The Constellation-X reflection grating spectrometer

Jean Cottam^a^, Webster Cash^b^, Kathryn A. Flanagan^c^, Ralf K.
Heilmann^c^, Gregory Y. Prigozhin^c^, Andrew P. Rasmussen^d^, George R.
Ricker^c^, Mark L. Schattenburg^c^, Eric Schindhelm^b^

^a^NASA/Goddard Space Flight Center, X-ray Astrophysics Laboratory,
Greenbelt, MD, USA 20771;

^b^University of Colorado, Center for Astrophysics and Space Astronomy,
Boulder, CO USA 80309;

^c^MIT Kavli Institute for Astrophysics and Space Research, Cambridge,
MA USA 02139;

^d^Stanford University, SLAC, Menlo Park, CA USA 94025

Abstract

The Constellation-X Reflection Grating Spectrometer (RGS) is designed to
provide high-throughput, high-resolution spectra in the long wavelength
band of 6 to 50 Å. In the nominal design an array of reflection gratings
is mounted at the exit of the Spectroscopy X-ray Telescope (SXT) mirror
module. The gratings intercept and disperse light to a designated array
of CCD detectors. To achieve the throughput (A~eff~ \> 1000 cm^2^ below
0.6 keV) and resolution ( \> 300 below 0.6 keV) requirements for the
instrument we are investigating two possible grating designs. The first
design uses in-plane gratings in a classical configuration that is very
similar to the XMM-Newton RGS. The second design uses off-plane gratings
in a conical configuration. The off-plane design has the advantage of
providing higher reflectivity and potentially, a higher spectral
resolution than the in-plane configuration. In our presentation we will
describe the performance requirements and the current status of the
technology development.

**Keywords:** X-ray, spectrometer, grating, CCD, Constellation-X

# INTRODUCTION

Constellation-X^1^ is one of NASA's "Beyond Einstein" Great
Observatories. It is designed to address such fundamental issues as the
physics of strong gravity, the evolution of black holes, and to study
dark matter throughout the universe^2^. Constellation-X will provide
high-resolution spectra across the x-ray band from 0.25 to 40 keV, with
orders of magnitude increased sensitivity over the current Chandra and
XMM-Newton spectrometers. The Hard X-ray Telescopes^3^ (HXT) will cover
the high energy band from 10 keV to 40 keV. For energies below 10 keV,
the performance requirements of the mission will be met using two
complementary spectrometers, the Reflection Grating Spectrometer (RGS),
which is a dispersive spectrometer with a resolving power that increases
with decreasing energy, and the X-ray Microcalorimeter Spectrometer
(XMS), which is a non-dispersive spectrometer with a resolving power
that increases with energy and is optimized for the Fe K emission region
around 6 keV. The XMS will be described in Kelley et al.^4^ In this
presentation we will describe the baseline design for the RGS and the
current status of the technology development.

# RGS Design

The basic design of the Constellation-X RGS is illustrated in Figure 1.
A Reflection Grating Array (RGA) is mounted at grazing incidence
relative to the converging beam exiting the Spectroscopy X-ray
Telescope^5^ (SXT). The gratings intercept and disperse photons to a
dedicated array of detectors, called the RGS Focal-Plane Camera (RFC),
where the energy resolution of the detectors is used to separate the
spatially overlapping spectral orders. The RFC consists of the
Spectroscopy Readout Camera (SRC), which images the diffracted light,
and the Zero-Order Camera (ZOC), which images the reflected light, and
will be used to anchor the RGS wavelength scale by tracking small aspect
drifts. Light not intercepted by the RGA passes through to the focus of
the telescope where it is imaged by the XMS. There are four identical
RGS systems in the Constellation-X mission.

