HARMONI: first light spectroscopy for the ELT: simulating the alignment of a three-mirror anastigmat

HARMONI is the first light visible and near-infrared (450 – 2450 nm, λ/Δλ = 3,000 – 17,000) integral field spectrograph for the European Southern Observatory’s Extremely Large Telescope. The path for scientific light includes pre-optics to set the field of view (4 – 60 mas), an integral field unit to reformat the rectangular field into four 540 mm long slits, and four spectrographs which collimate, disperse, and image the light. The spectrographs collimate the input slits using a system of three off-axis, aspheric mirrors which lack a common optical axis. We have demonstrated the alignment procedure of this three-mirror anastigmat in numerical simulations. We will initially align the collimator mirrors by measuring reference features with a coordinate measuring machine (CMM). We will then use an interferometer to measure the wavefront error (WFE) at positions along the input slit. These WFE measurements will be entered into a numerical model to optimize available compensators, and the corresponding adjustments will be applied to the collimator. This process of measuring WFE, using a model to determine optimal compensator adjustments, and applying the adjustments will be repeated until the performance has converged. As an initial demonstration of this method, we have simulated the full alignment procedure. The modelled systems include manufacturing errors, account for the uncertainty of values reported by mirror manufacturers, and simulate the CMM and interferometer alignment steps. Nearly all of 1000 instances meet our requirements after two iterations of the optical alignment, and all were found to satisfy our requirements after five iterations.