[论文解读] A New Forced Photometry Service for the Zwicky Transient Facility

Frank J. Masci, Russ R. Laher+7 more|arXiv (Cornell University)|May 25, 2023

Spectroscopy and Laser Applications被引用 11

一句话总结

本论文描述 ZTF 强制光度测量服务（ZFPS）、其批处理工作流、架构以及光度测量方法，用以从差分图像生成可发表光曲线。

ABSTRACT

We describe the Zwicky Transient Facility (ZTF) Forced Photometry Service (ZFPS) as developed and maintained by the ZTF Science Data System Team at IPAC/Caltech. The service is open for public use following a subscription. The ZFPS has been operational since early 2020 and has been used to generate publication quality lightcurves for a myriad of science programs. The ZFPS has been recently upgraded to allow users to request forced-photometry lightcurves for up to 1500 sky positions per request in a single web-application submission. The underlying software has been recoded to take advantage of a parallel processing architecture with the most compute-intensive component rewritten in C and optimized for the available hardware. The ZTF processing cluster consists of 66 compute nodes, each hosting at least 16 physical cores. The compute nodes are generally idle following nightly real-time processing of the ZTF survey data and when other ad hoc processing tasks have been completed. The ZFPS and associated infrastructure at IPAC/Caltech therefore enable thousands of forced-photometry lightcurves to be generated along with a wealth of quality metrics to facilitate analyses and filtering of bad quality data prior to scientific use.

研究动机与目标

• 动机：在 ZTF 数据中需要强制光度测量，以回收低于单次观测检测阈值的测量。
• 描述将 ZFPS 设计、部署与作为批处理服务的运行。
• 详细说明用于生成光曲线的光度测量方法、性能考量和数据质量指标。
• 为提交请求、检索结果和构建可发表光曲线提供用户指南。

提出的方法

• 批量提交工作流，单次请求可处理高达 1500 个天空位置。
• 基于 C 的多线程 PSF 拟合光度测量，以及对差分图像 stamps 的孔径光度测量。
• 将 PSF 与 stamp 进行五倍放大，以实现更精确的质心定位与通量提取。
• 质量标志、基线校正及不确定性验证，以确保可靠的光曲线。
• 将差分通量转换为标定等效星等，包含基线处理与上限计算。

Figure 1: (a) Number of forced photometry requests submitted versus calendar date since January 1, 2021. (b) Number of forced photometry lightcurves generated as a function of lightcurve time-span, also from requests submitted since January 1, 2021.

实验结果

研究问题

• RQ1 如何将强制光度测量扩展至在 ZTF 数据中每次提交处理大量天空位置？
• RQ2 在 ZTF 集群上规模化处理强制光度光曲线所需的性能、内存和计算资源？
• RQ3 差分图像的差分通量测量应如何处理、验证并转换为可用于科学用途的校准星等？
• RQ4 为了从 ZFPS 产出可靠的光曲线，需要哪些质量度量和基线校正？
• RQ5 对于提交、监控和检索 ZFPS 结果的用户，实际操作指南有哪些？

主要发现

• ZFPS 现已支持每次提交最多处理 1500 个天空位置 的批处理，相较初始版本有显著吞吐量提升。
• 该服务在一个 66 节点集群上运行，每个节点至少 16 核，利用并行处理以及用于 PSF 拟合光度测量的 C 内核。
• 光度测量在 25x25 像素的差分图像 stamps 上进行，放大至 125x125，输出中包含 PSF 拟合通量、通量不确定性和孔径光度。
• 强调质量过滤、基线校正和不确定性验证，以生产可信的光曲线，包括通过上限处理非探测。
• 光曲线包含每时段状态码和元数据，便于进行稳健筛选与分析以用于发表质量的用途。

Figure 2: ZFPS data flow and system architecture. Major components consist of a compute cluster managed by SLURM; an operations database for retrieving image inputs and metadata; a SQLite database for storage of ZFPS products and a web-server for serving them to users.