Hubble Space Telescope optics: problems and solutions

The Hubble Space Telescope suffers from spherical aberration caused by a spacing error in the null lens used to fabricate and test the primary mirror. Corrective optics must be installed on-orbit in order to enable many key programs. An analysis of imaging data obtained on-orbit gives the same results as measurements on the faulty ground test apparatus, so such optics can be designed with confidence. The assumed conic constant on the primary mirror for all the corrective optics, -1.0139(5), is consistent with measurements by four major independent methods. Aligning the new optics will be very demanding, because of the large slope of the wavefront to be corrected. If the images are to be diffraction limited, the pupil at the corrective element must be aligned to better than one percent of its diameter. Some other residual effects of the spherical aberration will remain after installation of the corrective optics, primarily in the pointing and collimation of the telescope.