The rotation method for the absolute testing of three flats is extended by adding a second rotation of one of the flats. This means that altogether five interferograms of pairs of flats (positional combinations) are evaluated: three basic combinations and two rotational combinations. The effect of random measuring errors is minimized by fully applying least-squares methods. Here the addition of a second rotation leads to a substantial increase of accuracy of the results and enables the lateral resolution to be further enhanced.
© 1993 Optical Society of America
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