Commissioning the SALT High Resolution Spectrograph’s iodine cell

Abstract

We report on commissioning the iodine absorption cell in the High Resolution Spectrograph (HRS) on the Southern African Large Telescope (SALT). The low-, medium- and high-resolution (LR, MR and HR) modes of this fibre-fed, dual-channel, white-pupil vacuum echelle spectrograph have been in use by the SALT consortium since 2014, but the high-stability (HS) mode requires exoplanet expertise not available in our community. The original commercial HRS iodine cell was unsuitable due to an excess of iodine so it was replaced with a suitable custom-built cell. This cell was characterised at high signal-to-noise, at a resolution of 106, using the Fourier Transform Spectrometer at the National Institute of Standards and Technology before incorporation into the HRS HS bench. A combination of calibration frames and on-sky data were then used to produce an HRS-specific version of an IDL software package that derives precision radial velocities (PRVs) from spectra taken through an iodine cell. Bright stars with highly stable RVs observed during a short engineering campaign in May 2018 demonstrate that SALT HRS is currently capable of delivering Doppler precision of 4-7m/s.