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    Re: Telescope danger to sight. Was: Venus transit ...
    From: Ken Muldrew
    Date: 2004 May 12, 17:10 -0600

    On 12 May 2004 at 17:03, George Huxtable wrote:
    
    > Recent discussions about danger to sight in looking at the Sun through
    > the telescope of a sextant have got me pondering about the physics and
    > optics involved. I have concluded that through a telescope the danger
    > of a retina-burn is no greater (and may indeed be much less) than when
    > looking at the Sun through a naked eye.
    
    > If the focal length of the eye is 15mm (a typical value, I
    > understand) then the Sun's direct image on the retina is about 1 mm
    > diameter. We know that the Sun's energy (light and heat) which passes
    > through the iris, if focussed on that 1mm spot, can be  immensely
    > damaging. That's what gives rise to retina burns, which literally ARE
    > burns, and can't be cured.
    
    I have a recollection, and I'll try to find the reference tonight,
    that viewing the sun with the naked eye only results in photochemical
    injury to the retina, not a heat-related burn. But I may be
    misremembering. Of course with a telescope, a burn becomes possible.
    
    > Many old navigators had such burns in their
    > "sextant eye", and blindness would often ensue. The worst instrument
    > for causing eye damage was the cross-staff, until someone had the idea
    > to attach a bit of smoked glass to the upper end of the cross-piece.
    
    The explorer David Thompson was blind in his right eye. He attributed
    it to working late by candle light but I don't think anyone believes
    his analysis.
    
    > What would happen if you looked at the Sun through, say, a x3
    > telescope?
    
    I think you have analyzed the situation correctly from an optical
    perspective. If heating is the cause of injury, though, then the
    larger area could lead to more damage due to the limited capacity of
    the eye to remove heat through blood flow. On the other hand, if the
    injury is photochemical (and even if true for naked eye viewing, it
    would only remain true for low magnifications, perhaps including
    typical sextant scopes) then Liouville's theorem tells us that there
    is no concentration of the brightness, so the injury will accumulate
    at the same rate but over the larger area of illumination.
    
    Ken Muldrew.
    
    
    

       
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