Mechanical components have a tendency to become dirty from the lubricants that are used during their manufacturing process, so that all sorts of organic material sticks to them. Even touching the components with a bare hand will leave behind a grease that will adhere to their surface. Of course, we avoid touching the components with a bare hand, and after machining all parts are washed. Yet even still, some of this contamination is left behind. In the vacuum of space, this “grime” slowly becomes volatilized (evaporates) from the surface it is on and can adhere to colder surfaces in the surrounding area. This presents problems if the surface it adheres to is the detector or the reflector surface. To avoid this situation, almost every device that will fly on-board the satellite is placed in a vacuum chamber and put under a high temperature over an extended period of time. In this way, the “grime” will evaporate and be removed from the components. This process is referred to as “baking out”.
This picture from ISAS is a view of the “baking out” process being carried out for the hard X-ray telescope housing (the container for reflectors). The housing is placed in the center of the vacuum chamber. After this, the door to the chamber is closed and a vacuum is realized. The telescope is then heated for an extended period of time. Once this is completed, the telescope may finally be put together.