When measuring the form errors of precision optics with an interferometer, the calibration of the reference wavefront is of central importance. Ball averaging, or random ball testing, has in recent years emerged as a robust method for calibrating spherical reference wavefronts in coverging beams. We describe a simple instrument, consisting of an air bearing and two electric motors, that can rotate the test ball around three axes as required for a ball averaging test. The performance of the instrument is demonstrated by isng it to calibrate a concave transmission sphere. Further we discuss the effects of image sampling at random locations or on uniform grids, and the effect of correlated measurements due to partial image overlap. Finally, we describe a method to determine the number of measurements which are sufficient for a ball averaging calibration.
Conference Dates: July 31-August 4, 2005
Conference Location: San Diego, CA
Conference Title: SPIE Optics and Photonics 2005 Conference
Pub Type: Conferences
ball averaging test, Transmission sphere calibration