Moose asked:
And how does that work with a cadioptric lens? I did some measurements
of mirror lenses and found that the 0f total aperture taken up by the
secondary mirror mount varies quite a bit, which might affect this
resolution thing, although it always seems to be properly subracted in
calculating the effective f-stop.
---------------------------------------
I think the principal fault introduced by the secondary is to add
another diffraction edge and reduce contrast. Insofar as contrast and
resolution are not totally separate then I suppose the actual resolution
is reduced somewhat. Since the photographic lenses lenses tend to be
relatively fast (f/5.6-8) compared to telescopes (f/10-15) and since
they need even illumination over a larger field of view the secondary
has to be fairly large.
The photographic lenses also tend to be Maksutov designs probably
because the secondary can be designed such that it's nothing but an
aluminzed spot on the back of the Maksutov corrector lens. No
additional mechanical support or adjustments required. However, I think
the Maksutov corrector lens is thick enough that chromatic abberation
could be a design concern. Also, over about 90-100mm diameter the
Maksutov lens begins to get very expensive to manufacture. The Schmidt
corrector plate is so thin and so nearly plano-parallel that chromatic
abberation is not a worry and (for the last 30 years or so) is
relatively inexpensive to build in large diameters.
Having said all this I just decided to check the optical layout of the
Oly 500mm reflex and see that it is not a Maksutov design. I was
surprised at the number of refractive elements.
Chuck Norcutt
Woburn, Massachusetts, USA
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