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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.

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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.

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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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