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Radio Astronomy

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Author Topic: Radio Astronomy  (Read 91 times)
Jennie McGrath
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« Reply #15 on: August 16, 2007, 11:16:39 pm »

Interferometric imaging in astronomy

Interferometry provides access to very-high-angular-resolution observations. It also importantly separates the issues of angular resolution and limiting sensitivity. A single mirror of diameter D has an angular resolution 1.22λ / D, and a collecting area for the flux of photons D2, so that there is a well-de­ned relation between resolution and sensitivity. Since astronomers necessarily study objects beyond their control, it is unlikely that this ­fixed relationship will be a good match to all but a subset of their observational requirements. It was for this reason that the growth of radio astronomy after 1945 depended so dramatically on the use of interferometric methods. The extreme imbalance between the excellent sensitivity of small ­filled apertures at long radio wavelengths and their poor angular resolution, which could be many tens of degrees, led naturally to the development of sparse arrays of widely separated telescopes.

In 1946 Ryle and Vonberg (Ryle and Vonberg 1946) constructed a radio analogue of the Michelson interferometer and soon located a number of new cosmic radio sources. The signals from two radio antennas were added electronically to produce interference. Ryle and Vonberg's telescope used the rotation of the Earth to scan the sky in one dimension. Fringe visibilities could be calculated from the variation of intensity with time. Later interferometers included a variable delay between one of the antennas and the detector as shown in the figure at the right.

In the figure radio waves from a source at an angle θ to the vertical must travel a distance δl further in order to reach the left-hand antenna. These signals are thus delayed relative to the signals received at the right hand antenna by a time cδl = casin[θ] where c is the speed of the radio waves. The signal from the right hand antenna must be delayed artificially by the same length of time for constructive interference to occur. Interference fringes will be produced by sources with angles in a small range either side of determined by the coherence time of the radio source. Altering the delay time δt varies the angle at which a source will produce interference fringes. The effective baseline of this interferometer will be given by the projection of the telescope positions onto a plane perpendicular to the source direction. The length of the effective baseline, shown at the bottom of the figure, will be

x = acos(θ)
where a is the actual telescope separation.

Technical difficulties delayed the growth of optical interferometry. The human eye is a sensitive detector but is not capable of quantitative photometric assessment of interferometric fringe patterns. When coupled with the need to record data at kHz rates, progress in optical synthesis imaging had to wait for the development of sensitive photon-counting detectors. Furthermore, the limits of mechanical stability had been reached with the 50 ft (ca. 15.2 m) beam interferometer constructed by Michelson and Pease in 1930 (Pease 1931). This method was extended to short-wavelength measurements using separated telescopes by (Johnson, Betz and Towns 1974) in the infrared and by (Labeyrie 1975) in the visible. This demanded micrometre-level metrology of variable optical delay lines, which was not feasible until access to stabilized lasers had become routine. In the 1980s the aperture synthesis technique was extended to visible light and infrared astronomy by the Cavendish Astrophysics Group, providing the first very high resolution images of nearby stars. In 1995 this technique was demonstrated on an array of separate optical telescopes for the first time, allowing a further improvement in resolution, and allowing even higher resolution imaging of stellar surfaces. The same imaging techniques have now been applied at a number of other astronomical telescope arrays, including the Navy Prototype Optical Interferometer, the the CHARA array, the IOTA array and will soon be applied at the VLTI and MRO Interferometer. A detailed description of the development of astronomical optical interferometry can be found here. Impressive results were obtained in the 1990s with COAST and NPOI producing many very high resolution images. Some scientists exaggerated the benefits of combining large diameter (adaptive optics corrected) telescopes for near-infrared interferometry, and this left many astronomers disappointed with new arrays utilizing small numbers of large telescopes which came online in the early 2000s. For details of individual instruments, see the list of astronomical interferometers at visible and infrared wavelengths.

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