Dynamic null lens for aspheric testing using a membrane mirror

Flexible aspheric surface testing is still a major topic in optical industry. Interferometric null testing of aspheric surfaces requires a compensator element that adapts the spherical wave of the interferometer to the individual surface under test. In this paper we present a new approach for testing aspheric surfaces that avoids static, individually manufactured null lenses: an active null lens that is based on a computer controlled membrane mirror. It can be adapted to different aspheric shapes, thus providing much more flexibility in optical shop testing. In this paper we are giving some details on the characteristics of the 25 min membrane mirror developed at TU Delft and introduce the setup of the active null lens test system. Measurements and the calibration algorithm of the setup are presented. (C) 2004 Elsevier B.V. All rights reserved.