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Thread: Radio telescope beams

  1. #1 Radio telescope beams 
    Forum Freshman
    Join Date
    Nov 2008
    What is a radio telescope beam?

    Also, how do radio telescopes display their detected data - do they form images?

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  3. #2  
    Moderator Moderator Dishmaster's Avatar
    Join Date
    Apr 2008
    Heidelberg, Germany
    The beam of a radio telescope (called "PSF = point spread function" for optical telescopes) is the opening angle of the smallest resolvable angular unit. The beam size is proportional to the wavelength and inverse proportional to the diameter of the telescope. This is why radio telescopes are usually that big compared to optical telescopes. A radio telescope usually has so-called "side lobes" that "look" in directions other than the boresight. Therefore, it can also pick up signals from sources that are outside the main field-of-view. Although optical telescopes also suffer from this effect, it is generally much weaker there and can be neglected.

    The left image shows the axisymmetric beam shape in one dimension, the boresight being up. You can see the sidelobes also pointing into other directions. The distance from the origin of the coordinate system represents the gain of the antenna/telescope, i.e. the beam shape shows you the sensitivity of the telescope for a given direction. It is the highest of course towards the boresight of the telescope.

    The beam shape of an ideal telescope should of course be circular when projected on the sky. But the actual shape is often distorted because of optical misalignments or the accuracy of the mirror surface.

    With interferometers (more than one telescope linked together) the beam of a single telescope represents the field-of-view of a combined single shot, while the resolution depends on the diameter of the telescope array.

    The data can be displayed in different ways, depending on the nature of the detecting device. The final images are usually the product of a software that interprets the data. In radio astronomy, they are normally called "maps" rather than images. But they can be presented in a form very similar to optical images or spectra. While there is a general format for optical and IR images obtained with CCD cameras that is called FITS, there is no general standard format for radio data. Every data stream of every instrument is typically very unique.

    Let me know, if you need more detailed information.

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