Flexible null test for form measurement of highly astigmatic surfaces.
We report the application of interferometry to the form measurement of a family of highly astigmatic optical surfaces. These measurements are based on a null test that employs a double-pass off-axis test arrangement with a tilted test surface and a reference sphere. This arrangement provides a perfect null test for an ellipsoid of revolution, or prolate spheroid. Its application is illustrated in detail in the presentation of results for the measurement of a specific family of eight differing surfaces that are incorporated into the K-Band Multi-Object Spectrometer Integral Field Unit. All surfaces measured here are sufficiently close to a prolate spheroid to justify its practical application. We discuss the application of the technique as a flexible low-cost approach for the generation of null interferograms in the measurement of a variety of complex surfaces.
Centre for Advanced Instrumentation, University of Durham, Sedgefield, County Durham, UK. Stephen.Rolt@dur.ac.uk
Stephen
S
Rolt
Andrew
A
Kirby