NavList:

on 8/2/06 23:00, George Huxtable at george@huxtable.u-net.com wrote:

>
> On 28 July, Ken Gebhart wrote, about the dangers of seeing glimpses of
> the Sun through the telescope of a sextant-|
>
> "On the other hand, it has been
> thoroughly vetted on the Nav-L list that such glimpses, even with
> higher
> power scopes do less eye damage than naked eye exposure. And even this
> is
> not very significant unless exposure is prolonged to the point of pain
> (solar eclipses excluded of course)."
>
> Frank Reed has questioned this view.
>
> As one who has contributed to this discussion in the past, with the
> aim of getting it discussed rationally, I think it's important that we
> consider the dangers carefully. My view is that Ken made light of
> those real dangers, to some extent, but this contrasts with others
> who, in the past, have overdramatised them.
>
> First, the intrinsic brightness of a Sun image focussed on the retina,
> as seen through a telescope, in terms of energy per square millimetre,
> can not be greater than if the Sun was being observed in the same way
> by the naked eye. Not brighter, but certainly bigger. It is known to
> be damaging to the eye to look at the Sun directly, and we
> automatically avert our view. We have developed a fast blink response
> to minimise that damage, and avoid a retinal burn; the iris closes
> down as well, but more slowly. Through a telescope, if such a retinal
> burn can occur, it will be of a larger patch, rather than a tiny spot.
> So, to that extent it can be more damaging. But it seems to me that
> the likelihood of damage is no greater than it was without that
> telescope. But even so, caution is called for, just as it is with
> naked-eye viewing of the Sun. Blindness was a notorious danger for
> navigators when they had to look straight into the Sun, in the days of
> the cross-staff.
>
> That argument related to the energy density of the focussed image at
> the retina. But also, we have to consider other parts of the eye. The
> energy density at the pupil of the eye is certainly increased by the
> presence of a telescope. I just do not know about the physiology
> involved here, and how robust is the cornea / iris / lens combination
> to a light overdose. My guess is that the most sensitive part of the
> eye, by a long way,  is the retina, but that is indeed no more than a
> guess, and I have no knowledge to back it up. If we take a typical
> sextant telescope as having a magnification of 3, then (ignoring light
> losses within it) the energy density of light incident on the pupil
> will have increased by 9 times, compared with direct light from the
> Sun. Is that likely to damage those structures within the eye? I just
> don't know, but I doubt it. Even so, caution is called for.
>
> Frank should compare the behaviour of a sextant telescope, with a
> magnification of 3x, and so a brightness increase of 9 at its exit
> pupil, with a "backyard telescope", which likely has a magnification
> of 30x or more, so can concentrate light by a factor of 900 (or could
> if there were no losses). Perhaps, with that, one could light a piece
> of paper. The other; well, try it. Take a sextant telescope out into
> bright sunlight, and put your hand by the exit pupil, where your eye
> would be. Can you feel any hot spot?
>
> All this was based on the assumption of full Sun falling directly on
> the full area of the objective. And where I would back the viewpoint
> that Ken Gebhart was proposing, is that he was referring to a very
> different state of affairs, of glimpses of sunlight getting round the
> edges of the shades and through the very margins of the objective into
> the eye. In that case, the efficiency of getting light from Sun to eye
> is greatly reduced, and I would not expect the odd flicker of Sunlight
> peeking through to be any real hazard, disconcerting though it may be.
>
> George.
>
> 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.
>
>
> >
Well, I didn't mean to make light of the dangers of looking at the sun
through a telescope on a sextant.  However, as someone involved with the
manufacture of sextants, I am aware of the possibility on product liability
litigation in this area.  So, I have collected any and all anecdotes and
opinions on the subject in order to arm ourselves for this possibility.

Apart from the scientific discussion of light, lenses and retinas, I am not
aware of any instances of actual eye damage occurring from use of a modern
day sextant (or for that matter, an astronomical telescope). If any list
members know of such things, I would appreciate any leads they may have.

Ken


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