As we saw in the last two chapters, image quality is degraded by aberrations introduced by the imaging system. If the input spherical (or flat) wavefront is not transmuted by the imaging system into a spherical converging wavefront, the point spread function size and shape departs from that of the ideal Airy pattern. The consequence is poorer resolution. Now suppose the optical system is perfect. Image quality will depend upon the sphericity (or flatness) of the input wavefront. If the input wavefront is deformed, then imagery will deteriorate. There are optical systems whose business is to measure the quality of beam wavefronts. Such systems are called wavefront sensors (WFS). They are used in two primary roles: either diagnostics, or beam clean-up. In the former role they act as information gatherers. In the latter role, the data generated is used primarily to modify the shape of an optical surface in such a way that the input warped beam wavefront is converted into an ideal output (spherical or flat) beam wavefront. This corrected beam is then suitable for use by the main optical system.
A wavefront sensor measures the shape (and irradiance distribution) of an unknown wavefront presented to it at some input aperture. This is usually done as a function of time. The wavefront sensor consists of an optical head, mechanical scanners, detectors, electronics, computer controlled data acquisition, and a sophisticated software program to fit the data, make various calculations (e.g., far-field performance), present graphical displays, and provide a data storage medium. It is an expensive instrument usually built for a particular application. It is not an off-the-shelf item. The primary fields of use are in imaging through the atmosphere, e.g., astronomy, and in high energy lasers.
The big difficulty in using a wavefront sensor in astronomy (or to examine Russian satellites) is that a very bright star must be in the field of view near the object of interest. Light from the star will pick up the wavefront error induced by the Earth's atmosphere. This light is gathered by a telescope and fed to a wavefront sensor. There must be enough power in this beam to allow decent signal to noise in the wavefront sensor measurement; otherwise, no viable correcting signal can be supplied to the deformable mirror.
If there is no bright star in the field-of-view then there are options for an artificial star. A powerful ground based laser (pulsed) is fed into the same telescope and the beam is focused on the thin sodium layer at the top of the earth's atmosphere. A small volume of the sodium emits light via resonant fluorescence scattering, thereby generating an artificial star. Light from this "star" propagates down through the atmosphere to the telescope and onto the wavefront sensor. Such work is being conducted by the Air Force at the Phillips Lab's Sandia Optical Range.
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