[论文解读] The BINGO Project III: Optical design and optimisation of the focal plane

F. B. Abdalla, Alessandro Marins+27 more|arXiv (Cornell University)|Jul 4, 2021

Radio Astronomy Observations and Technology被引用 1

一句话总结

本文提出了BINGO望远镜焦点平面的优化光学设计，采用交叉式Dragone结构中的双矩形喇叭阵列，实现了均匀、奈奎斯特采样覆盖的天区。该设计实现了-30 dB的偏振纯度、极低的旁瓣电平以及视场内的波束稳定性，从而支持从21厘米强度映射中精确测量重子声学振荡。

ABSTRACT

The BINGO telescope was designed to measure the fluctuations of the 21-cm radiation arising from the hyperfine transition of neutral hydrogen and aims to measure the Baryon Acoustic Oscillations (BAO) from such fluctuations, therefore serving as a pathfinder to future deeper intensity mapping surveys. The requirements for the Phase 1 of the projects consider a large reflector system (two 40 m-class dishes in a crossed-Dragone configuration), illuminating a focal plane with 28 horns to measure the sky with two circular polarisations in a drift scan mode to produce measurements of the radiation in intensity as well as the circular polarisation. In this paper we present the optical design for the instrument. We describe the intensity and polarisation properties of the beams and the optical arrangement of the horns in the focal plane to produce a homogeneous and well-sampled map after the end of Phase 1. Our analysis provides an optimal model for the location of the horns in the focal plane, producing a homogeneous and Nyquist sampled map after the nominal survey time. We arrive at an optimal configuration for the optical system, including the focal plane positioning and the beam behavior of the instrument. We present an estimate of the expected side lobes both for intensity and polarisation, as well as the effect of band averaging on the final side lobes. The cross polarisation leakage values for the final configuration allow us to conclude that the optical arrangement meets the requirements of the project. We conclude that the chosen optical design meets the requirements for the project in terms of polarisation purity, area coverage as well as homogeneity of coverage so that BINGO can perform a successful BAO experiment. We further conclude that the requirements on the placement and r.m.s. error on the mirrors are also achievable so that a successful experiment can be conducted.(Abridged)

研究动机与目标

• 设计BINGO望远镜焦点平面的最优配置，以确保在漂移扫描过程中实现均匀且奈奎斯特采样的天区覆盖。
• 最小化波束旁瓣和交叉极化泄漏，以实现对强度和极化信号的精确测量。
• 确保在焦点平面上的偏振纯度至少达到-30 dB，特别是在V斯托克斯参数中，以满足BAO探测的科学需求。
• 优化喇叭的定位与方向，以适应非平面焦点面，并在视场内保持波束性能。
• 通过评估机械公差（包括约10 cm的焦深）验证光学设计的稳定性，确保在运行条件下保持性能。

提出的方法

• 使用GRASP软件包对焦点平面布局进行建模，并模拟波束图案，包括两面镜的衍射效应。
• 采用神经网络与直接优化技术相结合的多目标优化方法，确定最优的喇叭位置与方向。
• 分析视场内波束响应、旁瓣电平和交叉极化泄漏，特别关注斯托克斯参数U、V和Q。
• 研究焦点面边缘处的光学像差与波束退化情况，发现最大衰减为0.5 dB，波束失真极小。
• 评估频带平均对最终旁瓣电平的影响，并确认在980–1260 MHz观测频带内波束性能保持稳定。
• 通过估算所需焦深（约10 cm）验证机械与热稳定性，以确保0.005 dB的焦点容差。

实验结果

研究问题

• RQ1 BINGO望远镜漂移扫描巡天中，焦点平面上何种喇叭排列可实现最均匀且奈奎斯特采样的天区覆盖？
• RQ2 波束旁瓣和交叉极化泄漏在视场内如何变化？是否可将其最小化以满足科学需求？
• RQ3 在交叉式Dragone结构的非平面焦点面条件下，焦点平面的最优形状与喇叭定位策略为何？
• RQ4 光学像差在多大程度上会劣化主波束与旁瓣结构，特别是在视场边缘？
• RQ5 能否在整个焦点平面上实现-30 dB的偏振纯度要求，特别是在V斯托克斯参数中？

主要发现

• 双矩形喇叭配置被确定为最优布局，确保在名义巡天时间后实现均匀且无间隙的天区覆盖，且满足奈奎斯特采样。
• 该设计在焦点平面的大部分区域实现了-30 dB的偏振纯度，唯一接近极限的区域位于δ = -25°处，此时V偏振达到-25 dBi。
• 旁瓣电平保持在可接受的低水平，波束失真局限于主瓣峰值附近几度范围内，且主瓣衰减在视场内无显著退化。
• 为维持0.005 dB的焦点容差，所需焦深约为10 cm，该值与机械公差及探测系统噪声预算兼容。
• 所有测试位置均满足椭圆度要求0.1，证实该光学设计在偏振测量中的鲁棒性。
• 波束性能在980–1260 MHz频带内保持稳定，频带平均对最终旁瓣电平的影响可忽略不计。