Is starlight a TEM00 gaussian beam or plane wave?

[Robin Lee]

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

 

[blue_leaf77]

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).

 

[Robin Lee]

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).

Thank you! A gaussian beam can be assumed to be a plane wave when the distance it has traveled 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.

 

[blue_leaf77]

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 traveled tremendous distance from the source star, e.g. on earth.

 

[Robin Lee]

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 traveled tremendous distance from the source star, e.g. on earth.

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.