Quote:
Originally Posted by Merlin66
Guys,
In "Telescopes, Eyepieces, Astrographs" by Hallock-Smith, Ceragioli and Berry
Chapter 6.2 details the design and construction of a TWO element APO (three wavelength corrected, Strehl +/_ 0.9 or better) using an ED element....
FPL53, Ohara flour-crown (the same as used in the SW ED series) combined with Schott's zinc crown ZKN7 as a flint element.
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What's the focal ratio though?