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18th of November 2021
Early Parkes image of Pictor A
The Parkes telescope was designed to be able to observe with high efficiency at 21cm, the wavelength of the astronomically immportant neutral hydrogen spectral line. But early experience indicated with could observe at shorter wavelengths (or higher frequencies). In a paper in 1965, Broten et al wrote: "Recent research in radio astronommy has shown the desirability of making observations at shorter and shorter wavelengths. Although designed originally for operation at a wavelength of 21cm, the Australian 210-ft telescope has given satisfactory performances at 11cm, and operation at still shorter wavelengths appeared possible... A wavelength of 6cm was therefore chosen for test and measurements were made with a crystal receiver having an intermediate frequency bandwidth of 10 Mc/s. ... the mean observed beamwidth is 4.1 minutes of arc, in close agreement with the theoretical value. At long wavelengths the aperture efficincy, calculated from the angular characteristics of the feed, mesh leakage and aperture blockage, is 62%. At 6cm and zero zenith angle, the measured aperture efficiency is 34.5%." Their paper included the image above of the nearby radio galaxy Pictor A. The isophotes indicate relative brightness (contour level 1.0 equals 2.6 K antenna temperature) and the direction of polarisation of the two components are also indicated. Previous ADAPs have shown more recent, higher resolution images of Pictor A, based on deep ATCA and X-ray observations on 28 Jan 2015 and 3 Nov 2015. (Image credit: Broten et al. 1965 Australian Journal of Physics, 18, 85)

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