A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Telescope danger to sight. Was: Venus transit ...
From: George Huxtable
Date: 2004 May 13, 11:36 +0100
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. ================================================================