Subaperture stitching is often adopted when measuring large size optic with interferometry method. To reduce the error of defocus in the process of stitching, an improved subaperture stitching method which can eliminate the high-order defocus error is presented based on the experimental study of defocused wavefront. Through detecting the wavefront difference with variable defocus, the actual defocus wavefront is proposed to replace the traditional paraboloid defocus basement so as to realize eliminating high-order defocus. A reflector sample is tested, the roughness parameter Ra is 1.252nm and 0.403nm respectively using the paraboloid and the actual decocus wavefront as the basement. The amendatory defocus basis is taken into the subaperture stitching process, for the two subaperture wavefront, different area of the actual defocus wavefront is used. A SiC super-smooth surface is measured with the improved subaperture stitching method, the Ra is 0.236nm after eliminating high-order defocus. Results show that it is a useful way to erase high-order defocus error without changing the low frequency information of test wavefront.
A 4+1 phase shifting algorithm is proposed for rotating-compensator spectroscopic ellipsometry (RCSE). The spectroscopic ellipsometric parameters are determined with five spectra, taken when the compensator is rotated at the detection angles of 0°, 45°, 90°, 135°, and an additional detection angle of 22.5°. There is no need to take the dark spectrum of the spectrometer for error correction using the new method, compared to Lee’s method  which also utilizes five spectra for RCSE. It also indicates the algorithm designed to suppress the second harmonic frequency component in traditional phase shifting algorithm is helpful to determine both fundamental and second harmonic frequency components. By taking an additional spectrum, both the two harmonic frequency components of spectra in RCSE are determined by the designed 4+1 phase shifting algorithm.