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    Re: Achromatic Telescope
    From: George Huxtable
    Date: 2003 Oct 7, 01:36 +0100

    Andrew Corl asked-
    >I was recently reading the journals of Lewis and Clark.  They mention
    >taking several telescopes with them on their voyage.  Further research
    >told me that one of the telescopes was an "Achromatic Telescope."  Could
    >anyone tell me (or refer me to a website) what this type of telescope
    >is.  If there are plans available for this I would appreciate that link
    >as well.  I am mainly interested in being able to see the moons of
    >Jupiter (gives you an idea of the magnification I am looking for).
    Response from George.
    I'm presently looking into some of the latitude observations of Lewis ans
    Clark. I wonder whether Andrew, too, has a special interest in their
    celestial nav.
    Early telescopes suffered from chromatic defect. Ordinary glass lenses have
    a focal length that depends on the colour of the light, so white light,
    being a mix of all colours from red to blue, can't all be focussed at the
    same spot. This muddies the sharpness of an image. By using clever
    combinations of crown glass and flint glass (which both show refraction
    changing with colour, but to differing extent) it's possible to make
    composite lenses which balance out this smearing, to a large extent. These
    are achromatic lenses.
    All modern telescopes will have achromatic lenses (some more so than
    others). Perhaps better, if you choose a reflector telescope, the mirrors
    are by their nature achromatic, and it's just the eyepiece that has an
    achromtic requirement. My guess is that all such eyepieces are achromatic
    nowadays. For Andrew's needs (seeing Jupiter's moons), I suggest he will
    find a reflector to be shorter, lighter, and cheaper that the corresponding
    refractor. The only real snag I can think of is that with most reflectors
    you have to look into them in an unnatural direction, whereas usually you
    look through a reflector toward the general direction of the star.
    contact George Huxtable by email at george@huxtable.u-net.com, by phone at
    01865 820222 (from outside UK, +44 1865 820222), or by mail at 1 Sandy
    Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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