A method is described for obtaining four simultaneous phase- stepped images from a Michelson interferometer in order to measure the Doppler shift of a spectral line. One of the Michelson mirrors is divided into quadrants and each quadrant coated separately, so the path difference varies by about (lambda) /4 from one quadrant to another. An image of the mirrors is formed outside the interferometer, where the light from the quadrants is diverted in different directions, and four separate images of the field of view are formed, one for each quadrant. For a given direction in the field of view, the fringe is sampled at four points on the interferogram separated by (lambda) /4 and from these four intensities, the phase of the fringe is calculated. Doppler shifts of the spectral line are seen as changes in the phase of the fringe. In earlier versions of the imaging Doppler Michelson technique, the sampling was done in sequence. Simultaneous sampling eliminates the errors caused by intensity variations during the measurement, making the technique useful for rapidly varying sources such as aurora.
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