# NavList:

## A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding

**Re: Eyesight dangers using telescopes**

**From:**Douglas Denny

**Date:**2009 Jun 29, 08:11 -0700

Mr. Huxtable, you do not convince me at all. Your pedantry astonishes me. You wrote: "There is confusion throughout Douglas' posting, between aperture-as-diameter, and aperture-as-area." I say : no there isn't. I have been careful to mention area as the the important parameter in this discussion about light flux energy and its importance with viewing the Sun through a telescope. I think it is quite clear throughout the debate that whenever apertures are being discussed the property which is important is area. It is you who has been concerned with the issue of exit pupil diameters of the telscope and entry pupil diameter of the eye. For the sake of brevity I have resisted challenging your 3mm criterion for the eye pupil which is a complicated issue in its own right. --------------- You wrote: "Increasing magnification decreases luminance squared." I don't understand that statement, which appears to be written backwards. If Douglas had written instead "luminance decreases as the square of the increase in magnification", I would go along with that." You are "going along" with what I already wrote. It is clear enough. (To me anyway). ------------- I wrote: "What happens with aperture changes with constant magnification? If a telescope is used of unity magnification, the maths says increase the aperture area and the luminance of the image increases. If aperture is doubled then L doubles. If aperture is quadrupled, then L is quadrupled. Use a telescope of unity magnification but aperture area double that of the pupil diameter of say 3mm and the luminance must double to 2L." You wrote: "No. That's where Douglas has it wrong. He has assumed the impossible." I still believe the maths states the case clearly. Luminance of the image is directly proportional to aperture of the objective, (and inversely proportional to square of magnification). I have already stated this a number of times. You seem to have had to have the aperture argument forced upon you. It should be obvious in my opinion. The magnifying lens burning paper provides experimental proof that anyone can try. ---------------- You wrote: "I think in a previous posting, Douglas eventually agreed with me (and with Johnson's book) that the ratio of entrance pupil diameter to exit pupil diameter of a telescope was equal to its magnification." I do not agree with this necessarly, or Johnson's book. I agreed I made an error in quoting what was in fact the radius of the exit pupil - a half-angle result from a ray diagram, forgetting the diameter is twice this. I query your result from Johnson's book Re. magnification which I think might be a simplification. Without the text before me from this book I cannot make head nor tail of the quoted result you made. Nor frankly do I wish to, as I ma satisfied with the result I calculated at 4.4 mm from a ray diagram based on Longhurst's definition of exit pupil. You say 5 mm from Johnson. Immaterial anyway at 0.6 mm difference, and a red-herring as I mentioned, as the angular subtense of the Sun (half a degree) at the exit pupil in my X 10 binocs with seven and a half degrees full field of view, is still much smaller than the 4.4 (or 5) mm diameter of that exit pupil. I deduced the exit pupil diameter from the definition in Longhurst's statement that the exit pupil in an astronomical telescope is the image of the objuective formed by the eyepiece... which indeed it is, if you draw the ray diagrams. ------------------- you wrote: "But double that diameter, and the diameter of its exit pupil is also doubled. However, now most of the light in that larger 6mm exit diameter pencil of now misses the hole in the iris, which remains at 3mm diameter." And this is where I think you are wrong. The exit pupil will, I agree, increase with increasing diameter of objective as you say (and I am aware of that and have made it clear in my assertions too) - but the limiting factor in this discussion about the Sun is the angluar subtense of the Sun which is the limiting factor with whatever the combination of magnification and aperture is, which in turn, gives half a degree at the exit pupil of the telescope, to give a diameter (if you insist) of say 3 mm to allow full entry into the entry pupil of the eye ......and this only if you want to work out the conbination of aperture/magnification that gives a _full_ solar disc from objective aperture to eyepiece exit pupil entering the eye. You are IGNORING the danger still exists for only a _fraction_ of the solar disc appearing focussed on the fovea to cause serious damage. (I have made that point before too). ----------------- I wrote: "So it is clear in my mind, for brightness of image in a telescope there are two parameters working in opposition: aperture increasing directly and magnification decreasing and by a square function."" You wrote: "No, the possible objective area and the total input energy increase by the magnification squared, as explained" I say no to you again.......... The "total input energy" presented to the telescope as a light wavefront per square metre is constant. The amount of that energy acccepted into the telescope is a function of objective aperture area. The amount appearing at the exit pupil is an inverse square function of magnification. The amount of solar disc entering the eye depends then on the maximum angular subtense allowable at the telescope exit pupil/eye pupil entry pupil due to the _combination_ of aperture/magnification effects. ==================== I had indicated I was not wishing to get sucked into a never ending diatribe about this issue. This is likely to be my last submission. I think it is quite clear now having been discussed in some detail. Finally though: As you are quite asserive to challenge this issue of Solar energy entering the eye causing damage, I ask you directly: are you suggesting that in any way it is accceptable to view the Sun through a telescope? Yes or no? Douglas Denny. Chichester. England. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---