High Efficiency Wideband Refractive Optics for ALMA Band-1 (35-52 GHz)
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We present the design, implementation, and characterization of the optics of ALMA Band 1, the lowest frequency band in the most advanced radio astronomical telescope. Band 1 covers the broad frequency range from 35 to 50 GHz, with the goal of minor degradation up to 52 GHz. This is, up to now, the largest fractional bandwidth of all ALMA bands. Since the optics is the first subsystem of any receiver, low noise figure and maximum aperture efficiency are fundamental for best sensitivity. However, a conjunction of several factors (small cryostat apertures, mechanical constraints, and cost limitations) makes extremely challenging to achieve these goals. To overcome these problems, the optics presented here includes two innovative solutions, a compact optimized-profile corrugated horn and a modified Fresnel lens. The horn profile was optimized for optimum performance and easy fabrication by a single-piece manufacturing process in a lathe. In this way, manufacturability is eased when compared with traditional fabrication methods. To minimize the noise contribution of the optics, a one-step zoned lens was designed. Its parameters were carefully optimized to maximize the frequency coverage and reduce losses. The optical assembly reported here fully complies with ALMA specifications.
KeywordsHorn antenna Dielectric lens antenna Optics
This work was supported partially by CONICYT grant CATA Basal PFB06; ALMA-CONICYT Project 31120005 and 31150012; FONDECYT projects 11140428 and 11151022; CONICYT Grant AIC-27; QUIMAL fund project 140002; and ESO-Chile Joint Committee. We thank National Instrument for the donation of software licences. Finally, we would like to especially thank José Pizarro for his support and manufacturing the horns and the lens used in this paper.
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