What are Active Optics?

Active Optics

For astronomers and physicists alike, the depths of space are a treasure trove that may provide us with the answers to some of the most profound questions of existence. Where we come from, how we came to be, how it all began, etc. However, observing deep space presents its share of challenges, not the least of which is visual accuracy.

In this case, scientists use what is known as Active Optics in order to compensate for external influences. The technique was first developed during the 1980s and relied on actively shaping a telescope’s mirrors to prevent deformation. This is necessary with telescopes that are in excess of 8 meters in diameter and have segmented mirrors.

Definition:

The name Active Optics refers to a system that keeps a mirror (usually the primary) in its optimal shape against all environmental factors. The technique corrects for distortion factors, such as gravity (at different telescope inclinations), wind, temperature changes, telescope axis deformation, and others.

The twin Keck telescopes shooting their laser guide stars into the heart of the Milky Way on a beautifully clear night on the summit on Mauna Kea. Credit: keckobservatory.org/Ethan Tweedie
The twin Keck telescopes shooting their laser guide stars into the heart of the Milky Way on a beautifully clear night on the summit on Mauna Kea. Credit: keckobservatory.org/Ethan

Adaptive Optics actively shapes a telescope’s mirrors to prevent deformation due to external influences (like wind, temperature, and mechanical stress) while keeping the telescope actively still and in its optimal shape. The technique has allowed for the construction of 8-meter telescopes and those with segmented mirrors.

Use in Astronomy:

Historically, a telescope’s mirrors have had to be very thick to hold their shape and to ensure accurate observations as they searched across the sky. However, this soon became unfeasible as the size and weight requirements became impractical. New generations of telescopes built since the 1980s have relied on very thin mirrors instead.

But since these were too thin to keep themselves in the correct shape, two methods were introduced to compensate. One was the use of actuators which would hold the mirrors rigid and in an optimal shape, the other was the use of small, segmented mirrors which would prevent most of the gravitational distortion that occur in large, thick mirrors.

This technique is used by the largest telescopes that have been built in the last decade. This includes the Keck Telescopes (Hawaii), the Nordic Optical Telescope (Canary Islands), the New Technology Telescope (Chile), and the Telescopio Nazionale Galileo (Canary Islands), among others.

The New Technology Telescope (NTT) pioneered the Active Optics. Credit: ESO/C.Madsen. Bacon
The New Technology Telescope (NTT) pioneered the Active Optics. Credit: ESO/C.Madsen. Bacon

Other Applications:

In addition to astronomy, Active Optics is used for a number of other purposes as well. These include laser set-ups, where lenses and mirrors are used to steer the course of a focused beam. Interferometers, devices which are used to emit interfering electromagnetic waves, also relies on Active Optics.

These interferometers are used for the purposes of astronomy, quantum mechanics, nuclear physics, fiber optics, and other fields of scientific research. Active optics are also being investigated for use in X-ray imaging, where actively deformable grazing incidence mirrors would be employed.

Adaptive Optics:

Active Optics are not to be confused with Adaptive Optics, a technique that operates on a much shorter timescale to compensate for atmospheric effects. The influences that active optics compensate for (temperature, gravity) are intrinsically slower and have a larger amplitude in aberration.

. Credit: ESO/L. Calçada/N. Risinger
Artist’s impression of the European Extremly Large Telescope deploying lasers for adaptive optics. Credit: ESO/L. Calçada/N. Risinger

On the other hand, Adaptive Optics corrects for atmospheric distortions that affect the image. These corrections need to be much faster, but also have smaller amplitude. Because of this, adaptive optics uses smaller corrective mirrors (often the second, third or fourth mirror in a telescope).

We have written many articles about optics for Universe Today. Here’s The Photon Sieve Could Revolutionize Optics, What did Galileo Invent?, What did Isaac Newton Invent?, What are the Biggest Telescopes in the World?

We’ve also recorded an entire episode of Astronomy Cast all about Adaptive Optics. Listen here, Episode 89: Adaptive Optics, Episode 133: Optical Astronomy, and Episode 380: The Limits of Optics.

