Subaperture stitching interferometry enables flexibleaspheric surface measurements

Subaperture stitching is now a well established technique for improving aperture coverage, accuracy, lateral resolution, and the aspheric departure that can be measured interferometrically. The Subaperture Stitching Interferometer (SSI®), introduced in 2004, has extended both the clear- and numerical-aperture coverage of off-the-shelf interferometers and transmission optics. Subaperture interferometry also enables the resolution of denser fringe fields and smaller spatial features than conventional interferometry. We have combined this increased spatial resolution with advanced calibration techniques to facilitate the measurement of mid-frequency surface features of flat, spherical, and aspherical surfaces with extreme (i.e. well below 1 nm rms) repeatability, reproducibility or cross test "accuracy". Even more recently, we have developed the SSI-A, capable of precisely determining aspherical surface figure without the need for auxiliary CGH, refractive, or reflective null optics. This paper covers these recent advances in subaperture stitching technology.