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    Re: Eyesight dangers using telescopes
    From: Frank Reed
    Date: 2009 Jul 3, 17:49 -0700

    I thought of one other example for the case of magnification and the 
    constantcy of "intensity" (which in this case is the apparent "surface 
    brightness" of a celestial object). Consider how we select shades for viewing 
    the Sun through a sextant. I'm sure most of us do the same thing... You hold 
    up the sextant towards the Sun's with the shades swung out of the normal 
    optical path and then starting with the darker shades, you experiment with 
    combinations until you get a comfortable low-intensity image. Then you swing 
    those shades into the optical path between the mirrors and look through the 
    telescope at the Sun. Even with a relatively high magnification scope at 7x, 
    the image of the Sun is every bit as "comfortable" to look at as it is when 
    the shades were being used "naked eye" while swung out of the optical path. 
    The image of the Sun has the same intensity of light across the disk --it is 
    only increased in total angular diameter. The telescope does not increase the 
    amount of energy or the brightness of the light on the fovea. It increases 
    the TOTAL amount of light entering the eye over a large area.
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