Flight Photon Counting EMCCD Development for the Roman Space Telescope Coronagraph Instrument

We describe the development of flight EMCCDs (electron multiplying charge coupled devices) for the photon counting camera system of the JPL Coronagraph Instrument (CGI) to be flown on the 2.4~m Nancy Grace Roman Space Telescope. Roman is a NASA flagship mission that will study dark energy and dark matter, and search for exoplanets with a planned launch in the mid-2020s. The CGI is intended to demonstrate technologies required for high contrast imaging and spectroscopy of exoplanets such as high speed wavefront sensing and pointing control, adaptive optics with deformable mirrors, and ultra-low noise signal detection with photon counting, visible-sensitive (350-950 nm) detectors. The camera system is at the heart of these demonstrations and is required to sense both faint and bright targets ($10^{-4}$ - $10^7$~counts-s$^{-1}$) adaptively at up to 1000 frames-s$^{-1}$ in order to provide the necessary feedback to the instrument control loops. The system includes two identical cameras, one to demonstrate faint light scientific capability, and the other to provide high speed real-time sensing of instrument pointing disturbances. Our program has evaluated the low signal performance of radiation damaged commercial EMCCD sensors and used those measurements as a basis for targeted radiation hardening modifications developed in partnership with the Open University (Milton Keynes, UK) and Teledyne-e2v (Chelmsford, UK). A pair of EMCCDs with test features was then developed and their low signal performance is reported here. The program has resulted in the development of a flight version of the EMCCD with low signal performance improved by a factor of 3.8 over the commercial one after exposure to $2.6\times 10^{9}$ protons-cm$^{-2}$ (10 MeV equivalent). The flight EMCCD sensors are contributed by ESA through a contract with Teledyne-e2v (Chelmsford, UK). In this paper, we will describe the program requirements, sensor design, test results and metrics used to evaluate photon counting performance.

本文介绍了为搭载于2.4米南希·格蕾丝·罗曼空间望远镜的JPL日冕仪（CGI，Coronagraph Instrument）光子计数相机系统所开发的飞行用电子倍增电荷耦合器件（EMCCD，electron multiplying charge coupled devices）。罗曼望远镜是美国国家航空航天局（NASA）的旗舰任务，计划于2020年代中期发射，旨在研究暗能量与暗物质，并搜寻系外行星（exoplanets）。CGI旨在演示系外行星高对比度成像（high contrast imaging）与光谱学（spectroscopy）所需的技术，例如高速波前传感（wavefront sensing）与指向控制、带变形镜的自适应光学（adaptive optics），以及基于光子计数（photon counting）的超低噪声信号探测与可见光敏感（350-950 nm）探测器。该相机系统是这些演示的核心，需能自适应地探测微弱与明亮目标（10⁻⁴至10⁷计数/秒），帧速最高可达1000帧/秒，以向仪器控制回路提供必要反馈。系统包含两台相同的相机：一台用于演示弱光科学探测能力，另一台用于高速实时传感仪器指向扰动。我们的项目评估了受辐射损伤的商用EMCCD传感器的低信号性能，并以这些测量结果为基础，与英国米尔顿凯恩斯的开放大学（Open University）及英国切尔姆斯福德的Teledyne-e2v合作开发了针对性的抗辐射加固（radiation hardening）改进方案。随后，我们开发了一对带有测试功能的EMCCD，并在此报告其低信号性能。该项目成功开发出飞行版EMCCD，在经受2.6×10⁹质子/平方厘米（10 MeV等效）辐射后，其低信号性能较商用版本提升了3.8倍。飞行用EMCCD传感器由欧洲空间局（ESA）通过与英国切尔姆斯福德的Teledyne-e2v签订合同提供。本文将详细介绍项目需求、传感器设计、测试结果，以及用于评估光子计数性能的指标。