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HARMONI is the first light integral field spectrograph for the ELT. It includes a core 'science instrument' -- the IFS -- supported by a range of other systems, in particular adaptive optics sensors for SCAO and LTAO. The latter was, for many years, treated as an entirely separate instrument with the ELT observatory architecture. A better understanding of the technical challenges, together with a changing political and funding environment, led to merger of the two projects in 2014. The project now rates over 400FTE with a commensurately large hardware budget.
The IFS part of the instrument, at least in function, remains largerly unchanged since 2009 when the consortium completed a Phase A study as part of the (then 42m) E-ELT instrument studies. The structure of the consortium was essentially fixed then, and many firm (and soft) contractual agreements and understandings limit the flexibility to match work to product. Over the years however, as the ELT project has evolved, the design and scope of HARMONI has changed and expanded. This has brought new partners into the consortium, changed the design concept of the instrument, introduced new interfaces, and updated requirements. To further complicate matters, as of PDR (late 2017) the final scope of the project is still open due to funding uncertainties.
All of these factors have made the development of a system architecture particularly challenging. The architecture of 2009 - whilst ultimately linked to the structure of the consortium - is no longer fit for the technical purpose. A revised system architecture, and the resulting product breakdown structure, have had to be carefully adapted to satisfy a wide range of constraints. It must be solid enough to allow the project to progress clearly, but flexible enough to deal with what changes may lie ahead.
We have applied systems engineering processes to develop and architecture which is clean and robust, whilst including some inevitable compromise driven by overall project considerations. The paper will describe the processes we have followed, how the architecture has evolved, and how we have dealt with constraints and compromises forced by the existing consortium structure. We will present the baseline architecture for HARMONI, and explain how this maps onto other areas of the project and the overall instrument development process. This is an example of system architecting in the real world of moving targets and immovable obstructions.
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