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    Re: Venus transit June 8 and sextants
    From: George Huxtable
    Date: 2004 May 12, 23:20 +0100

    Fred Hebard asked-
    >What's the problem with the shades that go over the eyepiece of the
    >telescope overheating and blowing up?  They were a fixture of sextants
    >for years and still are in some models.  Is there a history of this
    >occurring or is this speculation?  A piece of glass blew into the
    >cornea of my left eye and it was no fun.  Maybe the light from a
    >telescope could burn a hole in your retina, but lacerating your cornea
    >could be even more damaging to your eyesight.
    Response from George-
    Such a light-filter on the eyepiece is very dangerous. It's a small and
    very black disc which has to absorb nearly all of the energy that is
    collected by the objective, in a very small area. This can heat it enough
    for it to shatter in an instant without warning and let full Sun into your
    eye before you get a chance to blink.
    I am aware that such filters have been supplied for Newtonian astronomical
    reflectors, and deplore it. Whether there much risk of such heat-failure
    with the smaller telescopes that are fitted to sextants, I just don't know,
    but I would not be keen to try it.
    On the other hand, a filter fitted to the objective end of a sextant
    telescope (or any other telescope) is exposed to nothing stronger than
    being left out in full sunshine, so it is unlikely to suffer such
    contact George Huxtable by email at george---.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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