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From: George Huxtable (no email)
Date: Mon Oct 06 2003 - 20:36:29 EDT
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.
George,
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contact George Huxtable by email at , 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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