台大陣列七像素毫米波致冷接收機

Abstract

近年因毫米波望遠鏡的接收器靈敏度逐漸接近量子極限，前級接收機的研究以寬頻、廣角、與能觀測天文極化為主。尤其是多像素 (multi-pixel)，這種將幾十甚至幾百顆接收器放置在一起以增加接收面積的方法，能大幅加快毫米波望遠鏡的巡天速度。配備多像素接收機的干涉陣列將能同時達成高空間解析度與廣角觀測。因此，我們設計了一座寬頻 (80-116GHz)、能觀測天文極化 (dual-polarization) 的七像素前級接收機。在接收機裡，墊片極化器 (septum polarizer) 的頻寬被提升到了 42\% (77-118 GHz)，是以往的兩倍以上。極化器量測顯示其Stokes I/Q Leakage在 2\% 以內，Stokes Q/U Leakage在 1\% 以內。因墊片極化器比正交極化轉換器 (orthomode transducer) 適合觀測線型極化，配備墊片極化器的天文望遠鏡將更具優勢。同時我們也設計了一個有 40 dB 增益的 75-110 GHz 低噪音放大器，以及有 15 dB 隔離度的法拉第隔離器。最後我們針對十九像素接收機及其光學元件做了詳細的熱學計算。此論文討論了建立多像素接收機的各項細節，以為將來毫米波陣列的巡天觀測做準備。

While the current detectors in millimeter/sub-millimeter telescopes reach the quantum noise level, wide-band polarization measurement and fast mapping provide advantages. A telescope equipped with multi-pixel feeds directly increases its survey efficiency. We have developed a 7-pixel, coherent, wide-band (80-116 GHz), and polarization-capable front-end receiver, aiming for astrophysical observations. The critical receiver components include the septum polarizer and low noise amplifiers. The septum polarizer reported a 42\% bandwidth without any resonances, at least twice as wide as the previous polarizer's bandwidth. Polarizer measurements show that I to Q/U leakage is below ±2\% and the Q−U mutual leakage is below ±1\%. A single low noise amplifier provides a high gain of 40dB in 75-110GHz. We also estimate the thermal load of the 19-pixel cryostat and the receiver optics.