Sources:

Telescope’s Laser Pointer Clarifies Blurry Skies

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While it’s handy for us humans (and all of the other life on our planet for that matter), the atmosphere is almost universally cursed among astronomers. It’s great for breathing, but when it comes to astronomical observations of faint objects, all the atmosphere tends to do is muck up the view. In the past 20 years, development of adaptive optics – essentially telescopes that change the shape of their mirrors to improve their imaging capability – has dramatically improved what we can see in space from the Earth.

With a new technique involving lasers (Yes! Lasers!), the images capable with an adaptive optics telescope could be nearly as crisp as those from the Hubble Space Telescope over a wide field of view. A team of University of Arizona astronomers led by Michael Hart has developed a technique that helps calibrate the surface of the telescope very precisely, which leads to very, very clear images of objects that would normally be very blurry.

Laser adaptive optics in telescopes are a relatively new development in getting better image quality out of ground-based telescopes. While it’s nice to be able to use space-based telescopes like the Hubble and the forthcoming James Webb Space Telescope, they are certainly expensive to launch and maintain. On top of that, there are a lot of astronomers competing for very little time on these telescopes. Telescopes like the Very Large Telescope in Chile, and the Keck Telescope in Hawaii both already use laser adaptive optics to improve imaging.

Initially, adaptive optics focused in on a brighter star near the area of the sky that the telescope was observing, and actuators in the back of the mirror were moved very rapidly by a computer to cancel out atmospheric distortions. This system is limited, however, to areas of the sky that contain such an object.

Laser adaptive optics are more flexible in their usability – the technique involves using a single laser to excite molecules in the atmosphere to glow, and then using this as a “guide star” to calibrate the mirror to correct for distortions caused by turbulence in the atmosphere. A computer analyzes the incoming light from the artificial guide star, and can determine just how the atmosphere is behaving, changing the surface of the mirror to compensate.

In using a single laser, the adaptive optics can only compensate for turbulence in a very limited field of view. The new technique, pioneered at the 6.5-m MMT telescope in Arizona, uses not just one laser but five green lasers to produce five separate guide stars over a wider field of view, 2 arc minutes. The angular resolution is less than that of the single laser variety – for comparison, the Keck or VLT can produce images with a 30-60 milli-arcsecond resolution, but being able to see better over a wider field of view has many advantages.

In the image on the left, the cluster M3 appears blurry with the laser adaptive optics system turned off. Things are much clearer using the system, and individual stars in the cluster become visible, as can be seen in the image on the right. Image Credit: Michael Hart

The ability to take the spectra of older galaxies, which are very faint, is possible using this technique. By taking their spectra, scientists are better able to understand the composition and structure of objects in space. Using the new technique, taking the spectra of galaxies that are 10 billion years old – and thus have a very high red shift – should be possible from the ground.

Supermassive clusters of stars would also be more easily scrutinized using the technique, as images taken in a single pointing of the telescope on different nights would allow astronomers to understand just which stars are part of the cluster and which are not gravitationally bound.

The results of the team’s efforts was published in the Astrophysical Journal in 2009, and the original paper is available here on Arxiv.

Source: Eurekalert, Arxiv paper

Could An Amateur Astronomer Snap a Picture of an Exoplanet?

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Using their backyard telescope, today? No; however, this image of three exoplanets required just 1.5 meters (diameter; 60 inches) of a telescope mirror, not vastly larger than the biggest backyard ‘scope.

These particular exoplanets orbit the star HR 8799, and have been imaged directly before, by one of the 10-meter (33-foot) Keck telescopes and the 8.0-meter (26-foot) Gemini North Observatory, both on Mauna Kea in Hawaii; they are among the first to be so imaged, as reported by Universe Today in November 2008 First Image of Another Multi-Planet Solar System.

So how did Gene Serabyn and colleagues manage the trick of taking the image above, using just a 1.5-meter-diameter (4.9-foot) portion of the famous Palomar 200-inch (5.1 meter) Hale telescope’s mirror?

