Absolute interferometeric measurement of non rotational symmetric surface errors - a new evaluation concept

One of the basic problems of absolute interferometric surface measurement is the separation of interferometer- and surface errors. Non rotational symmetric surface errors are separated from the interferometer errors by taking measurements of the test surface at different rotational positions. The aquired surface maps are rotated back in the computer and added together, whereas the interferometer errors are averaged out. Since surface data is picked up at the locations corresponding to the pixel grid of the ccd camera, rotating the data set usually destroys the congruence of data points and pixel grid. Therefore, the rotated data field must be remapped to the pixel grid by interpolation. This interpolation causes corresponding errors. This paper presents a evaluation procedure, which is able to average out out non rotational symmetric interferometer errors without rotating the measurement maps, whereas corresponding interferometer errors are avoided.