The compensation of Fabry-Perot plate surface defects by the deposition of a dielectric film of appropriate thickness profile is discussed. The method of depositing a film over the reflecting coating on an etalon plate is often unsatisfactory, because the optical thickness of the etalon is insensitive to change in thickness of the dielectric film when the reflectance is a maximum. A dielectric film deposited before a reflecting coating produces a change in the optical thickness of the etalon equal to the thickness of the film and barely changes the reflectance. A method for depositing dielectric layers of suitable thickness profile is described. A slotted mask, prepared from an interferogram recording the surface defects, is used to produce an appropriate attenuation of evaporated dielectric material.
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