Optimal Coupling of Light to Spectrographs for Precision Radial Velocities

探测其他恒星周围的行星的一种经典且极具成效的技术是测量它们的引力赋予母星的微小反射运动。这种运动的特征是恒星光谱中存在微小的径向速度漂移。然而，要探测最有趣的岩石(或类地)行星(2010年《新世界，新地平线》十年调查列出的三大科学优先事项之一)，需要将测量精度提高到目前技术水平的十分之一，目前的测量精度约为每秒1米。因此，要探测到一颗可与我们的地球相媲美的行星，就需要对系统误差源进行特殊控制。这份名为《新世界，新地平线》的报告确实建议改进径向速度技术，将其作为陆基系外行星研究的首要任务。耶鲁大学的D.Fischer和他的同事正在研究这个问题的多个方面，涉及确定来自望远镜的光进入光谱仪的最佳耦合，光谱仪实际上是测量感兴趣的径向速度的仪器。为了控制光纤的照明变化，该光纤首先接收来自望远镜的光，然后将其导入光谱仪，八角形光纤将被研究其相对于狭缝或圆形光纤的输入模式的优越“扰乱”。以这种方式进行置乱会使输出强度对输入照明的细节不那么敏感。将对这一影响和其他影响进行彻底研究，例如如何将图像分割成不同的部分进行光谱分析，在全面研究完成后，预计结论将大大减少径向速度行星发现的系统误差源。将使用智利Cerro Tololo 1.5米望远镜上的高度稳定的交叉分散梯形光谱仪Chron以及夏威夷莫纳克亚Keck天文台的HiRes光谱仪对这项工作提出的建议进行现场测试。然而，这一结果将对所有以极高速度精度为目标的光谱仪产生影响。这项工作的资金是由NSF的天文科学部通过其先进技术和仪器计划提供的。

A classic and extremely productive technique for detecting planets around other stars is to measure the tiny reflex motion their gravity imparts to the parent stars. The signature of that motion is the presence of small radial velocity shifts in the stellar spectra. However, to detect the most interesting rocky (or earthlike) planets (one of the top three scientific priorities laid out by the 2010 Decadal Survey "New Worlds, New Horizons") will require an improvement in measurement precision by a full factor of ten over the current state of the art, which is roughly 1 meter per second. Extraordinary control of systematic error sources will therefore be required to detect a planet comparable to our Earth. The "New Worlds, New Horizons" report indeed recommended improvement of radial velocity techniques as the highest priority for ground-based exoplanet research.Dr. D. Fischer and coworkers at Yale University are investigating a number of aspects of the problem that involve determining the optimal coupling of light from the telescope into the spectrograph, the instrument that actually measures the radial velocity of interest. To control variations in illumination of the optical fiber that first accepts light from the telescope and then channels it into the spectrograph, octagonal fibers will be investigated for their superior "scrambling" of input modes relative to a slit or round fiber. Scrambling in this way makes the output intensity less sensitive to details of the input illumination. A thorough study of this and other effects, such as how the image is sliced into different contributions for spectral analysis, will be performed, and when the comprehensive study is completed, the conclusions are expected to lead to substantial reductions in the systematic sources of error for radial velocity planet finding. In situ tests of the recommendations emerging from this work will be carried out using the highly stable cross-dispersed echelle spectrometer CHIRON on the Cerro Tololo 1.5 meter telescope in Chile, as well as on the HIRES spectrometer at Keck Observatory on Mauna Kea, Hawaii. However, the results will have implications for all spectrographs targeted for extremely high velocity precision. Funding for this work is being provided by NSF's Division of Astronomical Sciences through its Advanced Technologies and Instrumentation program.