Optical designers are becoming increasingly aware of the importance of specifying and tolerancing slope errors on optical surfaces, especially aspheric surfaces. Slope errors can degrade system optical performance -in some cases even if the peak to valley surface figure errors meets the optical design tolerance analysis. With this awareness, more optical engineers are putting requirements for peak surface slope on optical element drawings. This puts pressure on optical fabricators to understand slope specifications and react to these requirements, and use the appropriate metrology instrumentation to ensure final system performance. This paper will discuss appropriate ways to specify slope errors, and the challenges and limitations of measuring slope errors with commercial interferometers. The optical designer should be aware of how slope errors are measured on Fizeau interferometers and should specify the spatial intervals of interest when tolerancing aspheric elements. Peak slope error measurement is prone to erroneous measurement errors due to surface contamination, environmental errors, and pupil focus. Finally, filtering has a strong influence on surface slope calculations. Practical examples of slope specifications and experimental results will be presented.
With the advent of advanced, automated fabrication techniques, the limiting factor in the production of complex aspheric space optics is increasing the ability to accurately measure and characterize optical surface parameters. In addition to increased accuracy, the metrology is required to keep pace with the shorter fabrication cycles of advanced figuring techniques such as computer-controlled polishing. As a result, there is an emphasis on:
1) Integrated testing approaches. Instruments such as interferometers must be interfaced with the polishing machine and the performance prediction software in order to minimize the time required to perform a full metrology cycle.
2) Redundancy in testing. Each surface parameter is measured in two independent, complementary test methods using unique test equipment. This minimizes the risk of systematic errors and reduces measurement uncertainty.
3) Overlapping measurement bandwidths. Test methods used to evaluate the surface errors are selected to ensure all spatial periods are adequately sampled and a complete characterization of the mirror surface is obtained.
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