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.,, the fiber link design (Fűrész et al.,, and the pupil slicer (Egan et al.,.

Fiber link design for the NASA-NSF extreme precision Doppler spectrograph concept "WISDOM"

“…Although it is often large telescopes (6-10m) that are equipped with such facility -but it is usually smaller telescopes that can commit much larger fraction of the observing time to a given program, which PRV observations really need and can significantly benefit from. For those smaller (2-4m) telescopes an AO system is rarely an option, but in the absence of AO pupil or image slicing can be used to decrease the instrument size and thus the overall budget (see section 2.1, [3]). …”

“…This is why the optical bench on the inside needs to be kinematically supported so such deformations do not impose stress and dimensional changes onto the optical bench. 3 But in case of an optical relay coupling an external fiber end to an internal fiber component through a window, it is important to make sure that the breathing of the vessel does not displace the vessel mounted outside collimator/optical assembly respect to the internal one. Our proposed solution completely avoids this issue, and has an inherently higher throughput than an optical relay -although often such optical coupling is the location where the double scrambler is implemented, and thus it doesn't really add throughput losses to the overall efficiency.…”

“…It is advised to use low shrinkage epoxy and/or mix the epoxy with silica powder (40-60% in weight) to match the CTE of the fiber and avoid stress induction while assembly and with changing temperature. (Although the wall of the vacuum vessel does have a <±0.1°C level temperature control 3 .) The probably more important aspect is to support the epoxy plug from the inside of the chamber, by filling epoxy to encapsulate the fiber inside of a bottomed hole that only has a tiny opening for the fiber to pass through (see Figure 9, right).…”

“…If there is one area, though, in which ways of improvements are not yet fully explored that is very cautious optical design and careful illumination control of the spectrograph. Our papers on WISDOM 3 and its optical design 4 elaborate on the former, while this paper is mainly concentrating on the latter, by describing a fiber optic link that is designed to provide high throughput and a very uniform, stable spectrograph illumination.…”

“…Thus in our response to the EPDS call the WISDOM design 3 (WIYN Spectrograph for DOppler Monitoring), while building on well established heritage and incorporating best practices, have addressed improvements of many PRV instrument subsystems. If there is one area, though, in which ways of improvements are not yet fully explored that is very cautious optical design and careful illumination control of the spectrograph.…”