Kenichi HIBINO^1*, Makoto ITOH², and Ryohei HANAYAMA³

¹National Institute of Industrial Science and Technology, Tsukuba, Ibaraki 305-8563, Japan
²Tokyo University of Science, Shinjuku, Tokyo 162-8601, Japan
³The Graduate School for the Creation of New Photonics Industries, Hamamatsu 431-1202, Japan

(Received October 23, 2013; Accepted January 14, 2014)

Two dominant systematic errors in the absolute interferometric test of high numerical-aperture sphericals are discussed and measured experimentally in pursuit of an accuracy of λ/100. Gravitational sag deformation of a 4-inch spherical concave surface was measured in a vertical phase-shifting Fizeau interferometer. The surface shapes of two identical transmission spherical concaves were measured via the two-surface comparison method using three positional measurements. One of the surfaces was then rotated around the optical axis and the interference phases were averaged to extract a rotationally symmetric component of the aberration. The gravitational sag was then determined by the aberration component independent of the rotation. The geometrical error in the phase shift is also estimated theoretically and corrected in the experiment. Experimental results show that the both errors amount to 6 to 7 nm peak-to-valley lengths, the magnitudes of which are comparable to that of the total aberration of the spherical surface.

Key words: interferometry (120.3180), instrumentation, measurement and metrology (120.0120), absolute spherical test, Fizeau interferometer, phase shift

^*E-mail address: k.hibino@aist.go.jp