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10th of March 2022
ASKAP looking up
As noted in a previous ADAP, ASKAP antennas are pretty big when you get up close to them! This photo from below shows the back-up supporting structure to the 12m diameter dish. What is harder to see is the novel three-axis drive system which allows the dish to be driven "up and down" (in elevation), "left and right" (in azimuth), and for the whole dish to rotate around its main axis. As ASKAP's Phased Array Feed (PAF) produces beams offset from the optical axis, it is necessary to track "parallactic" angle on the sky during an observation. This was traditionally done with an equatorial mount, at the cost of extra mechanical complexity. ASKAP instead uses a standard azimuth-elevation mount with the addition of a third axis of rotation for the reflector itself. While it would have been possible to track parallactic angle by continuously updating the electronic beamformer weights, or rotating just the PAF, the third axis exchanges a small amount of mechanical complexity for greatly reduced computational complexity at the time of both calibration and imaging, since it maintains the angular relationship of the PAF elements with respect to the support structure.


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