A mirror has been fabricated with the shape of an off-axis section of a paraboloid by grinding and polishing a sphere into a prestressed blank. The applied stresses were then removed allowing the mirror to spring into the desired paraboloidal shape. The 36-cm diam off-axis section deviated 9.9-microm rms from the polished sphere. The final surface deviated 0.03-microm rms from the desired off-axis section.
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Nelson et al. (1980) studied this question.