The X-ray Multi Mirror (XMM-Newton) 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 6
arcsec (FWHM). 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). Each
RGS consists of an array of reflection gratings which diffracts the
X-rays to an array of dedicated charge coupled devices (CCD)
detectors. The RGS instruments achieve high resolving power (150 to
800) over a range from 5 to 35 Å [0.33 to 2.5 keV] (in the first
spectral order). The effective area peaks around 15 Å [0.83 keV]
(first order) at about 150 cm2 for the two spectrometers.
Instrument Design
The RGS design incorporates an array of reflection gratings placed in
the converging beam at the exit from the X-ray telescope. The grating
stack intercepts roughly half of the X-ray light and deflects it to a
strip of CCD detectors offset from the telescope focal plane. The
undeflected light passes through and is intercepted by EPIC-MOS in the
telescope focal plane. Nine large format back-illuminated CCDs are
operated in single photon counting and frame transfer mode at a
temperature of -80 C. For each photon, the position and the energy is
measured: the position to determine the high resolution X-ray spectrum
as diffracted by the grating module, and the energy and position to
separate the contributions from the various overlapping grating orders
(and from the in-flight calibration source) and to reduce the
background.
The instrument consists of two identical chains with the following units:
Two Reflection grating Arrays units (RGA), directly attached to
the corresponding mirror assemblies.
Two Focal Plane Camera
units (RFC), each including a stand-off structure, a radiator and the
detector itself with its front-end electronics.
Two Analogue Electronic units (RAE), containing prime and redundant functions.
Four Digital Electronic units (RDE), two for each chain.
The relevant interconnecting harness between the different units.
Relative position of various units.
Design Parameters
First order blaze wavelength (lambdablaze)
15 Å
Blaze angle of facets (delta)
0.6989°
Graze angle on facets (gamma)
2.2755°
Angle of incidence (alpha)
1.5762°
Diffraction angle for blaze (betablaze)
2.9739°
Fraction of light intercepted
0.53
Central groove density (1/d)
645.6 lines/mm
Back illuminated CCDs
9
Pixel size; 3 x 3 OCB
27 x 27 µm
CCD image area
1024 x 384 pix
Operating temperature
nominally -80 C
The grating stack consists of 182 identical gratings, mounted at
grazing incidence to the beam in the classical configuration. The
gratings are located in a toroidal surface, formed by rotating the
Rowland circle about an axis passing through the telescope focus and
the first order blaze focus.
The dispersion equation for the spectrometer is given by:
m lambda = d (cos beta - cos alpha )
where "m" is the spectral order (-1, -2, ...), d is the groove spacing,
"beta" is the angle between the outgoing ray and the grating plane,
and "alpha" is the angle between the incoming ray and the grating plane
referred to above. The gratings are fabricated to have "blazed" groove
profiles, where the facets are all tilted with respect to the grating
plane. With this orientation, the light is primarily diffracted into
the "inside" spectral orders, where "m" < 0, so that "beta" >
"alpha". The diffraction efficiency is maximized when the incident and exit
angles on the facets are equal. Because the outgoing rays reflect at
larger angles than the incoming rays, neighbouring gratings within the
array vignet part of the diffracted light. The separation between the
gratings was chosen to be as close as possible without vignetting the
blaze wavelength. This implies that only a fraction of the light
exiting the telescope is intercepted by the Reflection Grating
Array. With the criterion above, this fraction is given approximately
by sin(alpha)/sin(betablaze) = 0.53 (ignoring obstruction by
structural elements).
Each Reflection Grating Array contains 182 identical diffraction
gratings (actually, one of the two RGAs contains 181 gratings due a
problem found during installation), each measuring about 10x20 cm. The
gratings are mounted in the classical configuration, in which the
incident and diffracted rays lie in a plane which is perpendicular to
the grating grooves. Because the beam is converging, the gratings are
not parallel, and they are oriented so that the graze angle of the
incident ray which intercepts the centre of the grating, "a", is the
same for all gratings in the array. The telescope aperture is filled
by rotating the Rowland circle about an axis passing through the
telescope focus and the first order blaze focus. At any given position
the grating grooves are nearly perpendicular to this plane. So as to
make the array contiguous, the gratings are therefore slightly
trapezoidal with their long edges parallel to the local converging
light. In addition, the groove density on the gratings is not constant
across the grating surface, varying by approximately ±10%. This is
because the incident beam is converging, and aberrations would result
if the line density were fixed. The groove density is approximately
646 grooves/mm at the centre.
The large grating substrates are very thin in order to keep the
obstruction of the beam by the grating edges within reasonable
limits. Early in the programme Beryllium was selected as the substrate
material because of strength and mass considerations. However, it
turned out to be very difficult to make Be-substrates with the
required flatness tolerance, and SiC is the actual substrate
material. The substrates are 1 mm thick with five stiffening ribs at
the back running in the direction of the X-ray beam in order to
maintain the required shape. The face sheets were fabricated to 1
wave (634.8 nm) and 10 waves flatness in the long and short
direction, respectively.
Detailed view of one of the RGAs
The gratings were replicated from an identical set of master gratings
onto the thin substrates. Master gratings produced by both mechanical
ruling and by holographic means were tested and evaluated in this
programme. Although holographic masters have been produced with
somewhat higher reflection efficiency and lower scattering than
mechanically ruled masters, unfortunately no suitable variable line
density holographic master was available at the time of the flight
model production. The gratings were covered with a 2000 Å gold
coating.
The grating array support structure was machined out of a monolithic
billet of vacuum hot-pressed Beryllium. Due to the importance of
minimizing residual stress in the grating support structure,
electrical discharge machining (spark erosion) was used in the initial
machining process.
