Wavefront lens corrector for studying flat surfaces

The method of manufacturing and the results of studies of a lens corrector that converts a spherical diverging front into a plane one and is intended for studying flat surfaces as part of an interferometer with a diffraction comparison wave is described. A feature of the corrector is the use of an aspherical convex surface with a maximum deviation of $\sim$200 $\mu$m from the nearest sphere. The first experimental results are presented, indicating the prospects for using ion-beam processing to improve the quality of the wavefront. After the procedure of ion-beam processing, the aberrations over the entire aperture of the corrector decreased by more than 4 times and amounted to the parameter of the height difference PV = 207 nm ($\sim\lambda$/3) and RMS = 19.2 nm ($\sim\lambda$ /33). On an area with a diameter of 80%, the aberrations fell to the nanometer level: PV = 65 nm ($\sim\lambda$/10) and RMS = 8.3 nm ($\sim\lambda$/76).