First light of a laser frequency comb at SALT

The High Resolution Spectrograph (HRS) on the Southern African Large Telescope (SALT) is a dual beam, fiber-fed echelle spectrograph providing high resolution capabilities to the SALT observing community. Its High Stability mode has a resolution of 65000, and its standard calibration sources are a ThAr lamp and an Iodine cell. The ThAr lamp’s emission lines cover two thirds of HRS’ red channel wavelength range, but with sparse and severely uneven spectral line spacing. The line intensity also ranges from only a few counts above the background noise to close to saturation. The Iodine Cell provides narrower wavelength coverage: from 500 to 620 nm (roughly a third of the red channel range), but with improved line spacing and greater homogeneity in line intensity.
Here we report on a Laser Frequency Comb (LFC) which provides complete coverage of the red channel, with comb lines uniformly separated in frequency space, with good intensity homogeneity and o ering unprecedented line occurrence (20 000 lines vs 400 ThAr lamp lines). The LFC is powered by a Nd:YVO4 laser centred at 1064 nm, which is frequency doubled to 532 nm. A mode-locked Ti:Sapphire laser oscillator, pumped by the Nd:YVO4 laser, generates sub-30 femtosecond laser pulses at a 1 GHz repetition rate and centred at 800 nm. The near-infrared pulses are then spectrally broadened into a super-continuum in the 2.7 micron core of a 50 cm long Photonic Crystal Fibre, yielding wavelength coverage from 550 to beyond 1000 nm. A Fabry-Pérot cavity then filters the laser modes such that the individual lines are 15 GHz apart, corresponding to ~10 pixels between successive comb lines on the HRS’s red detector. A narrow linewidth diode laser is stabilised to an atomic transition in a Rb gas cell (at a wavelength of 780.24nm) and co-coupled into the spectrograph with the frequency comb to provide an absolute fiducial point for wavelength calibration. The output beam of the frequency comb is fed into a 10 m transport fibre and directed into the HRS. This optical injection setup makes provision for effortlessly alternating between calibration sources: the ThAr lamp, the frequency comb, or both at the same time. The first on-sky observations were made 5 days after arrival at the telescope. Results obtained duringa week of operation include refinement of the wavelength calibration established using only the ThAr lamp, and a revised determination of the High-Stability mode’s resolution: 67000 rather than 65000. Expected results include: full calibration of the ThAr spectrum due to the possibility of obtaining frames that afford simultaneous detection of lines from the ThAr arc lamp and the LFC, improved understanding of the HRS stability and resultant radial velocity precision, and (from an engineering point of view) practical operational requirements for a LFC on a large telescope.