|
Metrology has historically been one of the most formidable hurdles in fabricating large, generalized aspheres. The heart of the problem is that exotic aspheres preclude the classical technique of interferometrically testing from the center of curvature with a spherical beam, driving the need for null correctors. Although the null corrector approach is feasible, the difficulty of certification and the fact that each corrector allows the testing of only a single aspheric form make it expensive and time consuming. The problems are of course much worse for large convex optics, where the center of curvature is not available. Finally, these problems only accentuate the problem of measuring figure during the transition from grinding to polishing. In this paper we discuss a new, non-interferometric instrument currently under development. Non-interferometric techniques are usually not chosen for aspheric metrology, either because they are too inaccurate, involve mechanical contact, or cannot accommodate a large test piece. Our approach is noncontacting and self-referencing, and can be easily expanded from its broadband capacity of 0.5 meter. The instrument consists of a three- dimensional coordinate reference system and a measurement head whose location is tracked within the reference system. The reference system involves no physical reference surfaces, but instead uses an arrangement of laser beams whose positions are monitored with position sensitive detectors. The measurement head uses a new autofocus technique that allows accurate testing even before the surface is polished. Using this head, there is no loss of knowledge during the tricky transition from grinding to polishing. We present here the overall instrument concept, as well as current status and expected performance levels.
|
