The field-compensation principle, which has been applied to interferometric spectroscopy independently by P. Connes and by L. Mertz, allows the useful solid angle accepted by an interferometer to be increased by an amount that can be very large. This paper is concerned with a particular application of this principle using the Michelson interferometer. Although the technique is difficult to utilize where a wide range of path differences is required, the interferometer takes an extremely simple form when constructed for a narrow range of path difference about a fixed central path difference. While such an instrument has a limited use in spectroscopy, there is one type of measurement which it is admirably suited to perform: the determination of the width of a single isolated atomic line whose analytical shape is known. A description is given of the theory and construction of a wide-angle Michelson interferometer now being used for the measurement of Doppler temperatures from the width of the 5577 Å atomic oxygen line in the nightglow and aurora. This line is known to be accurately gaussian in shape, and is well-isolated from other lines, making it an ideal subject for this instrument.
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