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22nd of February 2022
Another Lunar Occultation of 3C273 at Parkes!
Sixty years ago, the Parkes telescope played a critical role in the discovery of quasars, using a series of lunar occultations of the bright radio source 3C273 to pin-point its position, allowing the identification with an optical object. What had been assumed to be a star in our galaxy turned out to be a galaxy over 2 billion light years away, powered by a supermassive black hole at its centre. (See the Parkes 60th webpage for more information and reminiscences about that work.) Such lunar occultations are relatively infrequent, but there was one last weekend, and the Parkes observations were re-created, right down to the use of a chart recorder to plot the occultation in real time. Rather than the radio source blinking out as the moon moves in front of it, what is seen is the Fresnel diffraction pattern -- an increasing (on immersion) or decreasing (on emersion) sinusoidal pattern of fringes. The figure above shows a comparison of the immersion from August 1962 (inset) with the emersion recorded on the weekend - note that time increases from right to left on both chart recordings. Both traces clearly show both the core and jet components of 3C273. The event was recorded and is available on youtube. (Fast forward to the 4:10 mark for proceedings to start.)

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