Analysis of ALMA Optical System Designs

The Atacama Large Millimetre/submillimetre Array (ALMA) has been instrumental in advancing astrophysics, particularly in studying black holes and other cosmic phenomena. A key factor in ALMA's performance is the design of its receiver optics, which must maintain efficiency across a broad frequency range. The initial frequency-independent method for designing these optics proved inadequate, prompting the ALMA group to develop a revised approach that recalculates receiver optics parameters for improved accuracy. Both methods relied on the Gaussian approximation to estimate the field at the sub-reflector. However, the initial approach assumed the Gaussian solution to be exact, leading to inaccuracies in practical implementation. In contrast, the revised method considered a range of possible values around the Gaussian approximation, systematically selecting optimal parameters to enhance accuracy. This approach ensured proper Gaussian beam propagation from the feed horn aperture to the sub-reflector while optimizing edge taper and efficiency. Further refinements in mirror spacing minimized focal point deviations across the frequency band. To validate the revised design, physical optics techniques, including mode-matching and scalar wave propagation, were employed to accurately model beam propagation and assess performance. This paper presents a comparative analysis of the two receiver optics design methodologies, highlighting the shortcomings of the initial method and the effectiveness of the revised approach. By examining these strategies, we provide insights that can guide future advancements in astronomical instrumentation and high-sensitivity receiver design.