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: Jared Sherman
Date: 2004 May 12, 20:28 -0400
From: Jared Sherman
Date: 2004 May 12, 20:28 -0400
George, this is not relevant to the entral issue but I'd like to clarify something here.
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. I'll try to call my optometrist tomorrow and ask him about these things. He may not know naything about sextants, but he's retiring after a life of working with eyes and eye damage, and knows that side of the issue. There are some other issues which really haven't been touched. Those of IR and UV, and the fact that the eye is filled with a remarkably robust yet delicate fluid which is not easily replaced, and fronted with a 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. This is in fact the basis for LASIK surgery, ablating the cornea. I have no idea how large an issue that is but 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, rather than trying to explain exactly how and when the threshold for dmaage is passed. ----- Original Message ----- From: "George Huxtable" To: Sent: Wednesday, May 12, 2004 6:20 PM Subject: Re: Telescope danger to sight. Was: Venus transit ... Jared Sherman said- >George, I'm wondering if you glossed by something that deserves further >attention. > > 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 human eye typically has a diluted pupil of 5-6mm diameter, and rarely >more than 7mm. When contracted against bright light, that drops to something >like 1-2mm. (I'm less familiar with the fully contracted size.) > >So, a human eye viewing the sun has an "onbjectve lens" with a diameter of >2mm. >Compared to your telescope's objective lens of 15mm. > >But the scope on many sextants is well wider than 15mm, easily in the 30mm. >range. And "cheap" binoculars rapidly push past 42mm to 50mm objectives >while common telescopes used by birders here are 77-80+ mm lens now. > >The comparison then is of the human eye, gathering light, IR and UV >included, from a 2mm circle to a telescope objective gathering light from a >50mm objective. There's an incredible amount more light being gathered, and >even "cheap" coated optics boast a 95%+ transmission factor. > >Perhaps this is all that is needed to explain the warnings? ==================== Thanks for Jared's contribution, but I think he may be missing the point. But if he thinks I misunderstand what he says, I hope he will argue back. As I hope my last message made clear, it's not a matter that I am completely confident about, and I'm ready to be persuaded otherwise. But not (yet) by Jared's argument. Let's accept Jared's assumption that, it being a sunny day, the observer's pupil has shrunk down to 2mm dia And now, let's consider a x3 telescope with Jared's suggested objective of 50mm dia. Actually, I think one would be unlikely to come across such an instrument with a magnification of x3, for reasons that may become clear, but let's make that assumption anyway. We can compare the two situations, of with-telescope and naked-eye, in terms (in both cases) of the total sunlight energy which would be falling on a 50 mm dia disc, the size of the telescope's objective. That objective collects (50/2) squared times as much light as the eye pupil would collect, or 625 times as much, which is I think Jared's point. But where does that light go?. It would be shrunk into a light-pencil which was one-third of the diameter of the objective (because that shrinking by three unavoidably goes with the x3 magnification). So the light exiting the telescope eyepiece has a diameter of 50mm/3 or 16.7 mm, provided the eyepiece has sufficient diameter to let such a wide pencil through. So, we have a light-beam exiting from the telescope, 16.7mm wide, impinging on the eye. But remember, the eye pupil, we assumed, was only open to 2mm dia. So most of the light will strike all round the pupil and the eye and be lost, and only a tiny fraction of that wide beam of light will enter the eye. What fraction? Well, the area of the pupil divided by the area of the light-pencil, or (2/16.7)squared, or 1.44%. The rest will miss the tiny hole. And what area of the retina does that light fall on? A circle 3mm in dia, the Sun image magnified by 3 Now, take the telescope away, so direct sunlight shines into the pupil, which has 2mm dia. We already worked out that this would be only 1/625 of the total light that would otherwise have fallen on the telescope objective. But now this light is concentrated into a retinal area of only 1mm dia, because now there is no magnification. So we take 1/625 of the incident light, but allow an increase in this case by a factor of 9, because the light is concentrated into a retinal patch 9 times smaller. And we get 9 x 1/625 or 0.144, to compare with the figure 1.44% we arrived at for the telescope: Exactly the same, of course! The brightnesses in the two cases are the same, the only difference being that the telescope gives a 3x bigger (not brighter) image. And we have ignored any loss of heat and light in passing through the telescope. There are snags with Jared's chosen example. Nobody, I suggest, would make a 3x telescope with a 50mm objective. Even if it possessed an eyepiece 16.7mm diameter, to let out all the light that went in, nobody has an eye in which the pupil can expand to anything like that diameter, to take in all the light that emerged. Instead, telescopes are usually designed to match the expanded night-time eye-pupil of say 8mm. In that case, with 3x magnification, an objective bigger than 24mm would be pointless, and to make use of a 50mm objective, the magnification would need to be 6x. But in daylight conditions, when the pupil has closed down to a much smaller diameter, only a tiny fraction of the incident light can get in, as shown above. The iris is doing its job. A large-aperture objective is quite wasted, under those conditions. 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. ================================================================