Infrared observations of a multi-exoplanet star system HR 8799 (Keck Observatory)


They did it by working in the near infrared, and by combining two techniques – adaptive optics and a coronagraph – to minimize the glare from the star and reveal the dim glow of the much fainter planets.

“Our technique could be used on larger ground-based telescopes to image planets that are much closer to their stars, or it could be used on small space telescopes to find possible Earth-like worlds near bright stars,” said Gene Serabyn, who is an astrophysicist at JPL and visiting associate in physics at the California Institute of Technology in Pasadena.

The three planets, called HR8799b, c and d, are thought to be gas giants similar to Jupiter, but more massive. They orbit their host star at roughly 24, 38 and 68 times the distance between our Earth and the Sun, respectively (Jupiter resides at about five times the Earth-Sun distance). It’s possible that rocky worlds like Earth circle closer to the planets’ homestar, but with current technology, they would be impossible to see under the star’s glare.

The star HR 8799 is a bit more massive than our sun, and much younger, at about 60 million years, compared to our sun’s approximately 4.6 billion years. It is 120 light-years away in the constellation Pegasus. This star’s planetary system is still active, with bodies crashing together and kicking up dust, as recently detected by NASA’s Spitzer Space Telescope. Like a fresh-baked pie out of the oven, the planets are still warm from their formation and emit enough infrared radiation for telescopes to detect.

To take a picture of HR 8799’s planets, Serabyn and his colleagues first used a method called adaptive optics to reduce the amount of atmospheric blurring, or to take away the “twinkle” of the star. For these observations, technique was optimized by using only a small fraction of the telescope was used. Once the twinkle was removed, the light from the star itself was blocked using the team’s coronograph, an instrument that selectively masks out the star. A novel “vortex coronagraph,” invented by team member Dimitri Mawet of JPL, was used for this step. The final result was an image showing the light of three planets.

While adaptive optics is in use on only a few amateurs’ telescopes (and a relatively simple kind at that), the technology will likely become widely available to amateurs in the next few years. However, vortex coronagraphs may take a bit longer.

“The trick is to suppress the starlight without suppressing the planet light,” said Serabyn.

The technique can be used to image the space lying just a few arcseconds from a star. This is as close to the star as that achieved by Gemini and Keck – telescopes that are about five and seven times larger, respectively.

Keeping telescopes small is critical for space missions. “This is the kind of technology that could let us image other Earths,” said Wesley Traub, the chief scientist for NASA’s Exoplanet Exploration Program at JPL. “We are on our way toward getting a picture of another pale blue dot in space.”

Sources: JPL, Nature, Astrophysics Journal (preprint is arXiv:0912.2287)

Keck Telescope

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There are two Keck telescopes – Keck I and Keck II; together they make up the W.M. Keck Observatory, though strictly speaking the observatory is a great deal more than just the telescopes (there’s all the instrumentation, especially the interferometer, the staff, support facilities, etc, etc, etc.).

William Myron Keck (1880-1964) established a philanthropic foundation in 1954, to support scientific discoveries and new technologies. One project funded was the first Keck telescope, which was quite revolutionary at the time. Not only was it the largest optical telescope (and it still is) – it’s 10 meters in diameter – but is made up of 36 hexagonal segments, the manufacture of which required several breakthroughs … and all 36 are kept in line by a system of sensors and actuators which adjusts their position twice a second. Keck I saw first light in 1993. Like nearly all modern, large optical telescopes, the Keck telescopes are alt-azimuth. Fun fact: to keep the telescope at an optimal working temperature – no cool-down period during the evening – giant aircons work flat out during the day.

The Keck telescopes are on the summit of Hawaii’s Mauna Kea, where the air is nearly always clear, dry, and not turbulent (the seeing is, routinely, below 1″); an ideal site for not only optical astronomy, but also infrared.

The second Keck telescope – Keck II – saw first light in 1996, but its real day of glory came in 1999, when one of the first adaptive optics (AO) systems was installed on it (the first installed on a large telescope).

