HARMONI at ELT: Towards a final design for the Natural Guide Star Sensors system

HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 450nm to 2450nm with resolving powers from 3500 to 18000 and spatial sampling from 60mas to 4mas. It can operate in two Adaptive Optics modes - SCAO (including a High Contrast capability) and LTAO - or with NOAO. The project is preparing for final design review (FDR). The Natural Guide Star Sensors (NGSS) system of HARMONI provides wavefront and image stabilisation sensing for each of the four observing modes of the instrument, LTAO, SCAO, HCAO, and NOAO. It consists of five subsystems, three of which provide wavefront sensing (LOWFS, SCAOS and HCM), the remaining two (ESE and ISB) providing thermal and mechanical functions. To limit thermal background and to ensure the required stability, the sensors operate in a cold, thermally stabilized, dry gas environment. This paper presents the overall design of the system with emphasis on system analysis, assembly and test, and maintenance.