At 8:39 PM -0700 8/3/99, Ric L Ecker wrote:
>Chris..............give the Astro Physic scopes a
>try..................................they do build a true Apo with three
>elements, the Tak. that I've seen call them Apo but they are two element
>ED or fluorite which for the most reasonable is as close to a true Apo
>but not quite.............................Televue Apo (again 2-element)
>states they consider any scope that does not show purple color on Sirius
>at 200X is considered an Apo. Meade and Celestron both claim their
>scopes to be Apo and the smaller Televue Pronto and Ranger are considered
>Semi-Apo which is better than a achromate except an achromate at f/15 or
>higher. Sorry to ramble on but I have been doing some research...

Then you should know that the definition of "apochromatic" is a
technical one which involves three colors of light coming to focus at
the same plane and a lack of some other aberrations. It is
objective and possible to measure unambiguously. And it has nothing
to do with the number of elements in the objective lens or other
construction details. AFAIK, the Takahashi fluorites (and the Vixen
fluorites for that matter) easily meet the requirements. The "high
end" Meade refractors do not. I don't know if it is possible to make
an APO with ED glass or not, but Meade has not succeeded. And I'm
not sure about the other Televue products but the most recent
versions of the Genesis and the TV101 use 4 elements, not two, though
two of them are back by the focuser. I don't know if they are true
APOs but it should be possible (and the few that I've looked thru
seem to have as little color as the AP and Tak true APOs).


Bill Arnett
http://www.seds.org/billa/

"I know that I am mortal and the creature of a day; but when I search
out the massed wheeling circles of the stars, my feet no longer touch
the earth, but, side by side with Zeus himself, I take my fill of
ambrosia, the food of the gods." -- Claudius Ptolemaeus

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