HARMONI at ELT: architectural trade-off for the low order wavefront sensor subsystem

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 Reviews.

The Low Order Wave Front Subsystem (LOWFS) provides field stabilization and low-order wave front sensing in seeing-limited and LTAO observing modes, measuring the motion of the instrument focal plane relative to the telescope wave front sensors. A new set of requirements have been set for the LOWFS, expecting the micron acquisition and submicron accuracy tracking of two objects in a 400mm technical field, instead of the previous set of requirements requiring just one.

A trade-off process has been conducted to explore different architecture options. This process starts with the selection of the trade-off main criteria and metrics that will drive the decision. Among those metrics there are performance and functionality requirements, impact on cost and schedule, among others. Additionally, weights are allocated for each one of the metrics. Then, brainstorms methods have been applied to analyze the different architectures without any preconcluded assessment on each solution. A preliminary selection of 2 solutions is done and the selected architectures are further developed. Finally, a trade-off matrix is filled by experts to obtain the selected architecture, which is developed further in this paper.