The precision alignment of the grating array was achieved by
positioning the individual gratings against four, coplanar bosses
which were precision-machined into stainless steel alignment rails
mounted to the beryllium support structure. The gratings were held in
this position by sets of spring chips. Alignment of the rails was
achieved by interferometrically measuring the orientation and flatness
of the first grating inserted for each set of rails. The array is
supported from an attachment ring at the mirror assembly by three
kinematic, titanium flexures.
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.
Thermal requirements were an important driver for the design of the
focal plane camera. Three thermal nested shells constitute the
interior of the camera; the CCD bench, heatshield 1 and heatshield
2. The CCD bench contains nine back-illuminated CCDs mounted in a row,
following the curvature along the Rowland circle. The bottom of the
CCD bench interfaces to a cold finger which provides the necessary
cooling and ensures a uniform temperature of the bench on which the
CCDs are mounted. A second, outer heatshield serves mainly for thermal
purposes and was therefore made of thinner aluminum. An un-interrupted
heat flow between the heatsink, i.e. radiator, on one end and the CCD
bench on the other was necessary in order to avoid substantial
temperature drops over this path and thus to arrive at the lowest
envisaged temperatures on the CCD bench. This configuration allows the
CCDs to be operated as low as -120 C, whereas the normal operating
temperature is -80 C.
CCD Bench
The first shield also contains four internal calibration
sources. These consist of the alpha-emitter 244Cm, and an
Al target or a Teflon target, which produces Al K alpha (1487 eV) and
F K alpha (676.8 eV) fluorescent emission. These sources each
illuminate a small area of two CCDs, which is offset in the cross
dispersion direction from the source image. The use of the sources is
twofold, not only it allow energy gain monitoring, but also monitoring
of the efficiency variation with time. The latter is particularly
important since condensable materials will accumulate on the cold
detector surface. If too much contamination build-up takes place,
e.g. ice in the early phases of the mission or possibly worse,
hydrocarbons, the CCD bench can be heated-up, to boil-off
contaminants.
The detector is configured as a strip detector along the Rowland
circle with a radius of 335 cm. In order to cover the 5-35 Å
range, a length of 253 mm was required. The nine CCD-chips are
back-illuminated GEC/EEV devices with two 384 by 1024 pixels, of 27x27
µm2 each. The width of the detector was matched to
the height of the RGS spectrum (2.2 mm for 90 % energy width) and to
the spacecraft absolute pointing error.
For optimum scientific performance, the detector is operated in the
so-called frame transfer mode: the image is first accumulated in one
half of the CCD (image section) and then quickly (20 msec) transferred
to the other half (storage section) prior to read-out through two low
noise on-chip amplifiers. This doubles the read out speed and gives
redundancy in case of an amplifier failure.
Diffusion of the primary electron cloud, during its transport from the
X-ray absorption position to the bottom of the potential well,
generates split events, i.e. events in which the charge created by one
X-ray photon is spread out over two or more pixels. Without additional
split event processing, either on board or on the ground, the
existence of split events would give rise to efficiency loss at the
low energy side of the spectrometer, degraded energy resolution and an
increased background due to the need for wider energy band
thresholds. Split event handling can be performed in two different
ways, by on-chip binning (OCB) and by split event processing. The
resolution elements of the RGS (0.25x2.0 mm) are very much oversampled
by the CCD pixelsize of 27 µm2. On-chip (noise free)
binning of up to 3x3 pixels is possible without any penalty to the
spectral resolution of the RGS and is therefore the baseline,
resulting in a reduction of the read-out time and the read-out noise
(as most X-rays will be confined to two 3x3 bins).
Since CCDs have high efficiency for detection of optical light, a
filter for the rejection of optical photons was included. This
light shield was isolated from the Si by a MgF2 isolation
layer (about 26 nm thick).The CCDs were covered with a layer of
Al whose thickness depends on their position in the array. The Al
shield is 75 nm thick for the two CCDs closest to the optical pre-amp,
68 nm for the next three CCDs, and 45 nm for the four CCDs furthest
away from the optical axis. This gives a reduction in stray light
between 105 and 102 , equivalent to 1 electron
per pixel per readout.
Analogue electronics (RAE) and Digital electronics (RDE) Units
RAE
RDE
The front-end electronics (clock-drives and pre-amplifiers for the
CCDs) is part of the detector. The instrument is under control of the
Instrument Controller (IC, MA31750 processor) which is fully
programmable. The IC sets all bias settings of the CCDs and also
configures the Clock Sequence Generator (CSG) which controls the clock
drivers for each CCD. Different clock patterns can be loaded from the
ground. The nominal readout of each CCD is through two output nodes
resulting in different gains and offsets per read-out node of each
CCD. The signals are amplified close to the focal plane camera and
digitized in the RAE using a correlated double sampler (CDS).
All data are then transferred to the Data Pre-Processor. The
temperature of the CCD bench is controlled by the RDE and this unit
includes also the power converter (Power Supply Unit). All interfaces
with the spacecraft On Board Data Handling bus (OBDH) are channelled
via the IC. During periods when RGS is switched off (immediately
following launch and during eclipses) the spacecraft provides a fixed
power level to the CCD bench to avoid too much cooling. The control
chain is fully cold redundant (hence two RDEs) whereas the signal
chain is warm redundant. In case of a failure in one of the two signal
chains it is possible to read out the CCDs through one output node. In
case of the failure of a single CCD the others will not be affected
and the second camera would provide the required redundancy (although
the effective area will then be reduced).
The RAE was produced at SRON,
and the RDEs (2 per instrument) were produced at MSSL.
