Staring at the Sun with the Keck Planet Finder: The Solar Calibrator

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, disc- integrated sunlight to the recently commissioned Keck Planet Finder (KPF) at W. M. Keck Observa- tory. With SoCal, KPF acquires signal-to-noise ∼ 1200, R = 98,000 optical (445–870 nm) solar spectra in a 5 sec exposure at unprecedented cadence for an EPRV facility using KPF’s fast readout mode (< 16 sec between exposures). Comparing solar measurements 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 public EPRV solar dataset. On timescales of several hours, we find agreement at the 30–40 cm s−1 level, which is comparable to the combined noise floor of KPF and NEID. SoCal has been instrumental in assessing detector problems and wavelength cal- ibration towards optimizing KPF’s performance during commissioning. Long term SoCal operations will collect upwards of 1,000 solar spectra per six hour day using KPF’s fast readout mode, facilitating stellar activity studies on our nearest solar-type star.