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    Re: Basics of computing sunrise/sunset
    From: Douglas Denny
    Date: 2009 Jun 19, 02:17 -0700

    I disagree with you.
    Illuminance through an optical system depends on the relative sizes of 
    objective and exit apertures. Ignoring inefficiencies of the lenses, if all 
    the light entering passes through the exit aperture (and in properly designed 
    optical system it should) then the difference in intensity in/out is in the 
    ratio of the square of the diameters of the apertures.(ie their areas).
    If this was not so - explain please why it is that stars invisible to the 
    naked eye of aperture diam 6mm - because they are too faint (intensity too 
    low to stimulate the retinal cells),   become sudenly visible with a 
    telescope of aperture say, 100 mm diam?
    The answer is in the relative light gathering of the objective aperture being 
    bigger by far than that of the eye, and all that light becomes reduced to the 
    aperture of the eye through the exit pupil of the telscope.
    Douglas Denny.
    Chcihester. England.
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