Measurement Of Roof And Other Prisms

As an alternative to using an interferometer, Modulation Transfer Function (MTF) tests may be set up for evaluating any prism or combination of prisms. Advantages can frequently be obtained both in a reduction of the time to carry out the test and in the directly quantitative nature of the result which permits pass or reject decisions to be readily made. In the case of roof prisms performance can be affected by homogeneity of the prism material, flatness of the surfaces and accuracy of the 90 degree roof angle. Testing methods for other prisms ranging from the very weak prism, which is nominally a plane parallel window, to corner-cube retro-reflectors are described. The procedures are equally applicable to single mirrors and to fabrications of mirrors which perform the same function as prisms. The apparatus used for the MTF tests was evolved at MVEE for the testing of telescopic systems and operates at a single chosen spatial frequency scanning through all azimuths. The short focal length image analyser collimator is replaced by one of longer focal length for the tests on windows, prisms and mirrors. Where comparisons with interferometric testing are made, a Twyman-Green instrument of 140 mm working aperture has been used.