Reflection Grating Spectrometer

A reflection grating spectrometer works in a very similar way as an optical prism does, by spreading out the incoming light according to the wavelength of the photon. In fact, in the case of XMM, the reflection gratings are nothing more than mirrors with tiny grooves on them. The mirrors are covered with a gold layer, and there are 600 grooves per millimeter: so thin that one single human hair would cover 15 of them! Once dispersed out into a spectrum, the X-rays are focused on the two RGS cameras. These cameras share the telescope focusing plane with the EPIC cameras.

A view of an XMM-Newton Reflection Grating Array, illuminated by optical light in the same way as X-ray light would enter in flight.

XMM-Newton is the first X-ray mission to fly a reflection grating. It can identify lines of elements like iron, oxygen, silicon, and magnesium, which are common in astrophysical plasmas. Many institutions participated in the construction of the RGSs: Mullard Space Science Laboratory (MSSL), the Paul Scherrer Institute (PSI) in Villigen (Switzerland), Space Research Organisation Netherlands (SRON), and Columbia University (New York USA).