Deployable telescope structure
A major science driver for the design of IXO is a long focal length, since this allows a greater photon-collecting capability at higher photon energies. A focal length of 20 metres has been selected for IXO as a balance between science requirements and engineering constraints. As no current launch vehicle is capable of accommodating a payload that is nearly 24 metres long, IXO will have a deployable structure to position the instruments at the mirror focus after launch.
Two deployment mechanisms are under consideration: extending masts (NASA and JAXA) and stowable, articulated arms (ESA).
Three masts, similar to those used to deploy the ISS solar array wings over a distance of 35 metres, will be stowed in canisters located in the service module.
As the masts deploy from their canisters, they form a repeating series of cubic bays framed by vertical members (longerons), horizontal members (battens), and diagonal cross braces. The longerons and battens are made of graphite rods and the cross braces are stainless steel cables. Motors equipped with position encoders ensure that the three masts extend synchronously to deploy the instrument module.
Three tubular arms, each with two sections and three joints, are stowed on the outside of the service module, extending over the fixed telescope structure. After launch, the arms are released and a motorised joint in the centre of each arm pushes the instrument module away from the service module.
A shroud is required between the instrument module and the service module, to prevent straylight from entering the instruments. The shroud will either surround the deployment masts or be positioned inside the deployed, articulated arms.
The shroud will consist of multi-layer insulation blankets, pleated like camera bellows. The pleats allow the shroud to be stored in a canister on the service module prior to deployment. To minimise light leaks caused by micrometeoroid penetrations, the shroud will be made up of two concentric MLI blankets separated by 100 mm, forming a 'Whipple shield'.
A small-scale prototype of the shroud, constructed at NASA's Goddard Space Flight Center, has demonstrated that IXO's 12-meter shroud can be stowed in a stack just 200 mm tall.
Several baffles will be required inside the shroud, to further block stray X-ray photons. The baffles will be attached to the inside of the shroud and deploy with it.