Artificial Stars at Palomar
In a planetarium, artificial stars are projected onto a domed ceiling to give the audience the illusion of being out under the night sky. Astronomers at Palomar recently made an artificial star of their own in the real sky.
To make this artificial star, a laser was broadcast into the sky. This was the first step in creating a laser-guide star for use with a technique known as adaptive optics. Adaptive optics allows astronomers to correct for the fuzzy images produced by Earth’s moving atmosphere, giving them a view that often surpasses that of smaller space-based telescopes.
The Palomar Observatory currently employs the world’s fastest astronomical adaptive-optics system on its 200-inch Hale Telescope. The system is able to correct for changes in the atmosphere 2,000 times per second. Astronomers from Caltech, JPL, and Cornell University have exploited this system to discover brown dwarf companions to stars, to study the weather on a moon of Saturn, and to see the shapes of asteroids.
Currently, astronomers at Palomar can use the adaptive-optics technique only if a moderately bright star is sufficiently close to their object of interest. The adaptive-optics system needs a star to monitor and correct for the distortions produced by Earth’s atmosphere. Natural stars of sufficient brightness are rare enough that the technique can be used for only 1 percent of the sky.
Employing the laser will allow astronomers to place an artificial corrective guide-star wherever they see fit. To do so, they shine a narrow sodium laser beam up through the atmosphere. At an altitude of about 60 miles, the laser beam interacts with a small amount of naturally occurring sodium gas, making it glow. The reflected light from the glowing gas serves as the artificial guide-star for the adaptive-optics system. The laser beam is too faint to be seen except by observers very close to the telescope, and the guide-star it creates is even fainter. It can’t be seen with the unaided eye, yet it is bright enough to allow astronomers to make their adaptive-optics corrections. It uses the same sodium wavelength that we recommend for city street lights to minimize glare and maintain dark astronomy-friendly skies.
The new program will place Palomar in elite company as only the third observatory worldwide to deploy a laser-guide system. This laser will greatly expand the science performed at Palomar and pave the way for future projects on telescopes that have not yet been built.
”This is a terrific technical achievement, which not only opens up a bold and exciting scientific future for the venerable 200-inch telescope, but also demonstrates the next step on a path toward future large telescopes such as the Thirty Meter Telescope, “ says Richard Ellis, Steele Family Professor of Astronomy and director of the Caltech Optical Observatories. “The next generation of large telescopes requires sodium laser guide-star adaptive-optics of the type being demonstrated at Palomar Observatory,” he adds.
The Palomar adaptive-optics instrument was built and continues to be supported by the Jet Propulsion Laboratory as part of a Caltech—JPL collaboration.
Support for the adaptive-optics research at Caltech’s Palomar Observatory comes from the Gordon and Betty Moore Foundation, the Oschin Family Foundation, and the National Science Foundation Center for Adaptive Optics.
More information on the Palomar Adaptive-Optics Program can be found at: