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Detecting the effects of the atmosphere on a wavefront of light requires an instrument that both senses and measures variations in the phase or optical path length differences of the optical waves. Electronic imagers, film cameras, and human eyes all record the intensity of light falling on them, so phase variations must be transformed into intensity changes to be recorded. A wavefront sensor, then, is a tool for measuring differences in the optical path length over a wavefront by transforming the differences into intensity variations that can be recorded. Wavefront sensors are divided into two classes, direct and indirect (Geary 1995). Direct wavefront sensors measure the phase variation of the aberration in the wavefront. Two members of this class of sensor are the radial shearing and point diffraction interferometers. Indirect wavefront sensors measure the local tilt in the wavefront, or the differential wavefront. Examples of differential wavefront sensors are knife-edge tests, Shack-Hartmann sensors, and Shearing interferometers. This chapter explores wavefront sensors from a general perspective and then focuses on detecting the lowest-order modes important to image stabilization and beam steering. Wavefront sensors provide the first step in compensating for the aberrations present in the wavefront, i.e., detection of the aberrations.
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