Hubble Space Telescope primary-mirror characterization by measurement of the reflective null corrector

The reflective null corrector used to manufacture of the Hubble Space Telescope contains valuable information about the prescription of the primary mirror since an excellent null was achieved between the null-corrector wave front and the primary-mirror wave front. During the Phase I measurements, the leading cause of the spherical aberration, the field lens position error, was discovered and remeasured to an accuracy of +/-0.005 mm. To derive the conic constant of the primary mirror to an accuracy of +/-0.0003, we remeasured the parameters of the reflective null corrector that could contribute to the spherical aberration of the primary mirror.