A new, simple method for fabricating high performance sub-mm focal plane arrays by direct machining using shaped drill bits.

摘要

Large focal-plane arrays consisting of tens, hundreds or thousands of elements will dominate the next generation of sub-mm mapping instruments. While "CCD-style" filled-aperture bolometer arrays, such as SCUBA-2, are suitable for some mapping applications, heterodyne and sub-mm CMB polarisation mapping receivers, such as HARP-B and Clover, continue to consist of close packed arrays of feed horns. Such feed horn arrays are popular since they offer the high aperture efficiencies, low sidelobes, low stray light sensitivities and low-cross polarisations required for many astronomical applications. The problem of fabricating large numbers of high quality feeds at a low cost is becoming increasingly acute, and the cost of horn array fabrication is rapidly becoming a large fraction of the total instrument cost. In previous work, we have described novel multi-flare angled smooth-walled horns designed using a genetic algorithm. These horns have one or two flare-angle discontinuities and promise to be much easier to machine than corrugated horns at sub-mm wavelengths and yet offer similar radiation patterns. We have previously measured very good beam patterns for eJectroformed prototypes of these horns across a 17% bandwidth at 230 GHz. In this paper, we describe our fabrication process for these horns by direct machining into a block of aluminium using shaped drill bits. This is a new fabrication technique and is very rapid compared to the electroforming or direct machining of corrugated horns. We describe the construction of prototype 230 GHz horns using this technique and present experimentally measured beam patterns. We also present photographs of a split prototype horn and discuss the overall machining quality achieved.