Machining Accuracy for Large Optical Mirror using On-Machine Spherical Surface Referenced Shack-Hartmann System

A spherical surface referenced Shack-Hartmann method is studied for inspecting machining accuracy of large concave mirror. This method is so strong to the vibration environment for using as an on-machine inspection system during polishing process of large optics comparing with the interferometry. The measuring uncertainty of the system is shown as less than p-v 150 nm. On-machine measured surface profile data with this method is used for feed back control of the polishing time or depth to improve the surface profile accuracy of large concave mirror. Also, the spherical surface referenced Shack-Hartmann method is useful for measuring aspheric such as parabolic or hyperbolic surface profile, comparing that the interferometry needs a special null lens, which is to be a reference and difficult to fabricate.