Sean Goebel, a graduate student in astronomy at the University of Hawaii, has made this beautiful and fascinating time-lapse film of the observatories on Mauna Kea shooting laser beams into the night sky over the Big Island of Hawaii.
The lasers are part of the observatories’ adaptive optics systems, which compensate for distortions in light traveling through the Earth’s atmosphere. “Just as waves of heat coming off pavement blur out the detail of faraway objects,” explains Goebel on his Web site, “winds in the atmosphere blur out fine detail in the stars/galaxies/whatever is being observed. This is the reason that stars twinkle. The laser is used to track this atmospheric turbulence, and one of the mirrors in the telescope bends hundreds of times per second in order to cancel out the blurring.”
Adaptive optics make use of a guide star in the area of the sky near the object being observed. As light arriving from the guide star shifts, electronic circuits in the system automatically compute the minute adjustments to the deformable telescope mirror that are needed to cancel out the distortion.
There are, however, places in the sky where a natural guide star doesn’t exist close enough to the object astronomers want to observe. To solve this problem, the scientists create artificial guide stars using laser beams. For example, several of the observatories on Mauna Kea shine sodium laser beams into the upper atmosphere, where they interact with a naturally occurring layer of sodium atoms. The excited atoms give off light, creating a point source for the adaptive optics system to focus on. The powerful lasers must be used very carefully, says Goebel:
A typical laser pointer that you might use to point at stuff/exercise your cat is about 5 mW. That’s five one-thousandths of a watt. Not a whole lot of power. And yet it’s enough to blind airplane pilots. The lasers on the telescopes are in the range of 15-40 watts. The FAA calls a no-fly zone over the area when a laser is in use, and two people have to stand around outside in the freezing temperatures and watch for airplanes. Each of them has a kill switch to turn off the laser in case an airplane comes near. Additionally, the telescope has to send its target list to Space Command ahead of time. Space Command then tells them not to use the laser at specific times, ostensibly to avoid blinding spy satellites. However, you could calculate the spy satellite orbits if you knew where they were at specific times, so Space Command also tells the telescope to not use the laser at random times when no satellites are overhead.
Goebel captured the images for his time-lapse montage over a period of seven nights this past spring and summer. Conditions atop Mauna Kea, which rises to an altitude of over 13,000 feet above sea level, presented a challenge. Goebel had to contend with high winds, freezing temperatures and low oxygen. “Essentially everyone suffers from altitude sickness” on Mauna Kea, he says. “It’s not uncommon for tourists to step out of their vehicles and immediately pass out. Going from sea level to 14,000 feet in the span of a couple of hours will do that to you.”
For more on Goebel and his work, including technical specifications and examples of other work, visit his Web site.
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