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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

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