Extending the accuracy and range of an interferometer through subaperture stitching

Subaperture stitching is a well-known technique for extending the effective aperture and dynamic range of phase measuring instruments. Several commercially available interferometers can automatically stitch flat surfaces, but practical solutions for stitching spherical and aspherical surfaces are inherently more complex. We have developed an interferometer workstation that can perform high-accuracy automated subaperture stitching of spheres, flats, and mild aspheres up to 280 mm in diameter. The Subaperture Stitching Interferometer (SSI®) workstation combines a six-axis precision stage system, a commercial Fizeau interferometer of 4" or 6" aperture, and a specially developed software package that automates measurement design, subaperture data acquisition, and the mathematical reconstruction of a full-aperture phase map. The stitching algorithms automatically compensates not only for positioning errors caused by the stage mechanics, but also for systematic errors such as imaging distortion and reference wave. Methods for calibrating systematic errors are discussed, and our ability to perform absolute measurements is analyzed. We present SSI stitched data, and compare stitched measurements to a calibrated full-aperture measurement.