In two XMM telescopes a Reflection Grating Array (RGA) is mounted into
the optical path. Thereby, ca. 45% of the incoming light are dispersed
onto a camera in a secondary focus for high-resolution spectroscopy.
Ca. 42% of the light are focused in the primary focus, where the
incoming radiation is registered by an EPIC camera. The remaining 13%
are absorbed by the RGA support structure (ribs).
