Non-null interferometer for measurement of aspheric transmitted wavefronts

Testing in a non-null manner causes the test and reference rays in the interferometer to follow different optical paths through the system. The retrace errors generated by this difference are test dependent and must be calibrated independently for each test piece. Optical design software can be used to perform reverse optimization of the interferometer and data. An iterative reverse optimization process has been developed which eliminates weighting sensitivity and improves optimization efficiency. However, implementation of reverse optimization generates constraints on the interferometer design. These include constraints on lens parameters, system apertures, and component verification considerations. A Mach-Zehnder interferometer has been built for non-null transmitted aspheric wavefront testing. The large aspheric departures and steep wavefront slopes are detected and reconstructed using Sub-Nyquist interferometry (SNI). Experiments on several test parts were performed to verify the iterative reverse optimization process and extend the use of SNI to non-rotationally symmetric aspheric wavefronts depatures up to 200λ were characterized to λ/6 PV and λ/47 rms. The reverse optimization process was shown to remove up to 25λ of induced aberration from an aspheric measurement. The results indicate potential for application of the iterative method and its associated design constraints to routine aspheric testing.