William asked:
What is this reflex mirror lens? And what is the term Catatrioptic mean?
----------------------
Reflex simply means bending or folding the light path via reflection.
In a single lens reflex camera the light is bent away from the film
plane 90 degrees by means of a mirror and (optionally) a pentaprism.
The pentaprism's job is to turn the image another 90 degrees while at
the same time maintaining the image right side up and correct right to
left.
In a reflex mirror lens/telescope the light path is folded back on
itself by using two mirrors. The main mirror (the primary) is at the
back of the lens. It gathers the light and reflects and focuses the
light back toward the front of the lens to the secondary (small)
mirror. The secondary mirror magnifies (usually about 5X) the image
from the primary and reflects the image back to the rear of the lens
(usually) through a hole in the primary mirror.
So far, what I've described is called a Cassegrain system... a concave
primary mirror with a convex secondary mirror. Now we add the term
"catadioptric". A catadioptric instrument is one which combines
mirror(s) and refractive lens(es). A refracting lens is one where the
light is refracted (bent) by passing through the glass. A catadioptric
Cassegrain consists of a primary and secondary mirror with the addition
of a correcting refracting lens at the front. The correcting lens can
be designed to correct for many different optical abberations of the
uncorrected Cassegain (such as coma) but it also allows the designer to
use a spherical primary mirror. A spherical primary is cheaper to
manufacture than an aspherical design but would perform poorly by
itself. The correcting lens can be one of three designs (that I'm
familiar with)... Maksutov, Schmidt or Houghton. The Houghton design is
a two element lens and is probably only familiar to telescope
designers. The Masksutov and Schmidt designs are used in telephoto
lenses as well as telescopes.
The Maksutov uses a thick meniscus shaped lens and is usually designed
such that the back of the corrector lens and the secondary mirror have
the same curvature. Thus, the secondary mirror is nothing but an
aluminized spot on the back of the corrector lens and can never get out
of collimation (optical alignment). This is good for camera lenses
which tend to get knocked around a bit.
The Schmidt design uses a very thin lens which is flat on the back side
but has a curved front surface that looks a bit like the side view of a
sombrero. It would be difficult to see the curve though since it's only
a few thousandth's of an inch deep. The secondary mirror on a
Schmidt-Cassegrain is a separate convex mirror which is mounted through
a hole in the corrector plate and has adjusting screws for collimation.
Schmidt corrector plates used to be very difficult to manufacture and
were rare until about 1970. About that time Celestron figured out how
to grind and polish them much more cheaply than had been the case and is
now the principal purveyor of Schmidt-Cassegrain telescopes and
telephoto lenses.
For telephoto lenses the Maksutov is the most common design up to about
90mm diameter since the thick meniscus lens is easy to manufacture in
smaller sizes using conventional grinding and polishing equipment.
Above 90mm diameter one is usually talking about telescopes which are
optionally usable as telephoto lenses. Here, the Schmidt design is more
common due to manufacturing cost but Maksutov's are available up to at
least 175mm diameter (at very high cost).
The most common 35mm reflex lens is a 500mm f/8. At f/8 the aperture is
62.5mm and will almost certainly be a Maksutov design based on
ruggedness and manufacturing cost. I have a 500mm f/5.6 which is a 90mm
aperture. It's also a Maksutov. Note that the downside of a fast
Cassegrain is poor contrast. The faster the lens the larger must be the
secondary mirror to avoid severe vignetting. Unfortunately, the large
secondary also causes a lot of diffraction which reduces image
contrast. Can't have your cake and eat it too. Hope this answers the
question that you asked plus some of the possible follow-ons.
Chuck Norcutt
Woburn, Massachusetts, USA
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