Frequency-comb-calibrated Laser Heterodyne Radiometry for Precision Radial Velocity Measurements.

Disk-integrated observations of the Sun provide a unique vantage point to explore stellar activity and its effect on measured radial velocities. Here we report a new approach for disk-integrated solar spectroscopy and evaluate its capabilities for solar radial velocity measurements. Our approach is based on a near-infrared laser heterodyne radiometer (LHR) combined with an optical frequency comb calibration, and we show that this combination enables precision, disk-integrated solar spectroscopy with high spectral resolution (∼800,000), high signal-to-noise ratio (∼2600), and absolute frequency accuracy. We use the comb-calibrated LHR to record spectra of the solar Fe i 1565 nm transition over a 6-week period. We show that our measurements reach sub-meter-per-second radial velocity precision over a single day, and we use daily measurements of the absolute line center to assess the long-term stability of the comb-calibrated LHR approach. We use this long-duration data set to quantify the principal uncertainty sources that impact the measured radial velocities, and we discuss future modifications that can further improve this approach in studies of stellar variability and its impact on radial velocity measurements.