  The effects of the antenna far-field power pattern limits the imaging
performance of modern wide-bandwidth, high-sensitivity interferometric radio
telescopes. Given a model for the aperture illumination pattern (AIP) of the
antenna, referred to as the A-term, the wide-band (WB) A-Projection algorithm
corrects for the effects of its time, frequency, and polarization structure.
The level to which this correction is possible depends on how accurately the
A-term, represents the true AIP. In this paper, we describe the A-Solver
methodology that combines physical modeling with optimization to holographic
measurements to build an accurate model for the AIP. Using a parametrized
ray-tracing code as the predictor, we solve for the frequency dependence of the
antenna optics and show that the resulting low-order model for the Karl G.
Jansky Very Large Array (VLA) antenna captures the dominant frequency-dependent
terms. The A-Solver methodology described here is generic and can be adapted
for other types of antennas as well. The parameterization is based on the
physical characteristics of the antenna structure and optics and is therefore
arguably a compact representation (minimized degrees of freedom) of the
frequency-dependent structure of the antenna A-term. In this paper, we also
show that the parameters derived from A-Solver methodology are expected to
improve sensitivity and imaging performance out to the first side-lobe of the
antenna.
