A SELF-ADJUSTING TWYMAN-GREEN INTERFEROMETER FOR AN EXTENDED IMAGE FIELD

The quantitative determination of wave aberrations for high-quality im aging systems for off-axis use is an important challenge for modern op tical measurement techniques. Up to now sufficiently accurate faciliti es for measuring the wave aberrations of such imaging systems have not been available. This is especially true for systems with almost perfe ct imaging capabilities for an extended image height (e. g. photolitho graphic lenses). In the past the alignment of Twyman-Green interferome ters in an extended image field was delicate because the image point h ad to be brought to coincidence with the centre of the sphere with ext reme precision. A self-adjusting set-up was therefore developed, which determines the position of the image point in the image space and the n brings the centre of the sphere to this position with high accuracy. To realise this, a CCD-camera (with premagnifying microscope objectiv e) and a sphere were mounted, rigidly fixed to each other on a motoris ed xyz-stage. This unit was then calibrated by measuring the coordinat e differences between the centre of the sphere and the object point of the camera. The determination of the focus of the lens under test is then carried out on the basis of well defined mathematical calculation s and algorithms without any influence of the operator. Hereby a repro ducible and definitely quantitative alignment of the Twyman-Green set- up is achieved. The registration and analysis of the Twyman-Green inte rferograms is subsequently carried out with the help of a commercial i nterferometer unit. Measurement re suits for wave aberrations of a cal ibration lens at different image angles are presented.