HARMONI at ELT: characterizing spatial variations in transmission efficiency of volume phase holographic diffraction gratings

HARMONI is the first light visible and near-IR integral field spectrograph for the ELT. It covers a large spectral range from 470nm 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 Reviews. The diffraction gratings are the single biggest contributor to the instrument's throughput loss. Additionally, the range of operating modes and configurations available to HARMONI users will result in a varying beam footprint at the grating surface. HARMONI's four spectrographs will be equipped with 10 gratings each, requiring a bespoke automated test facility to qualify the transmission performance of all the HARMONI gratings. This paper describes the test bench, and presents transmission efficiency results for different HARMONI gratings.