M.A. Holdaway [1], Scott M. Foster [1]
July 9, 1996
Keywords: compact configuration, D array, mosaicing configuration, tapered illumination, uniform illumination, mosaicing, minimum spacing, Fourier plane coverage, UV coverage, surface brightness sensitivity
We quantify the decrease in mosaic image quality when the MMA dishes have a tapered, rather than uniform, illumination. The dynamic range and image fidelity decrease by about 30% for a given minimum antenna separation for maximum entropy mosaicing, worse for maximum emptiness mosaicing. While the decreased image quality is substantial, thermal noise will usually limit the dynamic range of MMA mosaic images so we can probably live with the decreased image quality.
Very small antenna separations are required by homogeneous array mosaicing. We can either design an antenna which allows short enough spacings without the possibility of collision, or we can rely upon some safety mechanism to prevent the antennas from colliding. John Lugten has suggested that a mechanical elevation limit of 30 deg will permit the conventional antenna to have separations as small as 1.30D, where D is the dish diameter, without the possibility of antenna- antenna collisions. With no elevation limit, the antennas must be 1.48D apart to completely avoid any antenna collision. (The minimum antenna spacings with and without elevation limits depend upon the details of the antenna design, and the current conventional design does not exactly reproduce the numbers I argue from in this report.) Mosaicing simulations with an 11dB tapered dish illumination indicate that this solution to the minimum separation problem will provide pretty good mosaic image quality over the entire declination range visible from the Chilean MMA site. It is possible to modify the design further to achieve 1.25D minimum antenna separations. Antenna configurations with a minimum separation of 1.30D do not generate mosaic images which are significantly worse than configurations with a minimum separation of 1.25D, so we conclude the design which yields 1.30D is probably adequate. Configurations with a 1.48D minimum dish separation perform much worse than the 1.30D minimum spacing arrays unless they are observing at low elevations and the baselines are significantly foreshortened. With an elevation limit of 30deg, the slant-axis design has a minimum antenna separation of 1.6D, which is not acceptable for mosaicing quality or for brightness sensitivity. If the slant-axis antenna is to be used for the MMA, a collision avoidance system must be used.
Armed with the 1.30D minimum antenna separation, we are able to make rough specifications for four different mosaicing configurations which will cover all elevations down to 10deg with little shadowing and good short spacings. This can be reduced to three mosaicing configurations by only optimizing down to 15deg. Due to packing considerations, the 1.30D minimum spacing will require that the zenith Darray be expanded from 70m to about 80m. We estimate that a move between adjacent mosaicing configurations requires moving 10-15 antennas, which will take three transporters between a few hours and a day, depending upon how many obstructing antennas must be temporarily moved to remove the target antennas. We need to explore the possibility of a grid of antenna foundations rather than discreet antenna pads to provide more flexibility for the D configurations.
[1] NRAO/TUC
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Last modified: 09 December, 1999
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