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About the dangers of making telescope observations close to the Sun, I
wrote.

> "As we have  discussed before, the telescope doesn't make a Sun image
> intrinsically  brighter; only bigger, on the retina. Nevertheless, the
> brightness of any Sun  image on the retina is enough to quickly cause
> irreverseable damage. Be  warned!"

and Frank has responded-

> I agree with that. Isn't that a bit of a change of heart for  you? I
> thought
> that you had previously argued that this fact that surface  brightness
> can't
> be changed by an optical system (which is very true) meant that  the
> threat of
> eye damage from looking at the Sun through a telescope  (specifically a
> sextant telescope, in previous discussions) was no greater than  the
> threat of eye
> damage when looking at the Sun for the same period of time  with the naked
> eye.

. No change of heart. When that subject has cropped up before, I have been
careful to state that
looking at the Sun with the naked-eye is itself dangerous, and must be
avoided. What I have pointed out is that a telescope can fry a bigger patch
of the retina, in the same time that a naked-eye Sun fries a much smaller
patch, though the intensity at the retina is no greater. I have also said
that that argument applies to the retina only, not to the exterior surfaces
such as the cornea. If that message hasn't come across, then I have failed
in my intention.

It is certainly true that when employing a large objective with great
magnification, as in Frank's case, there are considerable dangers in
overheating  the cornea, the iris and lens, also the eyelids (which will
close as the blink-reflex occurs).

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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