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  1. #1
    John Savard's Avatar
    John Savard Guest

    Default The "Half-Apochromat", if not the "semi-apo" has been around a long time...



    From

    http://alice.as.arizona.edu/~rogerc/...pter%204a.html

    part of a very interesting document also mirrored at ATMsite, many of
    the basic facts of the history of correcting for secondary color are
    noted.

    For a long time, except for using water or other liquids, the only
    practical material for making a lens that corrected for secondary color
    was calcium fluorite, F-Ca-F or Ca(subscript)2(/subscript). And this was
    only available in natural crystals, of which large ones were very rare.

    Thus, fluorite stayed in its original application of microscope
    objectives for a long time.

    In 1949, a method of growing synthetic fluorite crystals was published
    in a paper by D. Stockbarger (apparently, it was discovered during World
    War II, during which it was kept as a military secret); it wasn't until
    1968, apparently, when Canon started growing fluorite crystals
    presumably using an improved process that the material started showing
    up in consumer items. And, of course, as we know, glasses with
    characteristics similar to fluorite are now being made; they have the
    advantage of being less fragile, and of avoiding birefringence.

    But as this page also recounts, various attempts were made to produce
    lenses with correction for secondary color before that time. And no less
    a name than that of Zeiss was attached to some lenses called
    "half-apochromats", the first of which was designed by Konig.

    So I suppose we now have a standard for how good a "semi-apo" has to be;
    if the color error in a triplet apochromat is about 100 times less than
    that of an achromat, I suppose that a logarithmic scale, where a
    semi-apo has to be 10 times better than an achromat, this still wouldn't
    be too far-fetched.

    John Savard
    http://home.ecn.ab.ca/~jsavard/index.html

  2. #2
    Brian Tung's Avatar
    Brian Tung Guest

    Default The "Half-Apochromat", if not the "semi-apo" has been around a long time...

    John Savard wrote:

    Usually, the longitudinal chromatic aberration in a visual apochromat,
    expressed as a fraction of the focal length, is in the neighborhood of
    1 part in 20,000, give or take a factor of 2 or so, which makes it 10
    times better than a typical achromat. The Meade EDs, which I think
    have errors on the order of 1 part in 8,000, might make a good benchmark
    for a semi-apo.

    Brian Tung <brian@isi.edu>
    The Astronomy Corner at http://astro.isi.edu/
    Unofficial C5+ Home Page at http://astro.isi.edu/c5plus/
    The PleiadAtlas Home Page at http://astro.isi.edu/pleiadatlas/
    My Own Personal FAQ (SAA) at http://astro.isi.edu/reference/faq.txt

 

 

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