Design and Commissioning of an Iodine Cell for the ESPRESSO Spectrograph

Abstract

High resolution echelle spectrographs remain the backbone of precision Doppler radial velocity (RV) programs, detecting almost all known exoplanets within 50 pc. Precision Doppler RV spectrographs have traditionally fallen into two camps. Standard unstabilized echelles observe targets through an iodine absorption cell. The iodine spectrum is embedded on the target spectrum, and provides a wavelength scale and a record of the spectrometer point-spread-function (PSF). Super-stabilized spectrometers are placed inside a vacuum tank, temperature stabilized at the level of 0.001 deg C, and fed by two scrambled fibers. One fiber carries the target, the other the calibration source (ThAr, Fabry-Pérot, laser-comb). Both techniques have found hundreds of planets and produce sub m/s uncertainties on the highest resolution echelles currently available. Both techniques have advantages and disadvantages and can be combined with the goal of reducing the long-term Doppler RV uncertainty to the sub 10 cm/s level. We have designed, built, calibrated, and commissioned an iodine cell for the European Southern Observatory's (ESO) ESPRESSO spectrograph. The design and construction of the cell was carried out in 2022. The cell was calibrated at the National Institute of Standards Technology (NIST) Atomic Spectroscopy laboratory in early 2023 and was commissioned in May 2023. The commissioning run was limited to evening and morning twilight on VLT-UT2. Five main sequence dwarf stars ranging in spectral type from G to early K were observed between 4 and 6 nights spanning a total of ten nights.