Hans wrote...
The 1000mm/F11:
"The >>apochromat<< type front lens group provides minimal chromatic
aberration, so
both resolution and contrast in this lens are outstanding. The special
lens
configuration features two varieties of flint glass and makes use of the
tiny
difference in their refractive indices to correct for colour
aberrations. This
combination is known as Schubel, Flint >>Apo<< and makes an important
contribution to
upgrading the lens performance."
So the 1000mm is also something special; although the terms SD, ED or
EPD aren't
used, here we see the term >>APO<< which (I think) describes a lens
using special
glass.
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I think the term "APO" simply refers to an apochromatic lens (as opposed
to an achromatic lens). The following definitions are from the
Astronomics website at:
http://www.astronomics.com/Content/how_to_pick/terms.htm
Achromatic Doublet (Achromat): A two-element objective lens used in
refractor telescopes. Using two complementary glass types (low
refractive index crown glass for one element, and high refractive index
flint glass for the other) partially cancels out chromatic
aberration. While the word "achromatic" means "without color," it
actually refers to a refractor that is not totally free from spurious
color, but one that is corrected for chromatic aberration at two
wavelengths, or colors, of light.
Apochromat: A refractor telescope with a two to four lens optical system
that uses one or more elements of costly ED (Extra-low Dispersion) or SD
(Special Dispersion) glass and/or calcium fluorite crystal to virtually
eliminate chromatic aberration. Technically, any
lens system that is corrected for spherical aberration at two
wavelengths, or colors of light, and for chromatic aberration at three.
Chuck Norcutt
Woburn, Massachusetts, USA
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