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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
    Liouville.
    
    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
    >glass.
    
    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.
    
    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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