P3D Re: Mirror Lenses

[Brian Reynolds <reynolds@xxxxxxxxx>]

Larry wrote:
>
> Does anyone have experience with long focal length lenses that use
> mirrors ratherthan solid glass?  I notice 500 mm mirror lenses costing
> about $100 each being sold new.  They are lighter and smaller in
> diameter and in those regards would be very good for paired lens
> stereo.  Offhand it would seem that the reflected light of the mirror
> might reduce the available light, but I really do not know. Some of  the
> store people I have talked to say they are not as sharp as regular glass
> lenses, and yet there are those who say that if the lens is used at f 8
> - 22, they are just as sharp as glass lenses used at the same openings.
> I am interested in the experience of anyone who has used them.
> 

I have an Adorama Pro-Optics 500mm f/5.6 Maksutov (the optical
design).  The Pro-Optics Maksutov is made by the same factory that
makes Celestron's 500mm f/5.6 Maksutov.  Celestron imports it, they
don't actually manufacture it themselves.  I originally bought it to
be a long telephoto for my Pentax K1000, but now I use it mostly as a
small portable telescope (it uses standard 1.25" telescope
accessories).  As a telescope it produces very nice images, and is
much more user friendly than the C-90 (1000mm f/11) that I had before
it (e.g., it is easier to focus without causing vibrations because of
the design of the focuser, and the C-90 used 0.96" accessories).

"Mirror lenses" (sometimes called CAT lenses) are really catadioptric
telescopes.  They use a curved convex first surface mirror to bring
light to a focus (without chromatic aberrations), a curved (convex or
concave depending on design) first surface secondary mirror to
redirect the light beam back towards the rear of the telescope and a
corrector plate to correct for the spherical (instead of parabolic)
mirror surfaces.  The secondary mirror is often coated directly onto
the back of the corrector plate.

My CAT lens is lighter and more compact than the equivalent glass
lens, but (as with any design) there are always compromises.  CAT
lenses are fixed aperture.  If you want to use a specific shutter
speed you'll wind up using neutral density filters to do the
equivalent of changing the aperture (without effecting depth of
field).  Many CAT lenses have filter threads at the back end (where
you would be able to use smaller filters) just for this purpose.

There is no reason why a good CAT lens should not be able to focus as
sharply as a glass lens at the same aperture.  Of course the key word
in that sentence is good.  I wouldn't hesitate to buy a Pro-Optics or
Celestron Maksutov (of course I already did).  These are meant
primarily as spotting scopes, and are used by birders and amateur
astronomers (who tend to be much more critical of optics than
photographers).  The Nikon is probably also very good (and very
expensive).  I seem to recall hearing good things about the Canon.  I
would not buy a CAT lens from any other manufacturer.  Having seen one
that a friend had I definitely would not by a CAT lens (or anything
else for that matter) from Cambridge Camera in NYC (their house brand
name is Cambron).

Because CAT lenses are obstructed (the secondary mirror sits in the
middle of the light path for the primary) out of focus highlights will
look like doughnuts.  Many people hate this.  It doesn't bother me.
The secondary obstruction also causes a loss in contrast.  Although it
is possible to design a reflecting telescope so that the obstruction
does not cause a noticeable loss in contrast, none of the commercial
telescopes do so.

With good coatings on the mirror surfaces a reflector should be able
to give you over 90 percent of the incoming light.  Remember that
refractors (glass lenses) also scatter light (one of the causes of
contrast loss is due to lens flare), so you never get 100 percent of
the light.

You will need a sturdy tripod, just as you would for a long focus
glass lens.  I use a Bogen 3021 with a Bogen 3047 head.  If I were
serious about using this as a lens I would probably go for a sturdier
tripod (which I would also wind up using for my large format cameras).

The draw tube on the Pro-Optics Maksutov is threaded for a T-ring.
This is a standardized adapter for attaching cameras to telescopes
(and I think microscopes).  Because camera body designs differ (both
in lens mounting technique and thickness), you have to buy a T-ring
for your specific camera type.  It should cost about $20.  By the way,
this means you can use the one lens on all your different cameras by
simply getting the correct T-ring for each camera.  Other CAT lenses
will probably have different mounting methods.

Sky & Telescope magazine reviewed the Pro-Optics Maksutov in a January
issue a few years ago.  Yo might want to look up the article at the
library.

-- 
Brian Reynolds                  | "Humans explore the Universe with five
reynolds@xxxxxxxxx              |   senses and call the adventure science."
http://www.panix.com/~reynolds/ | - Edwin P. Hubble


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