Optimal Coupling of Light to Spectrographs for Precision Radial Velocities

Yale University, New Haven CT

Investigators

Abstract

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.