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Within the Space Situational Awareness Programme (SSA), ESA has initiated the assessment of two missions currently considered for implementation to enable enhanced space weather monitoring capabilities of ESA. These missions will position satellites at the Lagrangian points L1 and L5. Phases A and B1 with a focus on the L5 mission are now completed and the Bridging Phase studies are now initiated. The Lagrange mission to L5 considers four remote sensing optical instruments and several in-situ instruments to analyze the Sun, the energetic particle streams, the magnetic field and solar wind conditions, in order to provide early warnings of increased solar activity. Within this frame, an Extreme UltraViolet Imager (EUVI) instrument is under study to image the full solar corona. The overall Remote Sensing Instruments study will be led by RAL (GB). In particular, the EUVI study is carried out by three institutes - CSL (BE), ROB (BE) and PMOD/WRC (CH) - and is built upon the heritage of previous projects such as PROBA2/SWAP, ESIO and SolO/EUI to which CSL contributed. The work is shared between CSL, ROB and PMOD with the following responsibilities: - CSL: overall management, system study, optical engineering, thermal engineering, - PMOD: electrical engineering, mechanisms and mechanical engineering - ROB: instrument requirements, instrument operation, ground segments Several EUV imaging telescopes have been developed for scientific solar spacecraft, e.g. SOHO/EIT, TRACE, Proba-2/SWAP, STEREO/SECCHI-EUVI, SOLAR ORBITER/EUI. Another study called ESIO, based on an ESA General Support Technology Programme activity (GSTP), was also performed until 2015 jointly by the Centre Spatial de Liège (CSL) and the Royal Observatory of Belgium (ROB). The name of the instrument designed in that frame was the EUV Solar Imager for Operations (ESIO). The ESIO instrument was intended to provide solar EUV images for operational use as part of the ESA SSA (Space Situational Awareness) programme. The ESIO design is a low resolution, low data rate, compact solar EUV imager associated to a flux monitor for operation use in space weather monitoring and forecasting. The EUVI instrument intends to image the full solar disc, in a single spectral line as the requirement and multiple lines as a goal. The idea for this instrument is to combine heritages from SWAP, ESIO and SolO/EUI to benefit from the experience CSL gained during those projects. The optical design of its telescope originates from PROBA-2/SWAP: it consists of a two-mirror telescope imaging the Sun to a focal plane assembly. The actual structural solution also benefits from the SWAP heritage: a simple optical bench onto which the optical elements will be attached. Within Lagrange, the global philosophy is indeed to re-use as much as possible heritage from previous missions and limit the amount of new development. The previous accommodation of SWAP has however been updated to fit with the new optical design characteristics and benefits from some updates coming from SolO/EUI. The spectral selection is achieved with a set of aluminum foil filters, together with EUV reflective multilayer coatings deposited on the mirrors. The overall stack is specifically designed to provide reflectivity in the extreme ultraviolet range, and to achieve the spectral selection in the selected narrow band pass (1.5 nm at full width half maximum). The accuracy on central wavelength adjustment is within +/- 0.2 nm. The focal plane assembly comprises an EUV CMOS detector coming from the spare list of SolO/EUI. A decontamination heater located on the sensor will be used periodically to outgas the condensed layer that builds up on the cold sensor. Cleanliness of such EUV instrument is indeed critical as molecular deposits can absorb EUV radiation and dramatically decrease photon throughput. The focal plane assembly also includes the proximity electronics (FEE) required to readout the sensor and a thermal link to the cold space for the detector cooling down. An overview on the technical development and decisions taken will be presented during the presentation to highlight the tight relation between the design selection, user requirements, interfaces and cost/schedules aspects.
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