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 ∼200 μm 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 (∼λ/3) and RMS = 19.2 nm (∼λ /33). On an area with a diameter of 80%, the aberrations fell to the nanometer level: PV = 65 nm (∼λ/10) and RMS = 8.3 nm (∼λ/76).