For many years I'd see listings in surplus catalogs for Mangin
mirrors, used for searchlight reflectors. Eventually, I found out
that they are. As I recall, the front and back surfaces are
spherical, because spherical surfaces are easier to make than
parabolic (especially under time constraints in wartime--WW II).
Considering that the aluminizing* is on the back, the glass acts as a
refractor because the radii of curvature differ. I'd guess that the
aberrations are too great for imaging, but small enough for
Btw, I once tried imaging with a searchlight reflector that had no
obvious problems with its figure; it made a truly pathetic image!
*Silvering probably went out around 1950 or earlier; vacuum-deposited
aluminum is much more likely.
Nicholas Bodley |@| Waltham, Mass.
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