulence over the telescope.
5 - The fast and the furious – high-speed precision
To compensate for the turbulence measured by the AO tech, the deformable mirror needs to change shape faster than the changes in the atmosphere – at about a thousand times a second!
The deformable mirror of the ELT will be directly integrated within the telescope. It is 2.4m diameter and will contain 5316 actuators (points pushing and pulling on the back of the mirror to control the deforming process). HARMONI will use this large 2.4m mirror to reduce the optical distortion from several microns (the average human red blood cell is approximately 5 microns) to about 100nm (the approximate size of the influenza virus)!
6 - Extreme vs good AO
AO is really about capturing the best image possible.
Not to be confused with extreme ironing – that practice of ironing in the most unlikely of places – extreme AO is the name given to AO systems that are highly optimized to capture images, specifically from exoplanets. Exoplanets are planets that orbit stars, outside of our solar system – essentially stars that are not our sun. Typically, exoplanets appear very close to their parent star, which is used as the guide star. But because exoplanets are very far away (the closest is 4.25 light years away), the celestial light emitted from them is very faint. Astronomers therefore need powerful and highly sensitive AO to image this more distant celestial light.
But not all astronomy needs the power of extreme AO. ‘Good’ AO is less sensitive but this reduction in precision means it can use more guide stars to see more celestial targets in the sky. So whilst extreme AO targets a few celestial objects, good AO can capture many.
There is a special ratio that astronomers use called the Strehl ratio which determines the relationship between the best image and the actual image. HARMONI which uses ‘good’ AO will achieve a ratio of 70%, but will be able to target many science objects, not just exoplanets! By contrast SPHERE – another instrument which is an exoplanet imager on the VLT achieves a ratio of 95% ratio, but for very few objects close to the science target.
7 - Bigger is still better, even on nanosatellites
Nanosatellites are small satellites, used for both space research and increasingly by organisations with commercial goals. They are small because they weigh less than 10 kg – a 3U CubeSat (a particular type of nanosat), for example, is three 10cm x 10cm x 11.35cm units fixed together to create a small satellite that is about the size of a shoe box, or whisky box (depending on your preference). Their small size makes nanosatellites or CubeSats more affordable to build, especially for commercial companies. And they can ‘lift share’ on larger launches.
To increase the telescope size on a CubeSat (yes, bigger is still better), multiple mirrors are folded into the CubeSat before launch and deploy (in other words, unfold) while in space. To take an image, the individual mirrors need to act as if they were one single uncut mirror. To get a sharp image, AO is used to optimally move the mirrors into place after launch.
The UK Astronomy Technology Team AO team is developing the groundbreaking tech to do this – from the folding and deploying of the compact mechanical system to the entire AO system that measures the optical distortion and moves the mirrors into position.
8 - Not just astronomy
AO is not only used in astronomy, it’s also used in areas such as retinal imaging, laser communications, microscopy and more. In retinal imaging AO corrects for the optical distortion passing through the pupil and lens of the eye, to improve the quality of the images captured on the retina. In microscopy it corrects for the distortions caused by both the optics (the lenses) in the microscope, and the biological tissue being studied.
From the world’s largest telescopes to small nanosatellites, and to the tiny molecules in human tissue AO is the cool tech that lets us ‘see the unseen’ – with nm precision!
Teams at UK ATC and Heriot-Watt University are working together on AO for microscopy. They are combining two technologies – AO and multiple plane imaging (a type of microscopy that allows us to study the 3D dynamics in live cells) – with the aim of improving the image quality of large biological tissue samples. The final result will give high-resolution 3D images in real-time without the need for scanning through the biological sample.
Last updated: 29 May 2019