Title: Adaptive Optics
Podcaster: Scott Kardel
Organization: Palomar Observatory / Caltech
Description: Recently astronomers have been clearing the air and achieving remarkable results with a technique known as adaptive optics. In this pod cast I will explain the problem that adaptive optics needs to correct, how it is done on telescope’s like Palomar’s Hale Telescope and what is in store for the future.
Bio: Scott Kardel received his MS in Astronomy from the University of Arizona and his BS in Physical Science / Secondary Education from Northern Arizona University. For the last two and a half decades he has been working to bring an understanding of science and the universe to a wide range of audiences. In 2003 he became the Palomar Observatory’s first full-time person devoted to public outreach. There he works to bring Palomar’s rich history and story of exploration on the road and on the Net to a wide variety of groups throughout Southern California and beyond.
Today’s Sponsor: This episode of “365 Days of Astronomy” is sponsored by the AAS.
Welcome to another edition of the 365 Days of Astronomy podcasts. I am Scott Kardel of the Palomar Observatory.
Just about everyone has been outside at night and noticed the twinkling of the stars. The shimmering of starlight can make for a romantic evening, but frankly it is bad for astronomy. As the light of a star or other astronomical object passes through Earth’s atmosphere, with its many turbulent layers, it becomes rapidly distorted, producing the twinkling affect. You’ve likely seen the same result, if you’ve looked down a long road on a hot day. The images of the objects seen behind and above the road are distorted and wavy.
When astronomers use a telescope the affect of turbulent air becomes magnified.
You’ve probably heard that just about every astronomer would like to have access to a bigger telescope than they are already using. It is natural to want to push the limits and pull more information out of the universe. By increasing the size of the telescope it gathers more light, allowing fainter and often more distant sources to be detected. A bigger telescope should also give astronomers a higher resolving power, allowing them to see things in even finer detail. Unfortunately for ground-based telescopes, the atmosphere rarely allows a large telescope to achieve its full potential when it comes to resolving power. A 20-inch telescope will be far behind a 200-inch telescope when it comes to gathering light, but if they are looking through the same turbulent atmosphere they may have virtually the same resolving power.
Astronomer George Ellery Hale knew this well. In 1908 he was just finishing the 60-inch telescope on Mount Wilson and was already planning a 100-inch reflecting telescope for there when he wrote this:
“It is impossible to predict the dimensions that reflectors will ultimately attain. Atmospheric disturbances, rather than mechanical or optical difficulties, seem most likely to stand in the way. But perhaps even these, by some process now unknown, may at last be swept aside. If so, the astronomer will secure results far surpassing his present expectations.“
Hale knew that the atmosphere was a limiting factor. Astronomers have worked for generations to defeat the atmosphere by first trying to locate their telescopes in sites that not only are clear most nights, but also have steady skies that allow for sharp views of the universe. But even the best observing sites on the planet still do not allow a big telescope to achieve its full potential.
One way around the atmosphere is to place telescopes into space. It is never cloudy up there and the views of the stars are rock solid. Unfortunately it is very expensive to put observatories into space and we still have a lot of good telescopes here on Earth that astronomers have access to. So is there a way around the atmosphere from here on Earth?
45 years after George Ellery Hale wrote about the “process now unknown” that might allow telescopes to achieve their full potential Horace Babcock published a seminal article proposing the use of what is known as adaptive optics.
Adaptive optics is the technique that astronomers now use on large telescopes across the globe to defeat the turbulence of the atmosphere and achieve vastly improved resolution on ground-based telescopes.
Without getting into all the technical details an adaptive optics instrument looks at the light of a star, which should look like a pinpoint or small circle. It senses how out of round the image is and in real time then creates a feedback loop to a small mirror that can bend. This bendy mirror adjusts its shape so that the light reflected off of it and to a camera produces a vastly sharper image. Any object in the sky located near this guide star also can be resolved into a much sharper view.
Right now nobody does this with their primary telescope mirror. You certainly wouldn’t want to try that with, say the Palomar mirror that is two feet thick and weighs 14 and a half tons. At Palomar, instead the light is reflected down to the telescope’s Cassegrain focus where a six-inch deformable mirror is located. This mirror has 241 actuators that each can provide a tiny push or pull to it, changing its shape up to 2,000 times per second.
Basically the whole system senses the distortions caused by the atmosphere and corrects for them continuously. This can be done for much, much less money than launching telescopes into space. Space-based telescopes of course will always have a role in modern astronomy because an adaptive optics system cannot clear a cloudy night or allow the atmosphere to transmit the types of light it usually absorbs. But adaptive optics is helping to usher in a new era of giant telescopes.
Palomar’s Hale Telescope and others have been retrofitted to use adaptive optics. The new giant telescopes that are a few years away are being designed from the beginning to make use of adaptive optics. The Thirty Meter Telescope that will soon be built by Caltech and its partners will push the limits of light gathering and resolving power giving us our deepest and sharpest view of the universe yet.
Looking backward all the way to the time of Galileo every time there has been a new, bigger, better telescope there have been discoveries that no one has anticipated. Usually the new telescopes have provided a leap forward in light-gathering power. The Thirty Meter Telescope will provide a leap forward in light-gathering power and resolving power. By having its giant segmented mirror it will gather more light than any telescope on the planet or in space and by using adaptive optics it will out resolve all telescopes on Earth and space as well. It should be quite a spectacle and no one knows what new things will be uncovered.
For the Palomar Observatory, I am Scott Kardel wishing you “clear skies”.
365 Days of Astronomy
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Thanks for your informative and very professional post. This technology is going to be used for instance in the E-ELT (European Extremely Large Telescope)
I actually have a couple of questions, which may go a bit further than the scope of your post:
– I thought Adaptive Optics systems use wavefront sensors. Your article suggest telescopes are using the star shape as a feedback in the AO loop. Is this really the case?
– It also seems that the system you describe uses a single star as a reference for the adaptive optic correction. If that’s the case, how do you correct for the turbulences that are not “in front” of the star?