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Starts With A Bang

A Giant Leap Towards Defeating Astronomy’s Greatest Enemy: Earth’s Atmosphere

Telescopes from the ground are bigger, but have to fight the atmosphere. Here’s how to win.

In astronomy, seeing farther and fainter than ever before requires three simultaneous approaches.

First light, on April 26, 2016, of the 4LGSF (4 Laser Guide Star Facility). This is presently the most advanced adaptive optics system in use aboard a modern observatory, and helps astronomers produce, in many ways, superior-quality images to what even a space-based observatory like Hubble can obtain. For the next generation of ground-based observatories, improvements and new innovations will be necessary. (ESO/F. KAMPHUES)

1.) Building bigger telescopes, gathering more light and yielding higher resolutions.

A comparison of the mirror sizes of various existing and proposed telescopes. When the Giant Magellan Telescope and the Extremely Large Telescope come online later in the 2020s, they will be the world’s largest, at 25 and 39 meters in aperture, respectively. The largest space-based telescopes, like Hubble, Herschel, and even James Webb, are all significantly smaller. (WIKIMEDIA COMMONS USER CMGLEE)

2.) Upgrading your instruments, optimizing the data from each arriving photon.

The ESO’s Very Large Telescope (VLT) contains a new imaging instrument on it, SPHERE, which allows us to image exoplanets and protoplanetary disks around smaller, lower-mass stars at high resolution than ever before, and to do so rapidly as well. Improvements in instrumentation can give older telescopes a new lease on life. (ESO / SERGE BRUNIER)

3.) Overcoming the distortive effects of Earth’s atmosphere.

This 2-panel shows observations of the Galactic Center with and without Adaptive Optics, illustrating the resolution gain. Adaptive optics corrects for the blurring effects of the Earth’s atmosphere. Using a bright star, we measure how a wavefront of light is distorted by the atmosphere and quickly adjust the shape of a deformable mirror to remove these distortions. This enables individual stars to be resolved and tracked over time, in the infrared, from the ground. (UCLA GALACTIC CENTER GROUP — W.M. KECK OBSERVATORY LASER TEAM)

The easiest way to overcome the atmosphere is from space, avoiding it entirely.

The Hubble Space Telescope, as imaged during its last and final servicing mission. Although it hasn’t been serviced in over a decade, Hubble continues to be humanity’s flagship ultraviolet, optical, and near-infrared telescope in space, and has taken us beyond the limits of any other space-based or ground-based observatory. Going to space is one way to conquer Earth’s atmosphere. (NASA)

However, space telescopes are expensive, hard to service, and size/payload-limited.

The Extremely Large Telescope (ELT), with a main mirror 39 metres in diameter, will be the world’s biggest eye on the sky when it becomes operational later in the 2020s. This is a detailed preliminary design, showcasing the anatomy of the entire observatory. It is more than 10 times the diameter of any telescope launched to space, and will have 36 times the light-gathering power of even the James Webb Space Telescope. (ESO/L. CALÇADA)

Significantly larger telescopes can be constructed on the ground, where Earth’s atmosphere is unavoidable.

The summit of Mauna Kea contains many of the world’s most advanced, powerful telescopes. This is due to a combination of Mauna Kea’s equatorial location, high altitude, quality seeing, and the fact that it’s generally, but not always, above the cloud line. Even from a pristine location like this, however, Earth’s atmosphere cannot be avoided and must be reckoned with. (SUBARU TELESCOPE COLLABORATION)

Even at high altitudes, with smooth, dry air and cloudless skies, atmospheric distortion is severely limiting.

On the now under-construction Giant Magellan Telescope, each of the main seven primary mirrors will have their own secondary mirror, and there will be seven independent adaptive optics systems attached to the secondary mirrors themselves. Each segment will have 675 actuators and a segment positioner with six degrees-of-freedom to optimally focus and undistort the light. (GIANT MAGELLAN TELESCOPE — GMTO CORPORATION)

That’s where the science of Adaptive Optics comes in.

Exploded view of an adaptive secondary mirror segment that will be part of the GMT. It shows the key components, which include: the adaptive face sheet, rigid reference body, electromagnetic actuators, cold plate, and the 6-degrees-of-freedom segment positioner. (GIANT MAGELLAN TELESCOPE — GMTO CORPORATION)

A portion of any incoming light is immediately analyzed for identifiable distortions of known, point-like sources.

When light comes in from a distant source and makes its way through the atmosphere to our ground-based telescopes, we’ll typically observe an image like the one you see at the left. However, through processing techniques like speckle interferometry or adaptive optics, we can reconstruct the known point source at left, greatly reducing the distortion and providing an astronomers with a template to un-distort the remainder of the image. Adaptive optics is a remarkable technology, with the potential to compete with the ‘seeing’ quality from space. (WIKIMEDIA COMMONS USER RNT20)

Algorithms compute the shape of a mirror required to “undistort” that light.

As light comes into your adaptive optics setup, you must first create a copy of your light by using a device like a beam splitter, send half of it into an analyzer while you delay the other half by increasing its path-length, then create a deformed mirror designed to un-distort the delayed light and recover your pristine guide star, and then reflect your delayed light off of the adaptive mirror, producing the best images possible from the ground. (GEMINI OBSERVATORY — ADAPTIVE OPTICS — LASER GUIDE STAR; ANNOTATION BY E. SIEGEL)

A secondary mirror “adapts” its shape to counteract atmospheric distortion.

This star cluster, known as R136, is located some 168,000 light-years away and contains the most massive known stars in the Universe, with R136a1 weighing in at 260 times the mass of our Sun. This image was taken in the near-infrared with the MAD adaptive optics instrument at ESO’s Very Large Telescope, and could not have been this successful without adaptive optics technology. (ESO/P. CROWTHER/C.J. EVANS)

This clever scheme creates a crisp image that can surpass even Hubble’s capabilities.

A crowded, distant star field illustrates how resolution improves with the size of the primary mirror and the quality of adaptive optics. Without adaptive optics, natural seeing is highly distorted by the atmosphere. Smaller telescopes in space, such as Hubble, can surpass anything that the atmosphere distorts. With adaptive optics, however, a larger ground-based telescope can significantly outperform even Hubble. (GIANT MAGELLAN TELESCOPE — GMTO CORPORATION)

This decade, the GMTO and ELT will become Earth’s first 30-meter-class telescopes.

A side-view of the completed Giant Magellan Telescope (GMT) as it will look in the telescope enclosure. It will be able to image Earth-like worlds out to 30 light years away, and Jupiter-like worlds many hundreds of light years distant. GMT is slated to take its ‘first light’ image in later in the 2020s. (GIANT MAGELLAN TELESCOPE — GMTO CORPORATION)

The NSF just granted $17.5M to GMTO, including developing seven adaptive secondary mirrors working together, simultaneously.

Current technology has progressed to the point where exoplanets can be directly imaged, but only for gas giant worlds that are located far from their parent star, such as the four planets orbiting the star HR 8799 shown here. HR 8799 is located 129 light-years from Earth, but a 30-meter-class telescope could directly image rocky exoplanets around a nearby star like Alpha Centauri A or B. (J. WANG (UC BERKELEY) & C. MAROIS (HERZBERG ASTROPHYSICS), NEXSS (NASA), KECK OBS.)

Equipped with this novel technology, direct imaging of rocky exoplanets may finally become possible.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.


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