The fixed telescope structure is a 6.7-metre long, 3.4-metre diameter tube, which functions as a spacer to position the mirror assembly sufficiently far away from the service module so that the conical X-ray beam focussed by the mirror can pass through the centre of the service module. The term fixed denotes that the structure has no deployable component. The spacecraft’s solar arrays may be attached to the tube.

The fixed telescope structure could be manufactured from composite fibre-reinforced plastic, with a wall thickness of 1–1.5 mm and 20–40 mm tall ribs for stiffening. The materials and lay-up technique will be chosen to achieve a thermal expansion coefficient close to zero. This will maintain the focal length of the telescope in the various thermal environments encountered during ground testing and on orbit.

One of the primary design drivers for this spacecraft component is the required stiffness of the structure. The launchers impose a strict requirement on the frequency of the first bending mode.

	Above: Figure depicting different elements accomodated on the fixed metering structure. Credit: ESA Left: A finite element model of the fixed metering structure of IXO. Credit: ESA

The final constraint is that the service module must be large enough to allow the X-ray beam to pass through it. The X-ray beam exits the mirror assembly with a diameter of 3.8 metres for the silicon pore optics and tapers to around 100 mm diameter at the focal plane, some 20 metres away. The service module requires about 800 mm of space inside its walls to accommodate the avionics and propellant tanks. The maximum diameter of the service module is constrained by the launcher fairing to around four metres. Taking into account the clearance required inside each external panel, the hole for the X-ray beam can be 2.4 metres in diameter. This leads to the chosen 6.7-metre length for the fixed telescope structure.

	Mirror assembly
	Service module