NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Venus
From: George Huxtable
Date: 2005 Dec 12, 15:59 -0000
From: George Huxtable
Date: 2005 Dec 12, 15:59 -0000
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.