Results 1 to 1 of 1

Thread: Diffraction equation for telescope lens

  1. #1 Diffraction equation for telescope lens 
    Forum Professor
    Join Date
    Apr 2007
    In the circuitous haze of my mind
    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).

    Of all the wonders in the universe, none is likely more fascinating and complicated than human nature.

    "Two things are infinite: the universe and human stupidity; and I'm not sure about the universe."

    "Great spirits have always found violent opposition from mediocrities. The latter cannot understand it when a man does not thoughtlessly submit to hereditary prejudices but honestly and courageously uses his intelligence"


    Use your computing strength for science!
    Reply With Quote  


Posting Permissions
  • You may not post new threads
  • You may not post replies
  • You may not post attachments
  • You may not edit your posts