[论文解读] The Herschel-PACS photometer calibration: Point-source flux calibration for scan maps

Z. Balog, Thomas Müller+10 more|Repository of the Academy's Library (Library of the Hungarian Academy of Sciences)|Sep 24, 2013

Stellar, planetary, and galactic studies参考文献 9被引用 92

一句话总结

本文利用五颗标准星（α Boo, α Cet, α Tau, β And, γ Dra）作为绝对通量参考，为赫歇尔-PACS光度计的扫描图模式提供了高精度通量定标。通过校正仪器效应——特别是蒸发器温度漂移和望远镜镜面通量变化——相对光度测量精度在蓝、绿波段达到0.5%，红波段为2%，而绝对定标的主要限制因素是恒星大气模型的5%不确定性。

ABSTRACT

This paper provides an overview of the PACS photometer flux calibration concept, in particular for the principal observation mode, the scan map. The absolute flux calibration is tied to the photospheric models of five fiducial stellar standards (alpha Boo, alpha Cet, alpha Tau, beta And, gamma Dra). The data processing steps to arrive at a consistent and homogeneous calibration are outlined. In the current state the relative photometric accuracy is around 2% in all bands. Starting from the present calibration status, the characterization and correction for instrumental effects affecting the relative calibration accuracy is described and an outlook for the final achievable calibration numbers is given. After including all the correction for the instrumental effects, the relative photometric calibration accuracy (repeatability) will be as good as 0.5% in the blue and green band and 2% in the red band. This excellent calibration starts to reveal possible inconsistencies between the models of the K-type and the M-type stellar calibrators. The absolute calibration accuracy is therefore mainly limited by the 5% uncertainty of the celestial standard models in all three bands. The PACS bolometer response was extremely stable over the entire Herschel mission and a single, time-independent response calibration file is sufficient for the processing and calibration of the science observations. The dedicated measurements of the internal calibration sources were needed only to characterize secondary effects. No aging effects of the bolometer or the filters have been found. Also, we found no signs of filter leaks. The PACS photometric system is very well characterized with a constant energy spectrum nu*Fnu = lambda*Flambda = const as a reference. Colour corrections for a wide range of sources SEDs are determined and tabulated.

研究动机与目标

• 使用明确定义的恒星标准，为PACS扫描图建立一致且均匀的通量定标。
• 识别并校正降低光度测量精度的仪器效应，特别是温度漂移和镜面通量变化。
• 量化在系统误差校正后PACS仪器的内在光度测量重复性。
• 评估绝对定标的限制因素，主要源于恒星大气模型的不确定性。
• 为用户提供参考框架，以便通过自定义还原脚本重新处理数据，确保定标精度。

提出的方法

• 使用五颗K型和M型恒星作为基准通量标准，其光球模型定义明确。
• 将PACS扫描图测得的通量与参考波长处的模型预测通量进行比较，推导校正因子。
• 利用基准星的时间序列数据，对仪器效应——蒸发器温度漂移和望远镜镜面通量变化——进行建模并校正。
• 校正过程通过将通量比值归一化至参考点（f(x)/f(c) = 1）来消除随时间变化的系统趋势。
• 使用单一、与时间无关的响应定标文件，其有效性通过未检测到老化效应或滤光片漏光现象得到验证。
• 基于恒定 νFν = λFλ = const 参考光谱，推导并列出了适用于广泛源谱能谱分布（SED）的颜色校正。

实验结果

研究问题

• RQ1 在修正仪器系统误差后，PACS光度计的内在光度测量重复性如何？
• RQ2 温度漂移和望远镜镜面通量变化如何影响扫描图观测的定标？
• RQ3 K型与M型恒星模型之间的差异在多大程度上限制了绝对通量定标的精度？
• RQ4 通过仪器修正，能否将蓝、绿波段的相对定标精度提高至1%以下？
• RQ5 恒星大气模型的不确定性对最终绝对通量定标有何影响？

主要发现

• 在应用蒸发器温度和望远镜通量变化校正后，蓝、绿波段的相对光度测量重复性提高至0.5%，红波段保持在2%。
• 五颗基准星的观测通量比值与模型预测值一致，其平均比值为1.003 ± 0.019（蓝波段）、1.005 ± 0.017（绿波段）和1.006 ± 0.018（红波段），证实了定标的一致性。
• 光度测量的内在不确定性在蓝、绿波段已低于0.5%，表明PACS热释电探测器响应具有高度稳定性。
• 在整个任务期间未检测到探测器或滤光片的老化效应，亦无滤光片漏光迹象。
• 绝对定标精度受限于恒星大气模型5%的不确定性，导致绝对通量的总不确定性保守估计为7%。
• 校正过程有效消除了蓝波段的时间依赖性趋势，证实了仪器校正模型的成功。