WISDOM: the WIYN spectrograph for Doppler monitoring: a NASA-NSF concept for an extreme precision radial velocity instrument in support of TESS

Abstract

The Kepler mission highlighted that precision radial velocity (PRV) follow-up is a real bottleneck in supporting transiting exoplanet surveys. The limited availability of PRV instruments, and the desire to break the “1 m/s” precision barrier, prompted the formation of a NASA-NSF collaboration ‘NN-EXPLORE’ to call for proposals designing a new Extreme Precision Doppler Spectrograph (EPDS). By securing a significant fraction of telescope time on the 3.5m WIYN at Kitt Peak, and aiming for unprecedented long-term precision, the EPDS instrument will provide a unique tool for U.S. astronomers in characterizing exoplanet candidates identified by TESS. One of the two funded instrument concept studies is led by the Massachusetts Institute of Technology, in consortium with Lincoln Laboratories, Harvard-Smithsonian Center for Astrophysics and the Carnegie Observatories. This paper describes the instrument concept WISDOM (WIYN Spectrograph for DOppler Monitoring) prepared by this team. WISDOM is a fiber fed, environmentally controlled, high resolution (R=110k), asymmetric white-pupil echelle spectrograph, covering a wide 380-1300nm wavelength region. Its R4 and R6 echelle gratings provide the main dispersion, symmetrically mounted on either side of a vertically aligned, vacuum-enclosed carbon fiber optical bench. Each grating feeds two cameras and thus the resulting wavelength range per camera is narrow enough that the VPHG cross-dispersers and employed anti-reflection coatings are highly efficient. The instrument operates near room temperature, and so thermal background for the near-infrared arm is mitigated by thermal blocking filters and a short (1.7μm) cutoff HgCdTe detector. To achieve high resolution while maintaining small overall instrument size (100/125mm beam diameter), imposed by the limited available space within the observatory building, we chose to slice the telescope pupil 6 ways before coupling light into fibers. An atmospheric dispersion corrector and fast tip-tilt system assures maximal light gathering within the 1.2″ entrance aperture. The six octagonal fibers corresponding to each slice of the pupil employ ball-lens double scramblers to stabilize the near- and far-fields. Three apiece are coupled into each of two rectangular fibers, to mitigate modal nose and present a rectilinear illumination pattern at the spectrograph's slit plane. Wavelength solutions are derived from ThAr lamps and an extremely wide coverage dual-channel laser frequency comb. Data is reduced on the fly for evaluation by a custom pipeline, while daily archives and extended scope data reduction products are stored on NExScI servers, also managing archives and access privileges for GTO and GO programs. Note: individual papers, submitted along this main paper, describe the details of subsystems such as the optical design (Barnes et al., 9908-247), the fiber link design (Fűrész et al., 9908-281), and the pupil slicer (Egan et al., 9912-183)