PAPER: Understanding Systematic Errors Through Modeling of ALMA Primary Beams
Originally published in February 2017 in the IEEE Transactions on Antennas and Propagation, this paper analyzes the impacts of improper primary beam calibration on the ability of astronomers to make sensitive observations using the ALMA Observatory in the Atacama Desert of Chile. The primary beam defines the spatial receptivity of an individual antenna to the sky. Improper calibration of this beam can comprehensively impact observations, including distorting the shape of objects in imaging and even obscuring the existence of a source in the sky. In this analysis, we examine which common calibration errors are most impactful to an observation, and therefore when it is necessary to implement more costly calibration algorithms.
Shown here is the average imaginary component of a 12m DA-type aperture from the ALMA observatory. While the apertures analyzed in this paper are based on true measurements of the aperture illumination, they neglect the effects of support leg diffraction.
Abstract
Many aspects of the Atacama Large Millimeter Array (ALMA) instrument are still unknown due to its young age. One such aspect is the true nature of the primary beam of each baseline, and how changes to the individual primary beams affect astronomical observations when said changes are ignored during imaging. This paper aims to create a more thorough understanding of the strengths and weaknesses of ALMA through realistic modeling of the primary beams and simulated observations, which in turn can inform the user of the necessity of implementing more computationally costly algorithms, such as A-Projection, and when simpler, quicker algorithms will suffice. We quantify our results by examining the dynamic range of each observation, along with the ability to reconstruct the Stokes I amplitude of the test sources. These tests conclude that for the dynamic ranges of less than 1000, for point sources and sources much smaller than the main lobe of the primary beam, the accuracy of the primary beam model beyond the physical size of the aperture simply does not matter. In the observations of large extended sources, deconvolution errors dominate the reconstructed images and the individual primary beam errors were indistinguishable from each other.