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interferometers_feb16 - Interferometry Fundamentals Greg...

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Astronomy 423 at UNM Radio Astronomy Interferometry Fundamentals Greg Taylor University of New Mexico Spring 2011 based on a lecture by Rick Perley
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G. Taylor, Astr 423 at UNM Famous Interferometers
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G. Taylor, Astr 423 at UNM Outline Antennas – Our Connection to the Universe The Monochromatic, Stationary Interferometer The Relation between Brightness and Visibility Coordinate Systems Making Images The Consequences of Finite Bandwidth Adding Time Delay and Motion Heterodyning The Consequences of Finite Time Averaging
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G. Taylor, Astr 423 at UNM Telescopes – our eyes (ears?) on the Universe Nearly all we know of our universe is through observations of electromagnetic radiation. The purpose of an astronomical telescope is to determine the characteristics of this emission: Angular distribution Frequency distribution Polarization characteristics Temporal characteristics Telescopes are sophisticated, but imperfect devices, and proper use requires an understanding of their capabilities and limitations.
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G. Taylor, Astr 423 at UNM Beam Pattern Origin An antenna’s response is a result of incoherent phase summation at the focus. First null will occur at the angle where the extra distance for a wave at center of antenna is in anti- phase with that from edge. On-axis incidence Off-axis incidence
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G. Taylor, Astr 423 at UNM Getting Better Resolution The 25-meter aperture of a VLA antenna provides insufficient resolution for modern astronomy. 30 arcminutes at 1.4 GHz, when we want 1 arcsecond or better! The trivial solution of building a bigger telescope is not practical. 1 arcsecond resolution at λ = 20 cm requires a 40 kilometer aperture. The world’s largest fully steerable antenna (operated by the NRAO at Green Bank, WV) has an aperture of only 100 meters 4 times better resolution than a VLA antenna. As this is not practical, we must consider a means of synthesizing the equivalent aperture, through combinations of elements. This method, termed ‘aperture synthesis’, was developed in the 1950s in England and Australia. Martin Ryle (University of Cambridge) earned a Nobel Prize for his contributions.
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G. Taylor, Astr 423 at UNM If the source emission is unchanging, there is no need to collect all of the incoming rays at one time. One could imagine sequentially combining pairs of signals. If we break the aperture into N sub- apertures, there will be N(N 1)/2 pairs to combine. This approach is the basis of aperture synthesis. Aperture Synthesis – Basic Concept
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The Stationary, Monochromatic Interferometer A small (but finite) frequency width, and no motion. Consider radiation from a small solid angle d Ω , from direction s . X s s An antenna b c g / s b .
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