Early on in laser-beam-weapons development during the 1980s, it was apparent that changes in atmospheric density could play havoc with any collimated energy beam. The resultant adaptive-optics (AO) technology to correct these effects using computer-controlled, fast-acting deformable mirrors is now paying benefits in astronomical observation. Ground-based telescopes can now adjust for atmospheric effects, sharpening images many times over.
AO is a wavefront sensor measuring incoming-light wavefront distortions in real time. Most common is the Hartmann-Shack sensor, as supplied by United Technologies' Adaptive Optics Associates (AOA, Cambridge, MA). The first commercially-contracted system, ALFA (Adaptive optics with Laser For Astronomy), a joint project between AOA and the Max Planck Institut (Heidelberg, Germany), is now in use on the Calar Alto telescope (Almeria, Spain).
The Hartmann sensor is completely geometric and not dependent on coherence of the sensed optical beam. A 2-D array of small lenses, or lenslets, breaks up the incoming wavefront into an array of spatial samples called subapertures. Each subaperture is focused at a fixed distance behind the array. The lateral position of each focal spot depends on the local tilt of the incoming wavefront. Measuring all subaperture spot positions determines the incoming wavefront gradient. Reconstruction methods then estimate the shape of the original wavefront and thus the correction signals needed for the deformable mirror. AOA has developed a WaveLab(TM) software package to analyze and display sensor and wavefront data.
In an adaptive optics system, light from a point source in the field-of-view, a bright star or "artificial" laser spot on the iono-sphere, enters a primary telescope aperture. This light is split between a camera and the wavefront sensor. The sensor measures the wavefront distortion due to density effects and also image position. Correction signals are sent to a de-formable mirror to correct for turbulence and a fast-acting steering mirror to stabilize the image in jitter. AOA also makes its own mirror actuators or uses those from Xinetics (Littleton, MA).
"ALFA is only the second closed-loop, adaptive-optics system with a laser guide star in the world," according to Clive Standley, AOA principal scientist. The other was a university development project that took many years to bring about. With ALFA being commercially contracted, "we thought about the adaptive optics and all the astronomers had to think about was the specs for the system. It's more of a turnkey system where they don't have to worry about development."
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