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Robin Lee
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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
Robin
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 said: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).
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.blue_leaf77 said: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.
A TEM00 gaussian beam is a type of laser beam that has a Gaussian intensity profile and a transverse electromagnetic (TEM) mode with the lowest possible order. This means that the beam has a smooth and symmetric intensity distribution and a single peak in the center.
A plane wave is a type of electromagnetic wave that has a constant amplitude and phase across a plane that is perpendicular to the direction of propagation. In other words, the wave fronts are flat and parallel to each other, similar to the ripples on the surface of a calm lake.
Starlight can be considered as a combination of both a TEM00 gaussian beam and a plane wave. At the source, the light emitted from a star has a gaussian intensity profile and a well-defined spatial coherence, making it behave like a TEM00 gaussian beam. However, as the light travels through the atmosphere, it gets distorted and scattered, resulting in a random phase and amplitude distribution, similar to a plane wave.
To determine whether starlight is a TEM00 gaussian beam or plane wave, we can analyze its spatial intensity distribution and coherence properties. If the intensity profile is smooth and symmetric, and the coherence length is long, then it is likely a TEM00 gaussian beam. However, if the intensity profile is random and the coherence length is short, then it is more likely a plane wave.
Understanding the nature of starlight is important for various scientific and technological applications, such as astronomy, remote sensing, and laser technology. By knowing the properties of starlight, we can better analyze and interpret astronomical data, improve imaging and communication technologies, and design more efficient and precise lasers.