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    Re: Eyesight dangers using telescopes
    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
    I filter out messages with attachments or HTML.
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