P3D Apochromats

["Richard M. Koolish" <koolish@xxxxxxx>]

"Telescope Optics, Design and Evaluation" by Rutten and Venrooij has a section
on apochromat telescope objectives.  One short quote is:

    "The color aberration curve of a doublet can be made considerably flatter
    by using special glasses or fluorite.  But a less expensive alternative
    can be achieved when three different glasses are used.  Then it is possible
    to bring three colors, and with some combinations of glass, four colors
    into common focus."

Then follows a whole section on choosing the glasses for an apochromatic
triplet.


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