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Is starlight a TEM00 gaussian beam or plane wave?

  1. Jan 16, 2016 #1
    I am simulating a radio telescope and confused on what kind of source should I setup to simulate a star. Should it be a TEM00 gaussian beam or simply a plane wave?


    Cheers,
    Robin
     
  2. jcsd
  3. Jan 16, 2016 #2

    blue_leaf77

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    At astronomically far distances, the wavefront from stars can be well approximated as being spherical. Since the size of radiotelescope is much smaller than the radius of the wavefront which impinges on it, the wavefront portion received by the telescope can further be assumed as a plane wave. This wave will also be spatially coherent, as granted by van Cittert-Zernike theorem, which constitutes the fact that radiotelescope works (i.e. it does detect interference).
     
  4. Jan 26, 2016 #3
    Thank you! A gaussian beam can be assumed to be a plane wave when the distance it has travelled has become infinitely far since its radius of curvature increases proportionally to the distance it travels. Simulating a star as an infinitely far Gaussian beam in Zemax gives an awkward result though, I digress.
     
  5. Jan 26, 2016 #4

    blue_leaf77

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    I don't think it's a good idea to simulate the beam profile from stars to follow Gaussian nature, remember Gaussian optics was derived under the assumption that the beam is monochromatic and hence has perfect coherence everywhere. This is obviously not true in the case of starlight because the light emitted from stars is very incoherent at some distance near the star. The wavefront becomes coherent after it has travelled tremendous distance from the source star, e.g. on earth.
     
  6. Jan 26, 2016 #5
    You're right. Thanks for pointing that out. Now Zemax doesn't really offer a plane wave simulation for long wavelength simulation. I need to find a new tool.
     
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