Staring at the Sun with the Keck Planet Finder: An Autonomous Solar Calibrator for High Signal-to-noise Sun-as-a-star Spectra
-
1.
California Institute of Technology
Abstract
Extreme precision radial velocity (EPRV) measurements contend with internal noise (instrumental systematics) and external noise (intrinsic stellar variability) on the road to 10 cm s⁻¹ "exo-Earth" sensitivity. Both of these noise sources are well-probed using "Sun-as-a-star" RVs and cross-instrument comparisons. We built the Solar Calibrator (SoCal), an autonomous system that feeds stable, disk-integrated sunlight to the recently commissioned Keck Planet Finder (KPF) at the W. M. Keck Observatory. With SoCal, KPF acquires signal-to-noise ratio (S/N) ∼ 1200, R = 98,000 optical (445–870 nm) spectra of the Sun in 5 s exposures at unprecedented cadence for an EPRV facility using KPF's fast readout mode (<16 s between exposures). Daily autonomous operation is achieved by defining an operations loop using state machine logic. Data affected by clouds are automatically flagged using a reliable quality control metric derived from simultaneous irradiance measurements. Comparing solar data across the growing global network of EPRV spectrographs with solar feeds will allow EPRV teams to disentangle internal and external noise sources and benchmark spectrograph performance. To facilitate this, all SoCal data products are immediately available to the public on the Keck Observatory Archive. We compared SoCal RVs to contemporaneous RVs from NEID, the only other immediately public EPRV solar data set. We find agreement at the 30–40 cm s⁻¹ level on timescales of several hours, which is comparable to the combined photon-limited precision. Data from SoCal were also used to assess a detector problem and wavelength calibration inaccuracies associated with KPF during early operations. Long-term SoCal operations will collect upwards of 1000 solar spectra per six-hour day using KPF's fast readout mode, enabling stellar activity studies at high S/N on our nearest solar-type star.
Copyright and License
© 2023. The Author(s). Published by IOP Publishing Ltd on behalf of the Astronomical Society of the Pacific (ASP).
Acknowledgement
We gratefully acknowledge the efforts and dedication of the Keck Observatory staff, particularly Maylyn Carvalho, Rick Johnston, Matt Barnett, Derek Park, Jerry Pascua, Steve Baca, Bobby Harrington III, Danny Baldwin, Randy Ching, Hamza Elwir, Ed Wetherell, Justin Ballard, Todd Von Boeckmann, Chris Martins, Daniel Orr, Max Brodheim, and Kyle Lanclos. We thank Gábor Kovács for designing the enclosure electronics and providing troubleshooting guidance, and Gaspar Bakos for helpful design discussions and for facilitating the acquisition of the enclosure. We thank Kodi Rider for helping to coordinate SoCal operations at SSL in Berkeley. We thank Bradford Holden and William Deich for designing the KPF data structures and helping with FITS/KTL keywords. We thank Andy Monson and Andrea Lin for useful discussions about the tracker assembly, and for providing detailed solid models of key components of the NEID Solar tracker assembly.
We extend our deepest gratitude to the Kahu Kū Mauna (Guardians of the Mountain), the Center for Maunakea Stewardship's Environmental Committee, and the Maunakea Management Board for their thoughtful review and approval of the SoCal project permit. The summit of Maunakea is a place of significant ecological, cultural, and spiritual importance within the indigenous Hawaiian community. We understand and embrace our accountability to Maunakea and the indigenous Hawaiian community, and commit to our role in long-term mutual stewardship.
R.A.R. acknowledges support from the National Science Foundation through the Graduate Research Fellowship Program (DGE 1745301). The Solar Calibrator was supported in part by the Heising-Simons Foundation through grant 2022-3931, the Simons Foundation grant "Planetary Context of Habitability and Exobiology," and the Suzanne & Walter Scott Foundation.
Some of the data presented herein were obtained at the W. M. Keck Observatory, which is operated as a scientific partnership among the California Institute of Technology, the University of California, and the National Aeronautics and Space Administration. The Observatory was made possible by the generous financial support of the W. M. Keck Foundation. This research has made use of the Keck Observatory Archive (KOA), which is operated by the W. M. Keck Observatory and the NASA Exoplanet Science Institute (NExScI), under contract with the National Aeronautics and Space Administration. The research was carried out, in part, at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004).
Facilities
Keck I: (KPF)
Software References
astropy (Astropy Collaboration et al. 2022), astroplan (Morris et al. 2018), barycorrpy (Kanodia & Wright 2018), matplotlib (Hunter 2007), pvlib (Holmgren et al. 2018), pymodbus, pytransitions, (Neumann et al. 2022), scipy (Virtanen et al. 2020), socket, websockets
Files
| Name | Size | Download all |
|---|---|---|
|
md5:22c8b223633cb1780f1bdbd6a6eebfcd
|
71.0 MB | Preview Download |
Additional details
- ISSN
- 1538-3873
- National Science Foundation
- NSF Graduate Research Fellowship DGE-1745301
- Heising-Simons Foundation
- 2022-3931
- Simons Foundation
- Suzanne & Walter Scott Foundation
- W. M. Keck Foundation
- National Aeronautics and Space Administration
- 80NM0018D0004
- Caltech groups
- Astronomy Department, Infrared Processing and Analysis Center (IPAC)