In this paper several interferometer absolute testing methods to measure flat surfaces are compared. The main objective was to name a value for the achievable accuracy. Therefor four different methods were analyzed:
- 1. The three flat test, a method which is already used several decades to determine the quality of a flat surface. As a result, two absolute measured profiles, horizontal and vertical, can be calculated.
2. The multi rotation test, an extension of the classical three flat test. The big advantage of this method is a fully three dimensional map of the systematical error.
3. The systematical error calculated by the SSI-A. Hereby several subapertures are measured over the whole surface. The redundant information’s of the overlapping regions can be used to calculate the systematical error of the system.
4. The rotation of the transmission flat relatively to the interferometer. Thereby the rotation unsymmetrical errors can be calculated and subtracted.