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    Re: Telescope danger to sight. Was: Venus transit ...
    From: George Huxtable
    Date: 2004 May 13, 11:36 +0100

    My first mailing on this topic had compared the Sun brightness seen through
    a 3x telescope with Sun brightness seen through the naked eye.
    Jared Sherman had responded that some optical instruments had much larger
    objective lenses than the 3x15mm sextant telescope I had chosen as an
    example, and that these were capable of putting much more light into the
    eye. That is indeed true.
    To my objection-
    "There are snags with Jared's chosen example. Nobody, I suggest, would make
    a 3x telescope with a 50mm objective."
    he has replied-
    > I didn't suggest anything of the soft. We've been discussing sextant
    >telescopes and--from the wording of some of the posts--the larger issues of
    >"telescopes" or optics in general. My comments were directed to the broad
    >issue, that there are commonly available optics which folks may pick up to
    >look at the sun, with objective lenses commonly ranging from 30mm (sextants)
    >to 50mm (binoculars) and 80+mm (common spotting scopes and birding scopes)
    >and none of these even touch the range of "telescopes" for general
    >astronomical purposes. I think I've seen hundred dollar "astronomy" scopes
    >at the WalMart with larger objectives, and they don't ask for any skill
    >levels before selling them to the public.
    What he says is correct. But what he has missed out is this-
    To compress the light from a large objective into a small diameter (the
    pupil of the eye), it's unavoidable that the telescope has to have a
    magnification at least as great as the ratio of those two diameters. That's
    why 6x50mm or 7x50mm are such common oculars, and why you never see a
    3x50mm ocular. Much larger objectives can be used, but always, to go with
    the larger diameter, they must have a greater magnification.
    I had compared the brightness on the retina of direct-Sun, compared with
    x3telescope-Sun. But if you go to a bigger objective, and therefore a
    bigger magnification than x3, then that changes the calculated brightness,
    because the bigger magnification gives a bigger image on the retina. For
    example, if instead of my suggested 3x15mm telescope you used a 6x30mm
    instrument instead, then the objective would then collect 4 times the
    amount of light, pass it all through the pupil, and paint an image on the
    retina which had 4 times the area. The surface brighness of the Sun image
    on the retina remains unaltered, and exactly the same as it was for the
    naked-eye Sun.
    There seems to be no way round this, and I am grateful to Ken Muldrew to
    putting a name to Liouville's Theorem, which appears to back up my
    suppositions, though I haven't yet seen that theorem spelled out. I have a
    textbook, "A Treatise on Light", by R A Houston (1938 edn.) which on page
    359 shows that "the image of an extended object formed by a lens or mirror
    can never appear brighter than the object itself", but without mentioning
    Jared refers to the -
    >lens made
    >out of tissue (the cornea) which can be BURNED and damaged without any harm
    >coming to the retina all the way in the back. If the spot of the the sun
    >happened to focus and fall on the cornea itself--rather than back in the
    >retina--then the full image from the lens, unreduced by the iris and pupil
    >size, would be able to burn the cornea just like an ant under a child's
    Well, I did try to deal with the aspect of damage to the front end of the
    eye, rather than to the retina, saying in my first posting-
    "There's no doubt that with a single lens sunlight can be concentrated into
    a small spot. For example, with the telescope example above, the light
    intensity falling on and around the pupil will be 9x increased above the
    naked-eye case. And so, if the eye surface or the iris itself are easily
    burned by high intensity, that risk will be greatly increased when looking
    through a telescope. However, the intensity of the light that gets through
    the pupil to the retina itself, will not be increased."
    Jared concludes-
    >if there is any chance of
    >damage at all--no matter how small--who among us would want to risk losing
    >the vision in an eye, forever? Tha may be why the generic advice is simply
    >never to look at it...
    I don't disagree at all with Jared here, which is why my first posting
    included these words-
    "NOT, please be aware, that I am suggesting you should look directly at the
    Sun, either naked-eye or through a telescope. Avoid both!"
    He continued-
    >...rather than trying to explain exactly how and when the
    >threshold for dmaage is passed.
    Well, I disagree most strongly with THAT sentiment! It is anti-scientific
    obscurantism at its worst! Although it may indeed be true (and to me now,
    the evidence is accumulating) that a telescope does NOT increase the
    intensity of the Sun's image on the retina, only its size, it seems that
    Jared would rather not know that.
    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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