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Stargazing Telescope Question

  1. Nov 30, 2008 #1
    Hi there,

    Could somebody explain how illumination - by which I mean flux per unit area - depends on the mirror diameter of a telescope and/or its focal length?

    Is this different for point objects like stars and for extended objects like nebulae?

    Thanks in advance,

    Nick :-)
  2. jcsd
  3. Nov 30, 2008 #2


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    In simple terms the larger the aperture, the more light you can capture.
    It's a terrible shame to be a little photon thqat has made it accross the universe and then hits the ground just outside your telescope lens!
    The effect of focal length and magnification is a little more complicated, assumign you are talking about a viewing telescope (ie with an eyepiece rather than a camera)
    This article is a good introduction http://www.skyandtelescope.com/equipment/basics/3077091.html?page=1&c=y Sky and Telescope is a good source of articles on amateur astronomy.
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