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ay45c3-page17 - focal plane eyepiece D δθ δθ d p...

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Unformatted text preview: focal plane eyepiece D δθ δθ d p eyeball angle δϕ l L D is the aperture; L is the focal length of the objective lens; d is the exit pupil; l is the eyepiece focal length The basic idea is that the objective makes an image at the focal plane, and the eyepiece is simply a \magnifying glass" for looking at that image. From the gure you can derive M = magni cation = ' = L :  l Notice that it is useless to have d bigger than p (the pupil size of the human eye, no greater than about 9 mm when dark adapted) because the extra rays simply do not enter the eye! Tracing the rays back, the maximum useful size of D is D = Md  Mp : Bigger than this does not make a brighter image (Liouville's theorem, again). Smaller than this does make a dimmer image. How can this be true without violating Liouville? Answer: for ray bundles of diameter smaller than p, the eye is no longer a good approximation to a \surface brightness meter" | it is arti cially \stopped down." (In bright light, of course, the eye naturally stops down on its own, when the pupil contracts.) 41 ...
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