The established method to measure aspherical surfaces is interferometric testing with null optics, but due to economical
reasons the applications are limited. A special null optic has to be calculated, fabricated and qualified for each individual
type of asphere. This time- and money consuming method is only cost-efficient for large quantities or when tests require
high accuracy. We propose a new and flexible technique for measuring an ensemble of different aspheres with only one
measurement setup. The main idea is to use the wavelength as a tunable parameter. Because it is possible to change the
wavelength without introducing new errors by mechanical movements, the wavelength variation results in a higher
measurement flexibility without reducing the measurement accuracy.
We present the chromatic Fizeau Interferometer with a diffractive element as null-optic for the measurement of a set of
four aspheres. We will show the influence of unwanted diffraction orders and the expected measurement accuracy. As in
the monochromatic setup, especially the area around the optical axis is problematic and can not be measured with the
desired accuracy. The use of a small aperture stop on the optical axis is recommended because errors in other radial
domains are filtered as well. The results show, that the chromatic Fizeau interferometer makes the established
monochromatic method far more flexible and that different aspheres can be measured in the same setup.
The lateral resolution of an interferometer is limited mainly by the design of the optical arrangement as well as the size of the beam stop. For its characterization the MTF1,2 is not very useful. The height of a structure normal to the surface under test is transferred into a phase of a reflected wavefront. Since imaging mechanisms for intensity and phase are very different, we propose a Height Transfer Function (HTF) to describe the lateral resolution of interferometers. The HTF shows the quotient of the reconstructed and the original height of a sine-modulated surface structure as a function of the spatial frequency. The HTF can be measured with a test sample of varying periodical surface profiles and spacings. Simulations can be made using a combination of geometrical ray tracing and Fourier transformation techniques. Two different layouts of null systems for the test of an asphere are compared. A device to measure the HTF is shown along with results for a variety of different interferometers.
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 interferometer's 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.
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.
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.
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.
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 man's 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.
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.
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