How Perseid meteor shower helps produce amazing images of galaxies
The Perseid meteor shower and its ilk provide the sodium atoms in the middle atmosphere that light up when astronomers tickle them with a laser – a key step in adaptive optics, which are used by ground-based observatories.
The next time you leaf through one of those spectacular coffee-table books on astronomy, with their images of glowing nebulae or tightly wound spiral galaxies, don't forget to thank the Perseid meteor shower and its ilk for their contribution to those cosmic mug shots.
If those images came from ground-based observatories, it's likely the telescopes were using technology known as adaptive optics, which remove distortions that the atmosphere imparts to light from celestial objects.
What do meteors have to do with this? They provide the sodium atoms in the middle atmosphere that light up when astronomers tickle them with a laser. The tiny dot of light, invisible to humans on the ground, becomes the reference "star" that adaptive optics need to remove distortions.
The sodium is abundant. It's at the right altitude for use as artificial stars. It lights up at two closely spaced wavelengths within the yellow portion of the visible spectrum, which makes it brighter than other elements for a given amount of laser power. And those lasers are relatively inexpensive to make.
"It's a happy coincidence," says Dennis Wellnitz, a research astronomer at the University of Maryland in College Park, of the factors that have led to a technology that has revolutionized ground-based optical astronomy.
The sodium layer was first detected in 1929 by Vesto Slipher, an astronomer at the Lowell Observatory in Flagstaff, Ariz. He detected it as a weak glow, which has since become known as night glow. A decade later, researchers identified the source as meteors.
The sodium appears in a layer of the atmosphere known as the mesosphere, which starts about 31 miles up and, depending on latitude, rises to 53 miles above Earth. The sodium itself appears to be confined to the upper three miles of the mesosphere.
This is the layer where most meteors vaporize. Estimates of the amount of material entering this layer as meteors are as high as 15,000 tons a year.
The sodium atoms that meteors leave behind are energetic enough to emit a weak glow on their own. It doesn't take much to really light them up, researchers say.
Which is just fine for adaptive optics, which give ground-based telescopes abilities rivaling those of space-based telescopes.
Fuzzy blobs in images without adaptive optics become distinct points of starlight. Galaxies show far more detail. Objects that appear solitary without adaptive optics can reveal companions with the additional sharpness that adaptive optics provide.
Adaptive optics initially achieved this by taking light from a star that appeared near the object that astronomers wanted to observe and manipulating it to cancel the atmosphere's distorting effect. But bright stars aren't always so conveniently located.
Laser light tuned to excite the sodium atoms in the middle atmosphere generates an artificial star that is available wherever astronomers point their telescopes.
With a new generation of large telescopes on the drawing boards, with light-gathering primary mirrors measuring 30 to 100 meters across, the technology has started to advance – from approaches that in effect manipulate light after it passes through the telescopes to manipulations of the large mirrors that gather light or of the secondary mirrors that take the light from the main mirrors and deliver it to instrument packages bolted to the backs of the telescopes.
Small, tightly packed actuators push and pull on areas of mirror perhaps 12 to 20 inches across up to 1,000 times a second to compensate for atmospheric wiggle, Dr. Wellnitz says. Each push or pull involves movements measured in a few tens of billionths of a meter. With high-speed computers coordinating the action, the movements needed to keep the images as distortion-free as possible in effect take place as the light from a star or galaxy arrives.
This activity comes on top of the action of other actuators that ensure a mirror keeps its proper shape despite the shape-shifting influences of gravity and changing temperatures.
Although adaptive optics can give ground-based telescopes optical properties that can rival those of space-based telescopes, each has a role to play, Wellnitz suggests.
Putting a telescope into orbit is an expensive proposition. Just ask the National Aeronautics and Space Administration, which has been wrestling with the rising cost of its James Webb Space Telescope. The 6.5-meter infrared telescope is now slated for launch in 2018, even as ground-based astronomers are planning observatories with telescopes more than five times larger.
"On the ground, you can make much larger light buckets. Therefore, you can be more sensitive than space-based telescopes" to the faint light from distant objects, he says.
On the other hand, observatories are putting their ever larger telescopes in locations at high altitudes and in remote places to reduce the impact of weather or thickness of the atmosphere on observing. Many of these locations are tough to get to and work in.
In space, astronomers can get the most out of a space telescope's optics without having to go to adaptive optics. For some observations, such as those that require seeing objects via infrared light, space is the only place to be because the atmosphere is opaque to all but a few narrow bands of infrared light.
Weather, which can disrupt observations for ground-based telescopes, isn't an issue on orbit. Neither is a day-night cycle.
As ever larger telescopes are being built in ever more remote locations, usually with an eye toward reducing the thickness of the atmosphere, adaptive optics must be a key part of the system, says Chad Trujillo, who heads the adaptive optics program for the Gemini Observatory. The observatory built and maintains two eight-meter telescopes – one atop Mauna Kea in Hawaii and the other high in Chile's Atacama Desert.
"As telescopes get bigger they must look through a wider column of air," he said in a prepared statement. "The wider the column of air, the more turbulence in the air will distort the observed light."
Thanks to the sodium all those meteors leave behind, astronomers can throw that distortion in reverse.