# NavList:

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

**Re: Eyesight dangers using telescopes was: [8760] Basics of computing sunrise/sunset**

**From:**Douglas Denny

**Date:**2009 Jun 25, 12:47 -0700

I still disagree with Mr. Huxtable except in his correction of my miscalculation error of foveal area. Area is indeed 0.07 mm. not 0.28 mm. and I am sure he will appreciate the effect of this if anything is to potentially increase the I/R energy density at the fovea by a further four times, making my simple calculation an underestimate by this amount. He still seems reluctant, however, to accept light energy can be focussed. If the flux density of light cannot be focussed from an objective lens and hence increased by optical means onto a small area, then all of those people in the Lawrence Livermore facility concentrating huge aperture lasers onto a pinpoint of about one mm. for fusion research are going to be badly disappointed. Although he now agrees that all of the light energy into an objective (neglecting inefficiency of the optics) will emerge through the exit pupil of the optical device, he seems determined not to let that light into the eye from a telescope. I do not think he is being realistic about this or is familiar with the optics of the eye. One or two more points to try to shed light on the matter (sorry about pun):- The exit pupil of an astronomical telescope is the image size of the objective lens itself which forms the aperture stop. Take my simple cheap binoculars. Magnification ten; objective aperture 50 mm; focal length of objective 195 mm, focal length of eyepiece 19.5 mm. The aperture of the exit pupil(x) is therefore:- x/19.5 = 25/214.5 which = 2.2 mm. real, and at 19.5 mm. from the eyepiece. .....and hence is quite small enough to have the entrance pupil of the eye at this point and allow all the light pencil into the eye for a full field of view. This is normal for astronomical telescopes. The Galilean eyepiece however, produces a virtual exit pupil so there can be no field stop, no sharp boundary and the whole field cannot be seen at once, but the limiting factor is eventually the objective diameter. Magnification affects the field of view and illuminance at the image as normal with inverse functions with both. -------------- There is a simpler analysis for consideration with the eye however, for magnification and image of the sun. It is a curious co-incidence of nature that the diameter of the Sun, and the diameter of the Moon and the diameter of the fovea all subtend the same angle of about half a degree. This means that the image of the Sun, or Moon, covers almost exactly the fovea in the naked eye. If a telescope has unity magnification (1.0) then the energy entering the objective is equal to that emerging frmo the eyepice; and as just indicated, all of which can enter the eye. The energy enering the eye is now increased by the greater aperture of the objective compared to the eye pupil and is in proportion to the different aperture areas. The energy of that part of the image which is the Sun is fully covering the fovea - and increased by the larger aperture of the telescope compared to the eye pupil. Now consider the case if the magnification of the telescope used is say, two. Then the angle of emerging rays at the telescope is doubled, and image size on the fovea is doubled - but that is still half of the Sun's object size occupying the foveal area - and as discussed, the flux density of light from the full aperture of the objective is placed there. It might be half of the flux energy from the telescope if it had a magnification of one - but is still enormous when the difference in apertures between eye pupil size and objective lens aperture size are considered. Same argument if you increase the magnification of the telescope to four: you then have a quarter of the Sun's disc on the fovea, but still a massive flux increase in proportion to the objective aperture and eye pupil areas .. and quite enough to burn a hole in the retina. Enough of that. I have made my point. It is reckless to use telescopes looking at the sun - anytime. =============== I shall make another posting shortly and that involves the other thread with star distances and accuracy, but that will have to be later when I can find time to write about it and scan some documents which should give further interest/enlightenment. Douglas Denny. Chichester. England. --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To unsubscribe, email NavList-unsubscribe@fer3.com -~----------~----~----~----~------~----~------~--~---