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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
    heat-failure.
    
    George.
    
    ================================================================
    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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