In situ absolute surface metrology for a 600 mm aperture interferometer

•A novel absolute measurement method based on multisurface interference for a 600 mm aperture interferometer is proposed.•The proposed method is not necessary to replace the transmission flat with another flat and rotate the reflection flat about the z-axis, it just needs to rotate an auxiliary parallel flat about the z-axis.•The proposed method can realize the in situ absolute measurement of a 600 mm aperture reference flat surface error without losing the medium-frequency error.