The single laser adaptive optics system for the METIS

METIS, the Mid-IR instrument for the ELT will be operating an internal Single Conjugate Adaptive Optics System (SCAO), which will be the work horse AO system and mainly serve the science cases targeting exoplanets and disks around bright stars. In order to extend the sky coverage and brightness range of targets requiring AO correction to fainter stars, a Single Laser Adaptive Optics (SLAO) system is proposed. Although SLAO systems are currently in operation on 8-10 meter class (and smaller) telescopes, extending SLAO systems to the ELT increases the challenges associated with the cone effect and the spot elongation significantly. But since METIS will be operated at L-band, the requirements on required AO correction are fortunately reduced (with respect to other ELT instruments), making a SLAO system an attractive low-cost option for METIS. The METIS SLAO system will operate using an on-axis Laser Guide Star (LGS) and re-use the internal SCAO WFS for field stabilization and low-order correction (truth sensing), further reducing cost and complexity. In this paper we will present the current state of the design of the METIS SLAO system, and will address the challenges, like the impact of the cone effect, spot elongation and design constraints on the system. We will show that this system will likely provide a >60% SR in L-band over >50% of the sky, providing an attractive addition to the METIS SCAO system.