Date: March 9, 2010
Title: Adaptive Optics Saves Earth
Podcaster: Rob Sparks
Description: Astronomers use adaptive optics to overcome the limitation of Earth’s atmosphere. What about the captain of a starship?
Bio: Rob Sparks is a Science Education Specialist at the National Optical Astronomy Observatory. A lifelong astronomy enthusiast, he earned a B.A. in physics at Grinnell College and his M.S. at Michigan State University. He taught high school physics, math and astronomy for 11 years at schools on St. Croix, Florida and Wisconsin. He spent the 2001-2002 school year working on the Sloan Digital Sky Survey as a recipient of the Fermilab Teacher Fellowship. He spent the summer of 2003 at the National Radio Astronomy Observatory as part of the Research Experience for teachers. He has been working as a NASA Astrophysics Ambassador since 2002. He is a member of the Galileoscope Working Group for the International Year of Astronomy.
David Montgomery is a junior at the University of Arizona in Tucson, Arizona. He is currently working on a B.A. and B.S. in East Asian Studies and Optical Sciences and Engineering, respectively. In addition, he is employed with the National Optical Astronomy Observatory as an Education Outreach Student, and has worked on promoting Dark Skies and optical/astronomy related material to all audiences. His hobbies include music, coffee, and traveling.
Patrick Nepsky is a currently sophomore at the University of Arizona, where he is pursuing a B.S. in Chemical Engineering and in Mathematics. He has had a strong interest in astronomy and optics since he was young. During the 2007-2008 school year, he worked at Lowell Observatory in Flagstaff Arizona where he operated telescopes for public viewing. Now, he works at the National Optical Astronomy Observatory in Tucson, Arizona as an Education Outreach Student who teaches the public about basic scientific phenomenon.
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Sound Effect: Dramatic music, spaceship sound
Ensign: Captian, a Zarquon space cruiser has just entered orbit around the Earth.
Captain: Ensign, scan the ship.
Ensign: Captian, the Zarquon cruiser just fired its missiles at us!
Capatain: Prepare to fire the anti-missile laser!
Ensign: But captain, it won’t….
Captain: Just do it!
Ensign: Yes sir! Lasers charging….ready to fire at your mark.
Sound effect: Laser firing
Ensign: Captain, the laser is ineffective. The missiles are still coming toward us.
Captain: I don’t understand…these lasers worked flawless at the Battle of the Kuiper Belt.
Ensign: Yes, but we were out in space then. As I was trying to tell you, we are firing a laser through Earth’s atmosphere. We need to use the laser outfitted with adaptive optics.
Captain: What is this adaptive optics you are talking about?
Ensign: Hang on, I am going to save our lives first. Charging adaptive optics equipped laser…laser charged.
Captain: What are you waiting for?
Ensign: Orders to fire.
Captain: Oh, yeah. Fire!
Ensign: Laser firing.
Sound effect: Laser firing, explosions
Ensign: Captain, all incoming missiles have been destroyed. The Zarqoun cruiser is retreating.
Sound effect: Crowd cheering.
Captain: Okay, so how did you do that? Why didn’t our main laser work?
Ensign: Well, you see captain. The Battle of the Kuiper Belt took place in space, right?
Captain: Of course.
Ensign: In space, laser beams can travel great distances without being disrupted because there is no atmosphere.
Captain: Atmosphere? What are you going on about now?
Ensign: Okay, let me try to explain it this way. Have you ever looked up at the stars at night on Earth? What do you see?
Captain: A beautiful sky full of twinkling stars!
Ensign: And when you look at the stars from your cabin on the starship USS Kelvin, what do you see?
Captain: The stars don’t twinkle in space. That’s why we put telescopes in space like the famous Hubble Space Telescope.
Ensign: And that is why our laser didn’t work.
Captain: I don’t follow.
Ensign: Okay, stars don’t twinkle in space because there is no atmosphere. Starlight can travel thousands of light years and reach us totally undisturbed….until it hits Earth’s atmosphere. Earth’s atmosphere is turbulent and constantly moving. This turbulence distorts the starlight and causes stars to twinkle.
Captain: You mean that the starlight travels for trillions of trillions of miles and then gets distorted in the last second of its journey and that’s why stars twinkle?
Ensign: Yes, Captain. And that also causes the problem with our laser.
Captain: How so?
Ensign: When we fire our laser, it starts out as a nice tight beam of light. When we fire it from the surface of the Earth, the laser becomes distorted and the light spreads out as it passes through Earth’s atmosphere.
Captain: Just like the stars twinkling!
Ensign: Yes. And by the time the laser gets to the missile, the energy is spread out over such a large area, it cannot destroy the missile.
Captain: So how did this, what did you call it, “Adaptive optics” laser work?
Ensign: Well, I use a special system with a flexible mirror to distort the laser beam.
Captain: But I thought we wanted the beam NOT to be distorted.
