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Covering the Sky
A natural guide star travels through Earth's atmosphere only once, while the laser guide star must traverse it twice, up and back. Atmospheric effects cause the artificial guide star to appear larger than a comparable natural guide star, making it somewhat less accurate as a reference point. A natural guide star is about two times more effective as a wavefront reference than the laser guide star, although engineer Don Gavel and others at Livermore are working to make the laser spot smaller to improve its performance.
But the laser guide star makes up for its deficiencies by offering a significant improvement in sky coverage. This is because the laser can be pointed anywhere in the sky to make a bright artificial star.
Making Stars Bright
In the adaptive optics system, a wavefront sensor and a deformable mirror make corrections to light beaming into the telescope (Figure 4). The system must work quickly because the atmosphere between the telescope and the heavens typically blows by at about 10 meters per second, requiring a correction every few milliseconds.
A tilt-tip mirror, with its own sensor and camera, makes the initial correction to the incoming beam of light to stop it from dancing. Then the beam travels to a deformable mirror where the shape of the incoming wavefront is determined. A Shack-Hartmann wavefront sensor, which at Lick has 40 subapertures, examines part of the shape of the incoming wavefront. The sensor measures the difference between the actual shape and a perfect, flat wavefront. The measurements go into a computer that directs the activities of the deformable mirror. As the atmosphere blows by, the wavefront sensor and deformable mirror are in constant communication, searching for errors and correcting them.
The mirror at Lick has 127 electro-strictive actuators-each a tiny piston-arranged in a triangular pattern. Each actuator can raise or lower a part of the mirror surface by as much as 4 micrometers to straighten out the incoming light and make it all travel in the same direction. (This mirror is much like other deformable mirrors that Livermore has developed to correct wavefront aberrations in laser light in the 192-beam National Ignition Facility.)
The goal for astronomical adaptive optics is to flatten the guide star's incoming wavefront to achieve as perfect and hence as bright an image as possible. Once the system operators are satisfied with the image they have achieved for the guide star, the imaging camera, also known as the science camera, is turned on to gather data on the celestial object of interest.
Keck has a larger adaptive optics system than Lick because its telescopes are much larger. At Keck, the wavefront sensor has 241 subapertures and the deformable mirror has 349 actuators arranged in a square pattern. |