An optical element is fabricated and inspected by forming a spherical-segment
surface in a light-transparent blank at a first end thereof with a spherical-segment
center on an element axis of the light-transparent blank. A trial parabolic surface
is formed in a lateral surface of the light-transparent blank with a trial-surface
parabolic focus coincident with the spherical segment center. The trial parabolic
surface is inspected by directing a source light beam from a light source through
the light-transparent blank parallel to the element axis and against the trial
parabolic surface, whereupon a reflected light beam reflects back substantially
parallel to the source light beam but oppositely directed to the source light beam.
The reflected light beam is optically interfered with the source light beam to
produce an interference pattern. The shape of the trial parabolic surface is corrected
responsive to observations of the interference pattern to produce a final parabolic
surface having a final parabolic focus coincident with the spherical-segment center.