Ensign: Here’s the clever part. If we distort the laser beam just right, Earth’s atmosphere will focus it on the missile for us.
Captain: Like a giant lens! That’s brilliant! But how do you know the proper shape for the beam in the first place?
Ensign: We have to measure the distortion caused by Earth’s atmosphere. We can do this by measuring how a bright star twinkles using a wavefront sensor, also known as a Shack-Hartmann Sensor.
Captain: What’s a Shack-Hartmann Sensor?
Ensign: Here, let me show you a model of one.
Captain: That just looks like a large board with a bunch of holes with lenses in them.
Ensign: Yes, this is an array of lenses. Each of these lenses focuses an image of the star on a charge coupled device, or CCD chip. A CCD chip is what they use in digital camera. If Earth had no atmosphere, the image of each star should be right in the middle of each lens. However, Earth’s atmosphere makes those images move around. By measuring how those images move, we can calculate how Earth’s atmosphere is distorting the image.
Captain: But stars are continuously twinkling. Does that mean you have to keep measuring and changing the shape of the laser?
Ensign: Yes, Captain. We have to measure the distortions and change the shape of the laser hundreds of times a second. Therefore, we have to take very short exposures so we need a very bright star.
Captain: But what if there is not a bright star near the missile?
Ensign: We have that covered Captain. We have a special laser here we can shoot into the atmosphere. It excites sodium atoms in Earth’s upper atmosphere and creates an artificial star. We can measure the distortions caused by Earth’s atmosphere using this artificial star.
Captain: I hope this technology is secret!
Ensign: I am afraid not, Captain. Adaptive optics has lots of applications. Astronomers use it to remove distortions caused by Earth’s atmosphere in their images. We can get images from telescopes on the ground almost as good as those from the Hubble Space Telescope.
Eye doctors use adaptive optics technology as well. They can use it to calculate your prescription or to assist with LASIK eye surgery. Some people end up with better than 20/20 vision.
Captain: That’s amazing Ensign. Where did you learn all this?
Ensign: Starfleet Academy of course.
Narrator: Adaptive optics (or “AO”) technology plays a major role in modern astronomy. Adaptive optics systems are operating on telescopes including the Keck Telescopes, the Gemini Telescopes, the Palomar Telescope and many others.
Earth’s atmosphere limits our ability to see fine detail in astronomical images. Astronomers call this “seeing”. Seeing is measured in units called arc seconds. One arc second is 1/3600th of a degree. On a good night at a mountaintop observatory, the seeing is about 1 arc second (exceptionally good nights can be a little better). Seeing is frequently significantly worse than one arc second, especially if you are near sea level or have particularly turbulent atmosphere due to weather conditions.
If two stars are closer than 1 arc second apart, the telescope sees only one object. No matter how large you make your telescope, the atmosphere will prevent you from seeing two very closely spaced objects.
Astronomers use adaptive optics to overcome the limitation of Earth’s atmosphere. We can also overcome the effects of Earth’s atmosphere by launching telescopes into space. However, AO systems have several advantages. It is expensive to launch telescopes into space and the size of the telescope you can launch is limited by current rocket technology. The largest mirror launched into space is the 3.5 meter mirror on the Herschel Space Telescopes, which is relatively small by current standards. The largest space telescope in development is the James Webb Space Telescope which will have a 6.5 meter mirror. Both of these telescopes are smaller than the largest ground based telescopes.
Space telescopes are also very expensive to build and launch. You can build a ground based telescope equipped with adaptive optics for less money.
Finally, space based telescopes are difficult (if not impossible) to service. Astronauts have serviced the Hubble Space Telescope several times, but the newer space telescopes are being launched into orbits that the Space Shuttle cannot reach so they can never be serviced or repaired. Ground based telescopes can be repaired and have their instruments upgraded as needed..
The Gemini Planet Imager, or GPI, is a good example of a state of the art AO system. The GPI will have a deformable mirror with 4000 actuators capable of changing its shape 2,500 times per second with an accuracy of one nanometer (one billionth of a meter). The GPI will be able to detect planets orbiting other stars up to 150 light years distant. It is scheduled to begin observations in 2011.
The next generation of large telescopes is being designed to use adaptive optics systems. These telescopes include the Giant Magellan Telescope, the Thirty Meter Telescope, and the European Extremely Large Telescope. The resolution of these telescopes will be limited by their size, not Earth’s atmosphere.
The next generation of large telescopes will collect immense amounts of light and be able to reveal fainter and more distant objects than we have ever seen before. Adaptive optics will allow us to see much smaller details in the images and directly image planets orbiting nearby stars.
For more information on adaptive optics, I will have links on my blog at halfastro.wordpress.com.
This is Rob Sparks and I would like to thank my cast: David Montgomery as the Ensein and Patrick Nepsky as the Captain. Thanks for listening to the 365 Days of Astronomy podcast.
End of podcast:
365 Days of Astronomy
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