Very Stable Common-Path Interferometers and Applications

Common-path interferometers are subject to disturbance of the fringe pattern by certain types of vibration. A type of interferometer has been devised to overcome this by imaging the principal plane of the optics under test back onto itself with inversion. This, although removing most of the effects of vibration, restricts the amount of information obtainable, as asymmetric errors are seen as symmetric ones.One type of interferometer embodying this principle, using a Wollaston prism to give image shearing, is described, with an account of the relationship between surface errors and observed path differences when used to test a concave mirror. Methods of fringe control are outlined. Several other applications are described, and the sphere of usefulness of the principle in optical testing is outlined.