Introduction

The X-ray Multi Mirror (XMM) mission is the second of the four cornerstone projects of the ESA long-term programme for space, Horizon 2000. The payload comprises three co-aligned high throughput telescopes with a FOV of 30 arcmin and spatial resolution of about 16 arcsec. Imaging CCD detectors are placed in the focus of each telescope. Behind two of the three telescopes, about half of the X-ray light is utilized by the Reflection Grating Spectrometers (RGS). The RGS instruments achieve high resolving power (150 to 800 in the first spectral order) over a range from 5 to 35 Ångstrom (first order). The effective area peaks around 15 Ångstrom (first order) at about 150 cm2 for the two spectrometers. A full summary of the RGS performance can be found in 'the Reflection Grating Spectrometer on board XMM' by A. Brinkman et al.

The design incorporates an array of reflection gratings placed in the converging beam at the exit of the X-ray telescope. The grating stack diffracts the X-rays to an array of dedicated charge coupled devices (CCD) detectors offset from the telescope focal plane. This is illustrated in the figure below. (Rowland circle figure).


The cooling of the CCDs is provided through a passive radiator. A full description of the instrument and key parameters is given in the 'RGS User Manual'.


Each Reflection Grating Spectrometer consists of the following components:

Reflection Grating Array

A set of 182 reflection gratings is placed in the converging beam of the telescope at grazing incidence. The dispersion equation is given by:
  m * lambda = d * (cos beta - cos alpha)
where m is the spectral order (-1, -2), d is the groove space, beta is the angle between the outgoing ray and the grating plane and alpha is the angle between the incomming ray and the grating plane. The gratings have moderate to hight efficiency in first and second order. The groove density on the gratings is not constant across the grating surface to match the convergent beam.

The gratings consist of a silicon carbide substrate coated with 2000 Ångstrom gold. The gratings are trapezoidally shaped to fill the beam of the telescope. Flatness of the gratings is obtained by 5 stiffening ribs on their backside. The gratings are integrated in a be structure. This structure is mounted on the mirror via three flextures.


The Reflection Grating Array has been produced by Columbia University

Focal plane camera (RFC)

The diffracted X-rays will be detected with a strip of CCD detectors. The separation of spectral orders is accomplished by using the energy resolution of the CCDs. In addition this energy resolution provides the means for background suppression since it is required that events have the correct pulse height, corresponding to their spatial position in the spectrum. The CCD adopts as basic element a 27.97 mm x 25.4 mm chip with 27 micron squared pixels. The quantum efficiency of these CCDs is optimized using these in a back illuminated fashion (no absorption of soft X-rays in the gate structure). To suppress optical straylight, each CCD is has a thin Al layer on top of their backside.
Each CCD is operated in the frame transfer mode. A set of 9 of these chips is oriented tangent to the Rowland circle and the dead space between the CCDs is minmized. Since the optical design is nearly stigmatic, the spectrum of a point source loaced on axis for the telescope will appear as a line running down the centres of the image sections of the CCDs.


The CCDs have been procured from EEV Ltd (England). The front end electronics and the calibration of the CCDs has been performed by SRON.

To reduce noise and dark current the CCDs are operatored at -60 oC (begin of life) and down to -130 oC (end of life). Cooling is provided through a passive radiator (two stage). The detector housing also takes care of a stable and accurate position of the CCDs. The detector housing has been procured by the Paul Sherrer Institute.


Analogue electronics (RAE)

Whereas the front end electronics takes care of the first amplification, the analogue electronics which digitizes the CCD output using a correlated double sampler and a 12 bits ADC, has been located in a separate unit (RAE). This unit also contains some control functions and a programmable clock sequence generator (CSG). This CSG controls the clocking of the charge through the CCDs allowing for a large variety of readout patterns.


The RAE has been produced at SRON.

Digital electronics (RDE)

The digital electronics contains the instrument controller (MA31750 processor) which takes care of the control of the instrument. It sets the appropriate CCD voltages, controls the readout, collects housekeeping information, controls the CCD temperatures and interfaces to the spacecraft. In addition the digital electronics contains the digital signal processing unit which receives all pixels, rejects pixels which do not contain any X-ray related information (the majority) and can perform various onboard processing steps for the other pixels. This data processing unit is also based on a MA31750 processor. Finnaly the RDE includes the power converter, providing the required secondary voltages for the other units.


The RDEs (2 per instrument) have been produced at MSSL.