Bernd Doerband

Software concept for the new Zeiss interferometer

The new Zeiss interferometer works with a complex software package which can be run in two different modes : a workshop mode for simple pushbutton operation of standard measurement sequences, and a master mode well suited for the use in the laboratory with changing requirements for measurement and evaluation methods. Macros of arbitrary functional sequences can be created in the master mode and associated with function keys for the workshop mode. A description of the implemented features concerning data acquisition and display, control of the instrument as well as data evaluation and manipulation is given.

Aspheric optics at different quality levels and functional need

The design of optical systems gains profit from the application of aspheric elements: to correct for image aberrations, to reduce weight or system length or to cope with other functional needs like the number of reflections or the field size. Manufacturing, however, is suffering in practice from problems resulting from the reduced symmetry of the components as compared to spherical optics. Almost all large optical companies in the world are working on solving the problem of fabrication of accurate aspheric elements at reasonable cost. In this paper fabrication and metrology aspects of aspherical elements in the IR, visible, UV and EUV at Carl Zeiss and other companies will be discussed. The specifications achieved presently are ranging between some micrometers to nm for figure accuracy and 100nm to 0.1nm for microroughness.

High-precision testing of optical components

High quality lenses have reached a standarad where wavefront tolerances of a few nanometers for transmission and surface figure of individual components are necessary. The standard measuring tool in production is digital interferometry. To satisfy the demands of current and future production quality interferometruc equipment has to provide an overall accuracy below one nanometer under fabrication environment. This is reasonable, especially if work mans rule is applied demanding a tolerance/accuracy factor for metrology of ≥10:1. In this paper the limitations on interferometry for material and surface inspection as well as possible improvements are discussed.

Evaluation of rotational symmetric surface deviations by means of average radial profile

In ISO 10110-5 the rotational symmetric surface irregularity of an optical surface is determined form a set of rotational symmetric Zernike polynomials. For a large variety of aspheric optical surfaces which are manufactured by means of CNC-controlled machines this method leads to inadequate results. CNC-controlled fabrication methods often cause ring-shaped deviations described precisely by calculating the integrals over circles concentric to the axis of rotation. The final result is called average radial profile and shows the cross section over the typical ring- shaped deformations.

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.