Andrew asked:
Are all the typical mirror lenses 'catadioptric' - I thought that only
applied to those Vivitar style 'solid-cat' types?
------------------------------
Catadioptric simply means a design that incorporates both mirror and
lens elements. Most mirror lenses (goes for telescopes as well) have a
fast (about f/2) spherical, primary mirror at the back end of the tube.
Such fast glass suffers greatly from coma so the coma is corrected by
the addition of a corrector plate. This is the front lens element that
you see. The corrector plate can be either thin and nearly flat
(Schmidt design) or thick with a deep convex curve (Maksutov design).
Light enters through the corrector plate, reflects off the primary
mirror and is then reflected off a secondary mirror back to the film or
eyepiece. The secondary mirror is also a magnifier and serves to turn
the f/2 primary into the final focal ratio of the lens (say f/5.6 or
f/8). In the Schmidt design the secondary mirror is mounted in a holder
on the back side of the corrector plate via holes drilled through the
corrector plate. The Maksutov design sometimes does this as well but,
more commonly, the secondary mirror is designed such that its curvature
is the same as the curvature on the back side of the corrector plate.
The secondary mirror is then simply an aluminized spot on the back side
of the corrector lens.
I've never examined a 'solid-cat' but suppose that the entire lens is a
solid cylinder of glass with the primary and secondary mirrors being
aluminized coatings at either end of the cylinder. Don't know if the
cylinder is glued up from more than one piece of glass or not.
Most camera lenses are of the Maksutov design. A Masksutov corrector is
easy to make in the less than 100mm diameter required for a 500mm
f/5.6. Above 100mm diameter they start to get expensive very fast due
to the difficulty of casting a large, thick lens blank that is free of
bubbles. This is also part of the reason why large optics use mirrors.
Mirror blanks do not have to be free of bubbles except near the front
surface where a bubble might be opened up by grinding in the curve.
Someone commented the other day about severe vignetting from a Celestron
(whose focal length I don't recall). Celestron makes mostly telescopes
with Schmidt design optics. (Back in the 60's they made a manufacturing
breakthrough and figured out how to automate the grinding and polishing
of the complex, shallow curve in a Schmidt corrector plate.) It may be
that the lens in question was meant primarily as a telescope and
secondarily as a camera lens. As a telescope you only need to avoid
vignetting across a diameter of 25mm or even less. This allows for a
smaller secondary mirror and less obstruction. Fully lighting the full
43mm image circle of the 35mm film diagonal would take a large
secondary.
Chuck Norcutt
Woburn, Massachusetts, USA
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