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    Re: Telescope danger to sight. Was: Venus transit ...
    From: Jared Sherman
    Date: 2004 May 19, 17:38 -0400

    George-
     < I wonder whether glass lenses in a telescope are more effective than
    plastic
    ones in reducing the heat content of sunlight.>
    
     I don't think so. In photographic enlargers we used to use a special "HAG"
    heat absorbing glass to block infrared from the optical path from the bulb.
    And the classic Nikon 35mm camera lenses had an extra mark on the focal
    scale, for use with infra-red film. WIth IR the focal point of the lens
    changed, but they were still used for IR photography.
    
    That might be another point to consider when viewing the sun. The IR image
    of the sun may be focused differently from the visible light image, so the
    greatest point of heat may not match the visible image.
    
     Plastic-vs-glass is more of an issue with UV light, if I remember
    correctly.
    
    
    

       
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