The initial implementation the ALMA primary beam model was done as part of the technology demonstrator work (#49). In the future we may want to consider implementing a Gaussian or parabolic primary beam to avoid larger errors near the edge (p=0.1) of the primary beam.
Current CASA implementation
ALMA : Airy disks for a 10.7m dish (for 12m dishes) and 6.25m dish (for 7m dishes) each with 0.75m blockages (Hunter/Brogan 2011). Joint mosaic imaging supports heterogeneous arrays for ALMA (Hunter/Brogan 2011).
From Todd Hunter:
Regarding the existing primary beam model in CASA, the Airy function is not the best representation of the ALMA beam. Unlike the VLA (and ngVLA) antennas, the ALMA antennas do not have shaped optics that attempt to produce a near-uniform illumination pattern (to maximize aperture efficiency). Instead, the receiver feed horn patterns illuminate the dish with a Gaussian-like pattern that drops off to -12 dB at the edge of the subreflector (see Section 5.1.2 of the 2007 TICRA report: ALMA-80.04.00.00-026-A-REP AEDM link Todd Personal Link). This produces a cleaner beam with much lower sidelobes in exchange for lower (~10%) aperture efficiency. While the scaled Airy pattern of CASA (i.e., using 10.7m for a 12m dish) reproduces the sensitivity profile to within about 1% over the p>0.2 part of the primary beam, the error increases rapidly to >~10% as you reach the p=0.1 level. For RADPS-ALMA, implementation of either a Gaussian or a parabolic primary beam shape would reduce this discrepancy. See attached plots.
See ALMA Memo 456.
The initial implementation the ALMA primary beam model was done as part of the technology demonstrator work (#49). In the future we may want to consider implementing a Gaussian or parabolic primary beam to avoid larger errors near the edge (p=0.1) of the primary beam.
Current CASA implementation
From Todd Hunter: