The EIS structure has been jointly developed by Birmingham and McLaren Composites. It uses a panel construction to form the instrument box with all the optical subsystems mounted on a single 3.5m long optical bench. The panels are made from thin carbon fibre reinforced plastic (CFRP) laminates with a sandwich core of aluminium honeycomb. When screwed together, this construction produces a rigid and dimensionally stable telescope structure which is both extremely strong and light weight. The laminates are manufactured from a CFRP pre-preg of high modules unidirectional carbon fibres with an advanced cyanate ester resin system called RS3. The pre-preg material is produced by YLA in the USA and was chosen because it offered high stability and low out-gassing properties as well as space heritage. For EIS, the out-gassing properties of the materials is particularly important because of the wavelength working range and so a great deal of attention has been paid to minimising all sources of contamination from the structure. To provide extremely low structural distortion, each panel has been designed for a low coefficient of thermal expansion (CTE) by precisely tailoring the fibre lay-up in the laminate. Measurements carried out on representative test samples have show CTE values well within the design limits with typical values of less than 1 part per million over the working temperature range. Finally, a novel approach to attachments uses machined solid blocks of carbon fibre embedded inside the panel to provide locations for the attachment of all the various subsystems. Using this technique, extremely strong and stable location points can be produced. Attachments like this are essential if the optical elements in EIS are to retain their precise position during the high vibration loads encountered at launch. The figure above shows a 3D CAD drawing of the flight model instrument.