NavList:
A Community Devoted to the Preservation and Practice of Celestial Navigation and Other Methods of Traditional Wayfinding
Re: Eyesight dangers using telescopes
From: Paul Hirose
Date: 2009 Jun 30, 15:03 -0700
From: Paul Hirose
Date: 2009 Jun 30, 15:03 -0700
According to astronomer Andrew Young, "But in fact, according to the review of such injuries [from viewing the Sun without protection] published by Istock in 1985, 'the vast majority of solar retinal injuries occur as a result of viewing a solar eclipse without adequate protection.' So it usually requires the special conditions of an eclipse near totality, in which the low level of general illumination allows the pupil to open up instead of contracting (as it normally does when looking at the Sun), to push the visual system over the threshold for damage in a brief exposure... While there are a handful of cases of solar retinopathy produced by staring at the Sun outside of eclipse, these are nearly all associated with bizarre religious practices, drug use, mental illness, or other abnormal and rare circumstances." "Now, let's consider the hazards of using optical magnification. This introduces hazards of two kinds: a larger solar image on the retina, and a brighter illumination of the instrument's exit pupil. (The exit pupil of a telescope, also known as the 'Ramsden disk', is the little circle of light, behind the eyepiece, through which you see into the instrument.) "The larger solar image on the retina produces more heating than in naked-eye observation, as shown by the calculations of White et al. Still assuming an eye pupil diameter of 3 mm, they find that a 25x telescope would produce a retinal temperature rise of 12°C in one second, and 34°C in 10 seconds. Both of these numbers exceed the threshold for retinal thermal damage. However, they assume the Sun in the zenith; for the Sun only 5° above the astronomical horizon, the heating rates are smaller by a factor of 4, which would push even the 10-second telescopic observation (just) below the threshold for thermal damage. The smaller image produced by low-power binoculars would be safer still. "Clearly, both the retina and the iris are below the threshold of injury when the Sun is viewed through binoculars within a few degrees of the astronomical horizon, but not when it is higher in the sky. "However, telescopic observations, using higher magnifications, can easily be hazardous to the iris of the eye, even near the horizon. For a frightening story of a close encounter with this danger, read William Bunker's first-hand account." Of course, sextant telescopes are nowhere near 25x, and a flash of Sun through the scope (perhaps caused by forgetting to use an index shade) will cause you to reflexively blink and look away in much less than one second. In addition, the usual scope is the Galilean type, which has inferior light grasp compared to the Keplerian design used in ordinary telescopes and binoculars. As for photochemical damage, Young says, "The photochemical hazard depends only on the image brightness, which (by a well-known theorem of optics) cannot be increased by an optical system. So, on the whole, using optical aid cannot significantly increase the photochemical hazard to the retina, and (if the instrument's exit pupil is small, and/or the instrument's transmission is significantly less than unity) may even decrease it. "I have argued above that the retina will not be damaged photochemically if the Sun is within a few degrees of the astronomical horizon. This conclusion remains true if optical aid is used, as any optical instrument (e.g., binoculars) can only make the retinal image dimmer, not brighter. "In particular, the glass lenses used in binoculars and telescope eyepieces strongly absorb the shorter wavelengths that are responsible for photochemical damage." "I don't intend to minimize the seriousness of solar eye damage; victims often lose the ability to read normal-sized print, for example. But draconian pronouncements that 'you should NEVER look at the Sun' or assertions that you can become permanently and completely blind are an over-reaction to the actual hazard. Sunsets can be viewed safely, both with the naked eye and with binoculars, and most people are already aware of this." Young supports his assertions with numerous citations from books and journals. http://mintaka.sdsu.edu/GF/vision/Galileo.html -- --~--~---------~--~----~------------~-------~--~----~ Navigation List archive: www.fer3.com/arc To post, email NavList@fer3.com To , email NavList-@fer3.com -~----------~----~----~----~------~----~------~--~---