Frank Schillke

Critical aspects of testing aspheres in interferometric setups

Several critical aspects when testing aspherical surfaces by means of interferometry are reported. The aspheres are in the range of some microns to some 100 microns to some 100 microns of deviation from best fitting spheres. The required accuracy is a few nm. A compensating system has to be used to transform the interferometers spherical wavefront to a wavefront that fits the asphere under test. Some special aspects will be discussed that are different to the test of spheres. Effects on the wavefront from lateral distortion, field curvature, and alignment of the compensating system need to be handled. The most problematic item up to now is the limited accuracy of the rotational symmetrical errors. The calibration of those errors still is much more uncertain than calibrations of unsymmetric errors or complete calibrations of spheres.

Off-axis telescopes: the future generation of Earth observation telescopes

In 1991 Carl Zeiss started a program to develop powerful telescopes for airborne and spaceborne earth-observation telescopes. This article summarizes some of the main result of this program. To emphasize the importance of these activities a short historical review was added.

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.