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Light Cone

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Posted by Robert May on September 26, 2001 03:34:37 UTC

Actually, the light cone shown by NEWT and a lot of others starts out at the primary mirror as the diameter of the mirror and gradually converts to the max diameter that the EP will see. The cone really should be drawn to a point at the focal plane. This is also the point at which the aperture stop is put by the process of focusing an EP on the scope as the aperture stop is the focal plane of the EP.
FWIW, MOST EPs have an aperture stop although sometimes it's not quite obvious that it's there. Short FL EPs tend to have the aperture stop as the end of the mounting of the lens inside. I have several EPs in the 4 to 12mm range that work this way. Other EPs have a seperate ring that is either pressed or screwed into the barrel for the aperture stop.
Remember that the eye needs parallel light to be able to see anything. The EP's lenses takes the expanding cone of light from the focal plane (it's past the focal plane so it's expanding again) and turns it parallel again for your eyes. Note that the Galilian telescope uses a negative FL lens inside the focal plane to make the light parallel again.

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