STDWeb的多夜测光重复性：基于单天区14夜观测 campaign 的经验误差预算
Multi-night photometric repeatability of STDWeb: an empirical error budget from a 14-night campaign on a single field
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中文总结 AI 辅助
本研究以 STDWeb 测光管道的 14 夜观测数据为基础，发现其多夜测光重复性误差源于历元相关色项，通过引入色项修正可显著提升重复性并消除相关误差。
中文摘要 AI 辅助
STDWeb等基于网络的测光管道被小型天文台及专业-业余网络广泛应用于各类测光科学研究，包括变星、超新星、系外行星凌星、暂现源后续观测等领域。管道报告的正式星等不确定度仅描述单帧单次归算的结果，而 campaign 科学研究则需要同一星、同一天区在连续多夜观测中的离散度。本研究针对爱因斯坦探针触发源 EP-WXT 01709274151 天区，对 2026 年 7 月的 14 夜观测数据开展经验测量：采用 STDWeb 对 157 个类盖亚 G 波段曝光数据进行统一归算，通过 101 颗恒定天区恒星（G 星等范围 10.5<G<14.5）进行分析。正式误差的中位数为 8.1 毫等（亮星半样本为 6.7 毫等），单夜内的经验差分离散度与该值匹配（χ_within≈1）。若直接使用管道的 mag_calib（位于仪器伪波段），每颗恒星会产生 7-9 毫等的夜间偏移，campaign 离散度增至 11-12 毫等，正式误差约为真实值的一半（χ_camp=1.5-1.7）。研究表明，该全部额外误差均为与历元相关的色项：通过分析星表系统星等 m_sys = mag_calib + 色项×(BP-RP)，可将夜间项压缩至 3.4 毫等，使 χ_camp 恢复至 1.0-1.1，将以盖亚为基准的单星绝对重复性从 43 毫等提升至 9.5 毫等，夜间零点峰峰值从 130 毫等缩小至 11 毫等，残差的大气质量与色相关性消失。从光变曲线本身拟合的恒星光色可将盖亚 BP-RP 复现至 0.027 毫等（相关系数 r=0.98），且得出完全一致的误差预算：该修正无需外部色星表。研究还提供了闭合方差分解、实用建议，并对在管道外部重新推导零点的做法提出警示。
英文摘要
Web-based photometric pipelines such as STDWeb are routinely used by small observatories and professional-amateur networks across the full range of photometric science: variable stars, supernovae, exoplanet transits, transient follow-up. The formal magnitude uncertainty they report describes a single reduction of a single frame; campaign science instead requires the scatter of the same star, same field, night after night. We measure this empirically on the field of the Einstein Probe trigger EP-WXT 01709274151, observed on 14 nights in 2026 July: 157 Gaia-like G exposures reduced homogeneously with STDWeb, analysed through 101 constant field stars (10.5<G<14.5). The median formal error is 8.1 mmag (6.7 mmag for the bright half), and within a single night the empirical differential scatter matches it ($χ_{\rm within}\simeq1$). Used naively - taking the pipeline's mag_calib, which lives in the instrumental pseudo-band - each star acquires a night-level offset of 7-9 mmag, the campaign scatter grows to 11-12 mmag, and the formal error is roughly half of the true one ($χ_{\rm camp}=1.5$-$1.7$). We show that this entire excess is the epoch-dependent colour term: analysing the catalogue-system magnitude $m_{\rm sys}$ = mag_calib + colour term x (BP-RP) collapses the night term to 3.4 mmag, restores $χ_{\rm camp}=1.0$-$1.1$, and improves the absolute Gaia-anchored per-star repeatability from 43 to 9.5 mmag, with the nightly zero-point peak-to-peak shrinking from 130 to 11 mmag and the airmass and colour correlations of the residuals vanishing. Stellar colours fitted from the light curves themselves reproduce Gaia BP-RP to 0.027 mag (r=0.98) and yield an identical budget: the correction needs no external colour catalogue. We provide the closed variance decomposition, practical recommendations, and a cautionary note on re-deriving zero points outside the pipeline.
