用laser频率梳测量和表征HARPS的谱线轮廓
Measuring and characterizing the line profile of HARPS with a laser frequency comb
摘要：
目标。研究了高精度径向速度行星探测器（HARPS）的二维谱线轮廓，测量了它随探测器位置和谱线强度变化的变化。谱线轮廓及其变化的特征对于达到10^{-10}或3.0 cm/s的波长尺度的精度是很重要的，这是探测类太阳恒星周围宜居带中的地球孪星所必需的。方法.我们使用了激光频率梳（LFC）与未解决的和未混合的线探测仪器线的轮廓。我们将LFC光（经各种中性密度滤光片衰减后）注入HARPS的目标光纤和参考光纤，研究了谱线轮廓随谱线强度的变化。我们应用矩分析来测量线的位置，宽度和偏斜度，以及表征由摄谱仪和探测器引起的线轮廓失真。在此基础上，我们建立了一个模型来校正点扩散函数失真，通过跟踪光束在两个光纤中的轮廓。结果我们证明了线轮廓随检测器上的位置和作为线强度的函数而变化。这与HARPS检测器上的电荷转移低效率（CTI）效应一致。谱线位置的估计主要取决于谱线轮廓，因此谱线振幅的变化会有效地改变谱线的测量位置，从而影响仪器波长标度的稳定性。我们推导并应用校正函数来重新校准和减轻这种影响，将其降低到与光子噪声一致的水平。
英文摘要：
Aims. We study the 2D spectral line profile of HARPS (High Accuracy Radial Velocity Planet Searcher), measuring its variation with position across the detector and with changing line intensity. The characterization of the line profile and its variations are important for achieving the precision of the wavelength scales of 10^{-10} or 3.0 cm/s necessary to detect Earth-twins in the habitable zone around solar-like stars. Methods. We used a laser frequency comb (LFC) with unresolved and unblended lines to probe the instrument line profile. We injected the LFC light (attenuated by various neutral density filters) into both the object and the reference fibres of HARPS, and we studied the variations of the line profiles with the line intensities. We applied moment analysis to measure the line positions, widths, and skewness as well as to characterize the line profile distortions induced by the spectrograph and detectors. Based on this, we established a model to correct for point spread function distortions by tracking the beam profiles in both fibres. Results. We demonstrate that the line profile varies with the position on the detector and as a function of line intensities. This is consistent with a charge transfer inefficiency (CTI) effect on the HARPS detector. The estimate of the line position depends critically on the line profile, and therefore a change in the line amplitude effectively changes the measured position of the lines, affecting the stability of the wavelength scale of the instrument. We deduce and apply the correcting functions to re-calibrate and mitigate this effect, reducing it to a level consistent with photon noise.
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