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Thread: Diffraction equation for telescope lens

  1. #1 Diffraction equation for telescope lens 
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    The equation for the diffraction limit of a telescope (or lens system) is
    R = (1.22 x wavelength)/Diameter of mirror, where R is the radius of the Airy Disk.

    It surprises me that this equation is so simple. Is there a more detailed expanded form? It is my understanding that over a greater period of time, light will diffract more and more with itself, forming more interference. It is well known that the greater the diameter of a laser beam, the less the beam will diverge over an equal distance as a smaller diameter beam. Wouldn't this mean then that the length of the image beam from the objective mirror to the final imaging point on the CCD (or your eye) would further determine the resolution of the system? What if you use an aperture to reduce the light intensity at some point; would the aperture not cause more diffraction and therefore distortion? What about the diameter of the beam path after the objective mirror; surely they have to reduce the diameter of the initial light reflection (how else could it fit through the hole in the mirror of a Cassegrain reflector), but shouldn't the diameter of this beam matter like the diameter of a laser beam does?

    I was putting some thought into this because I realized that in order to fully exploit the resolution of the objective mirror you would need a large beam path, requiring that other components like all of the lenses (and beam splitters for a 3 CCD system) would need to be prohibitively large. Do they really use 1 foot diameter lenses within Hubble? I doubt it for some reason; which means that the beam path diameter does not matter....possibly because it is not coherent light?

    It could also be that 'natural' self inflicted internal diffraction within the light beam results in perfect dispersion, preventing visible airy disks from forming. Equal distortion is no distortion at all. If that is true...then only the aperture would cause non-linear distortion due to the shape of the wave coming off of the edges (like in the double slit experiment).


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