Wolfgang Wiedmann

High-precision interferometric testing of spherical mirrors with long radius of curvature

Surface deviations of spherical mirrors from a best fitting, mathematically ideal sphere were measured to an absolute precision of 0.25 nm rms. Because of the long radius of curvature, a Hindle-type arrangement was used as interferometric setup, resulting in a test arm length of about 1.4 m. A special calibration procedure was implemented to eliminate systematic, setup-dependent errors. A very fast data acquisition technique was combined with real-time wavefront averaging to eliminate the effects of random errors, such as wavefront variations due to the turbulent atmosphere in the beam path. For the evaluation of one mirror surface, all in all 400,000 individual wavefront measurements at 400 x 400 points were combined, requiring an overall measurement time of only one to two days.

Testing large plane mirrors with the Ritchey-Common test in two angular positions

A mathematical algorithm is given and explained in detail that can be applied to a Ritchey-Common test in two angular positions to calculate deviations. The algorithm comprises a transformation form wavefront to mirror coordinates and a weighting function. Influences of the interferometers misalignments are removed by fitting appropriate functions in the mirror plane. Functionality and accuracy have been checked by simulations and experiments. As an example one of the M3-mirrors of the Very Large Telescope fabricated by Carl Zeiss is shown.

Interferometric testing of the VLT tertiary mirrors

The tertiary mirrors of the Very Large Telescope, one of the most powerful astronomical telescope systems, were manufactured and tested at Carl Zeiss. These components are lightweight elliptical plane mirrors with diameters of 1250 mm and 880 mm for the long and short axis, respectively. A particular challenge of this project was the outer rim specification of 200 nm peak-to-valley mirror surface deviation. This value had to be obtained under all operational load cases differing in the influence of gravity on the lightweight structure of the mirror. The mirror had to be tested on its support cell. For the absolute calibration of the large plane mirror surface a Ritchey- Common test was performed at two different angular positions. The test setup was adapted as close as possible to the operational position of the mirror in the telescope. A special algorithm for the calculation of the surface figure error from the wavefront data sets was developed. The results and special challenges of the absolute calibration procedure of the mirror surface will be presented and discussed.