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As written by an astronomer and a professor of optometry:



-- Richard Langley

On Wed, 12 May 2004, George Huxtable wrote:

>Recent discussions about danger to sight in looking at the Sun through the
>telescope of a sextant have got me pondering about the physics and optics
>involved. I have concluded that through a telescope the danger of a
>retina-burn is no greater (and may indeed be much less) than when looking
>at the Sun through a naked eye. This has surprised me, somewhat. Let me try
>my arguments out and see if someone can find a snag and disprove what I
>say.
>
>NOT, please be aware, that I am suggesting you should look directly at the
>Sun, either naked-eye or through a telescope. Avoid both!
>
>The Sun subtends an angle of (damn near) half a degree. Looked at directly,
>this gives rise to an image half a degree wide at the retina. If the focal
>length of the eye is 15mm (a typical value, I understand) then the Sun's
>direct image on the retina is about 1 mm diameter. We know that the Sun's
>energy (light and heat) which passes through the iris, if focussed on that
>1mm spot, can be  immensely damaging. That's what gives rise to retina
>burns, which literally ARE burns, and can't be cured. Many old navigators
>had such burns in their "sextant eye", and blindness would often ensue. The
>worst instrument for causing eye damage was the cross-staff, until someone
>had the idea to attach a bit of smoked glass to the upper end of the
>cross-piece.
>
>What would happen if you looked at the Sun through, say, a x3 telescope?
>Well, the unescapable facts of life about optical instruments tell us this-
>That for the telescope to put light into the pupil of the eye, opened to a
>certain diameter (say 5mm) it can collect that light only from an area of
>the objective which is that same diameter, multiplied by the magnification.
>So that's an objective of 15mm diameter, in this case, or 9x the area of
>the pupil of the eye. If the objective is bigger than that, any extra light
>it collects will miss the entry pupil, being excluded by the iris. The
>result is that at most 9x as much light (and heat) than before is passing
>through the pupil (though we must allow for some losses of light and heat
>in passage through the telescope's optics).
>
>And what happens to that light? Well, it's divergence (the angular size of
>the Sun's image) has been increased by the x3 magnification, to 1.5
>degrees. Now it paints a disc-image on the retina which is 3x the size it
>was before, so now there's a 3mm diameter image of the Sun on the retina.
>This is a 9x greater area than the naked-eye image of the Sun was, over
>which that 9x increased amount of light is now spread. So its intensity, in
>terms of light-and-heat energy that falls on each square millimetre, is no
>greater than it was with the naked eye, and less if we allow for energy
>losses in passage through the lenses.
>
>I think it may be true that most such retina damage is caused by heat
>rather than by light, which is why they tell us that a nearly-black frame
>of silver in black-and-white film is effective, whereas the same darkness
>of colour film, created by dyes, does little to reduce the infra-red. I
>wonder whether glass lenses in a telescope are more effective than plastic
>ones in reducing the heat content of sunlight.
>
>The conclusion, that a magnified telescope Sun image on the retina is no
>more intense than a naked-eye Sun image, seems to contradict all my
>axperience of a burning-glass, with which, as a small boy, I used to be
>such a menace on a sunny day. I explain the distinction to myself like
>this. 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.
>
>If that's true, it should not induce any feeling of complacency when
>observing the Sun through a telescope. Although it implies that a burn-spot
>won't appear more quickly than with the naked eye, it also implies that any
>such burn-spot will be correspondingly bigger. The lesson is, then, that
>you could fry your whole retina in the same time that it would otherwise
>take to fry a small spot!
>
>The analysis above is presented rather tentatively. Optics isn't my
>subject; but to me the conclusions seem inescapable. I would welcome
>support or dissent from anyone.
>
>George.
>
>
>================================================================
>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.
>================================================================
>


===============================================================================
 Richard B. Langley                            E-mail: lang@unb.ca
 Geodetic Research Laboratory                  Web: http://www.unb.ca/GGE/
 Dept. of Geodesy and Geomatics Engineering    Phone:    +1 506 453-5142
 University of New Brunswick                   Fax:      +1 506 453-4943
 Fredericton, N.B., Canada  E3B 5A3
     Fredericton?  Where's that?  See: http://www.city.fredericton.nb.ca/
===============================================================================

   

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