[论文解读] On-orbit performance of the Gaia CCDs at L2

C. Crowley, R. Kohley+20 more|arXiv (Cornell University)|Sep 14, 2016

CCD and CMOS Imaging Sensors被引用 25

一句话总结

本论文分析了盖亚探测器在拉格朗日L2点轨道上运行前两年内106个CCD的在轨性能。尽管在扫描方向上存在由辐射引起的电荷转移不完整性（CTI），但CTI退化程度比发射前预测低约一个数量级，这是由于有利的辐射条件和电子捕获位点的填充所致，从而确保了任务恒星测绘目标的高精度天体测量。

ABSTRACT

The European Space Agency's Gaia satellite was launched into orbit around L2 in December 2013 with a payload containing 106 large-format scientific CCDs. The primary goal of the mission is to repeatedly obtain high-precision astrometric and photometric measurements of one thousand million stars over the course of five years. The scientific value of the down-linked data, and the operation of the onboard autonomous detection chain, relies on the high performance of the detectors. As Gaia slowly rotates and scans the sky, the CCDs are continuously operated in a mode where the line clock rate and the satellite rotation spin-rate are in synchronisation. Nominal mission operations began in July 2014 and the first data release is being prepared for release at the end of Summer 2016. In this paper we present an overview of the focal plane, the detector system, and strategies for on-orbit performance monitoring of the system. This is followed by a presentation of the performance results based on analysis of data acquired during a two-year window beginning at payload switch-on. Results for parameters such as readout noise and electronic offset behaviour are presented and we pay particular attention to the effects of the L2 radiation environment on the devices. The radiation-induced degradation in the charge transfer efficiency (CTE) in the (parallel) scan direction is clearly diagnosed; however, an extrapolation shows that charge transfer inefficiency (CTI) effects at end of mission will be approximately an order of magnitude less than predicted pre-flight. It is shown that the CTI in the serial register (horizontal direction) is still dominated by the traps inherent to the manufacturing process and that the radiation-induced degradation so far is only a few per cent. Finally, we summarise some of the detector effects discovered on-orbit which are still being investigated.

研究动机与目标

• 评估盖亚106个大面阵CCD在恶劣L2辐射环境中的在轨性能。
• 监测并表征平行（扫描）方向和串行（水平）方向的辐射诱导电荷转移不完整性（CTI）。
• 评估读出噪声、电子偏移量以及热点像素随时间的演变稳定性。
• 利用在轨数据和校准结果验证发射前的辐射模型。
• 识别并追踪异常探测器效应（如电荷阻挡栅极和串扰），以便在数据处理中加以缓解。

提出的方法

• 利用定期在轨校准活动（包括电荷注入）实时诊断CTI的演化情况。
• 通过专用校准序列监测电子偏移量和读出噪声，并将其漂移与焦面温度变化相关联。
• 利用电离辐射追踪和两年运行期间的热点像素监测分析辐射损伤。
• 将在轨CTI测量结果与地面辐照CCD的发射前FPR（全像素响应）数据进行比较，以估算任务末期的性能。
• 利用欧洲航天局/ESTEC的盖亚试验台验证在轨观测结果，并支持缺陷像素的追踪。
• 将结果整合进全局校准流程，以校正科学数据中的非均匀性和系统性偏差。

实验结果

研究问题

• RQ1 与发射前预测相比，L2点的辐射对盖亚CCD在扫描方向上的电荷转移效率（CTE）退化程度如何？
• RQ2 串行寄存器中的辐射诱导陷阱如何影响CTI？制造相关的缺陷与空间辐射的贡献分别是什么？
• RQ3 读出噪声和电子偏移量随时间的演变如何？它们与温度变化有何关联？
• RQ4 太阳质子事件和宇宙射线如何影响星上自主检测链路和探测器性能？
• RQ5 热点像素形成和电离辐射损伤的长期趋势如何？它们是否对数据质量构成威胁？

主要发现

• 所有106个CCD的读出噪声性能保持优异，前两年内未观察到显著退化。
• 电子偏移量稳定，观测到的漂移与焦面温度变化高度相关。
• 定期校准活动成功缓解了线内读出依赖的偏移非均匀性，已执行九次。
• 通过定期电荷注入，清晰诊断出平行扫描方向的辐射诱导CTI，仅发现两次与太阳质子事件相关的阶梯式上升。
• 由于太阳活动水平较低且电子捕获位点已填充，预计任务末期扫描方向的CTI将比发射前预测低约一个数量级。
• 串行寄存器CTI仍主要由预先存在的制造缺陷陷阱主导，辐射诱导退化对天体测量器件仅占1–3%，对较厚的红光探测器则为3–7%。