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    Re: Venus
    From: George Huxtable
    Date: 2005 Dec 12, 15:59 -0000

    About the dangers of making telescope observations close to the Sun, I
    > "As we have  discussed before, the telescope doesn't make a Sun image
    > intrinsically  brighter; only bigger, on the retina. Nevertheless, the
    > brightness of any Sun  image on the retina is enough to quickly cause
    > irreverseable damage. Be  warned!"
    and Frank has responded-
    > I agree with that. Isn't that a bit of a change of heart for  you? I
    > thought
    > that you had previously argued that this fact that surface  brightness
    > can't
    > be changed by an optical system (which is very true) meant that  the
    > threat of
    > eye damage from looking at the Sun through a telescope  (specifically a
    > sextant telescope, in previous discussions) was no greater than  the
    > threat of eye
    > damage when looking at the Sun for the same period of time  with the naked
    > eye.
    . No change of heart. When that subject has cropped up before, I have been
    careful to state that
    looking at the Sun with the naked-eye is itself dangerous, and must be
    avoided. What I have pointed out is that a telescope can fry a bigger patch
    of the retina, in the same time that a naked-eye Sun fries a much smaller
    patch, though the intensity at the retina is no greater. I have also said
    that that argument applies to the retina only, not to the exterior surfaces
    such as the cornea. If that message hasn't come across, then I have failed
    in my intention.
    It is certainly true that when employing a large objective with great
    magnification, as in Frank's case, there are considerable dangers in
    overheating  the cornea, the iris and lens, also the eyelids (which will
    close as the blink-reflex occurs).
    contact George Huxtable at george@huxtable.u-net.com
    or at +44 1865 820222 (from UK, 01865 820222)
    or at 1 Sandy Lane, Southmoor, Abingdon, Oxon OX13 5HX, UK.

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