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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.
In this paper, we present the design and prototyping of the HARMONI Adaptive Optics Calibration Unit (AOCU). The AOCU consists of a set of on-axis sources (covering 0.5-2.4 μm) with a controllable wavefront shape. It will deploy into the instrument focal plane to inject calibration light into the rest of the system. The AOCU supports all-natural guide-star wavefront sensors for SCAO, HCAO, and LTAO.
The AOCU will be used to calibrate the WFSs, the internal interaction matrices of HARMONI, measure and compensate NCPAs between AO dichroics and the science detectors, and calibrate the pointing model zero position. The illumination assembly of the AOCU will consist of six diffraction-limited sources and a resolved source coupled into fibres. Because of the wide range of wavelengths and the spatial separations requirements, we use two endlessly single-mode fibres and a multimode fibre. In addition, several LED sources need to be coupled efficiently into the single-mode fibres. In this paper, we present the general AOCU design using off-the-shelf with a focus on the illumination and source module.
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