NavList:

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.


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