2004 saw another first for the Keck telescope – a laser guide star AO system, which gives the Keck telescopes a resolution at least as good as the Hubble Space Telescope’s (in the infrared)!

And in 2005 the two Keck telescopes operated together, as an interferometer; yet another first.

To learn more, I suggest that you start with the official W.M. Keck Observatory website! Revolution in Telescope Design Debuts at Keck After Birth Here is a 1992 Lawrence Berkeley Lab article which captures some the excitement of those early days; and The Keck Telescopes viewed from the North puts the Keck telescopes in the Mauna Kea context.

Universe Today has covered the Keck telescopes, many times, in many different ways; for example, Keck Uses Adaptive Optics for the First Time, Binary Icy Asteroid in Jupiter’s Orbit, and New Technique Finds Farthest Supernovae.

Astronomy Cast has a couple of episodes on the Keck telescopes; check them out! The Rise of the Supertelescopes, and Adaptive Optics.

Cool – Literally – Extrasolar Planet Imaged

Yet another planet outside of our Solar System has been directly imaged, bumping the list up past ten. Given that the first visible light image of an extrasolar planet was taken a little more than a year ago, the list is growing pretty fast. The newest one, planet GJ 758 B is also the coolest directly imaged planet, measuring 600 degrees Kelvin, and it orbits a star that is much like our own Sun. GJ 758 B has a mass of between 10-40 times that of Jupiter, making it either a really big planet or a small brown dwarf.

Unlike many of the other directly imaged planets, GJ 758 B resides in a system remarkably like our own Solar System – the star at the center is Sun-like, and the orbit of the planet is at least the same distance from its star as Neptune is from our own. Current observations put the distance at 29 astronomical units.

“The discovery of GJ 758 B, an extrasolar planet or brown dwarf orbiting a star that is similar to our own sun, gives us an insight into the diversity of substellar objects that may form around Solar-type stars,” said Dr. Joseph Carson, from the Max Planck Institute for Astronomy. “This in turn helps show how our own Solar system, and the environments that are conducive to life, are just one of many scenarios that may be the outcome of planet or brown dwarf formation around Sun-like stars.”

Another object, labeled “C?” in the image above, could potentially be another companion to the star. Further observations will be required to determine whether the object in fact orbits the star or is merely another star in the background of the image which is not part of the system.

The mass of the star still has yet to be exactly determined, thus the 10-40 Jupiter mass range. It is 600 degrees Kelvin, which corresponds to 326 Celsius and 620 Fahrenheit, about the hottest temperature that a conventional oven can reach. Though this may seem hot, it’s actually pretty cool for an extrasolar planet. Even though it is so far away from its Sun that, like Neptune, it receives very little warmth from the star it orbits, GJ 758 B is in a stage of formation where the contraction of the planet due to gravity is converted into heat.

A size comparison of the GJ 758 system and corresponding members of our own Solar System, with the Earth for reference. Image Credit: Credit: MPIA/C. Thalmann
A size comparison of the GJ 758 system and corresponding members of our own Solar System, with the Earth for reference. Image Credit: Credit: MPIA/C. Thalmann

Dr. Markus Janson from the University of Toronto, a co-author of the paper announcing the imaging, said, “This is also why the mass of the companion is not well known: The measured infrared brightness could come from a 700 million year old planet of 10 Jupiter masses just as well as from a 8700 million year old companion of 40 Jupiter masses.” The paper detailing the results will be published in Astrophysical Journal Letters, but is available here on Arxiv.

The planet was imaged using the Subaru Telescope’s new High Contrast Instrument for the Subaru next generation Adaptive Optics (HiCIAO) instrument, which utilizes the technology of adaptive optics to eliminate the interference of our atmosphere that blurs images in ground-based telescopes. The imaging of GJ 758 B is part of the commissioning run of the HiCIAO instrument, which plans to take a larger survey to detect extrasolar planets and circumstellar disks in the next five years.

Source: Max-Planck Institute for Astronomy