The X-ray Multi Mirror mission (XMM-Newton) is the second of the four cornerstone projects of the ESA long-term Horizon 2000 programme. It was launched on December 10, 1999. XMM-Newton contains three co-aligned high throughput telescopes with a FOV of 30 arcmin and a spatial resolution of about 16 arcsec. Two of these X-ray telescopes carry the Reflection Grating Spectrometers (RGS). SRON, as Principal Investigator, was in charge of the development and construction of the RGS spectrometer.
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 description of the instrument and the key parameters is given in the RGS User Manual.
- A 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 CCD detectors offset from the telescope focal plane. This is illustrated in the figure at the top right.
Each Reflection Grating Spectrometer consists of the following components:
Reflection Grating Array (RGA)
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.
RGS Focal-plane Camera (RFC)
The diffracted X-rays are 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 the 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 has a thin Al layer on the backside.
Each CCD is operated in the frame transfer mode. A set of 9 of these chips is oriented tangently to the Rowland circle. 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.
In order to reduce noise and dark current, the CCDs are operatored at -60 oC (begin of life), 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.
RGS 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.
RGS Digital Electronics (RDE)
The RDE includes 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, and controls the CCD temperatures and interfaces to the spacecraft. In addition, the RDE 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 an MA31750 processor. Finally, the RDE includes the power converter, providing the required secondary voltages for the other units.