![](media/image1.jpeg){width="6.733333333333333in" height="2.75in"}

> Fig 1: Illustration of one Constellation-X RGS design. Light, incident
> from the left, is focused by the SXT mirrors. The RGA gratings,
> represented by the flat rectangles, intercept part of the focused beam
> and disperse it to the RFC detectors, represented by the blue squares.
> Light that is not intercepted by the gratings is imaged by the XMS
> system at the telescope focus. The geometry is highly exaggerated.
>
> Table 1: The performance requirements and goals for the Reflection
> Grating Spectrometer.

+---------------------------+---------------------+--------------------+
| **Parameter**             | **Requirement**     | **Goal**           |
+---------------------------+---------------------+--------------------+
| Bandpass                  | 0.25-2.0 keV        | 0.1-2.0 keV        |
+---------------------------+---------------------+--------------------+
| Resolving Power           | ≥ 300               | ≥ 3000             |
|                           |                     |                    |
| (for E ≤ 0.6 keV)         |                     |                    |
+---------------------------+---------------------+--------------------+
| Throughput                | \>1000 cm^2^        | \--                |
|                           |                     |                    |
| (for E ≤ 0.6 keV)         |                     |                    |
+---------------------------+---------------------+--------------------+

The performance requirements and goals for the RGS are given in Table 1.
To meet these requirements, two possible grating configurations have
been considered, an in-plane, or "classical" configuration, and an
off-plane or "conical" configuration. Illustrations and the diffraction
equations for an in-plane and off-plane grating are shown in Figure 2.
For the in-plane configuration, a grating is illuminated such that the
incident ray lies perpendicular to the grating grooves. By illuminating
it at grazing incidence, the projected line density for an in-plane
grating can made to be very high, thereby increasing the diffraction and
hence the spectral resolution. In-plane gratings are used on the
XMM-Newton Reflection Grating Spectrometer^6^. For the off-plane
configuration, a grating is illuminated such that the incident rays are
nearly parallel to the grating grooves. An off-plane grating has a
higher efficiency than the in-plane grating since the incident and
diffraction angles relative to the facet are nearly identical and the
surface acts like a mirror. Although there are advantages to each, the
Constellation-X project is currently exploring off-plane grating
configurations for the RGS. With higher efficiency, fewer off-plane
gratings can be used to satisfy the same effective area requirement.
Because off-plane gratings disperse light along a shallow cone, they can
be packed much more closely than in-plane gratings, which must be spaced
to allow the full range of diffraction angles without significant
vignetting. Off-plane gratings can therefore be arrayed to maximize the
spectral resolution. Each telescope mirror shell produces an image on
the detector. By positioning gratings across a narrow angular subset of
the mirror module, or sub-aperturing, the mirror image on the RFC
detectors will appear as a bow-tie instead of a full circle. This
narrowed line spread function greatly increases the spectral resolution.
The advantages of off-plane gratings, including a description of
sub-aperturing are given in Cash, and McEntaffer et al.^7,8^ Raytrace
results for Constellation-X grating configurations are given in
McEntaffer et al., Rasmussen et al., and Flanagan et al.^8,9,10^

![](media/image2.png){width="4.929861111111111in"
height="2.3986111111111112in"}

Fig 2: Illustration of an in-plane (left) and off-plane (right)
reflection grating. In-plane gratings are illuminated perpendicular to
the grooves and disperse light along an arc as shown. For an incident
angle of relative to the grating normal, and a line spacing of d, the
in-plane grating will disperse photons of wavelength, , in the plane of
incidence according to the given equation, where is the dispersion angle
and m is the spectral order. Off-plane gratings are illuminated along
the groove and disperse light along the arc as shown. For an incident
angle relative to the grating grooves and an azimuthal angle of along
the cone of half-angle the off-plane grating will diffract photons
according to the given equation, where is the azimuthal angle along the
dispersion cone.

The RGS design that is currently under consideration consists of
off-plane gratings arrayed over two 75^o^ segments on opposite sides of
the outer shells of the SXT. The gratings are integrated using a modular
approach with roughly 13 gratings per module, and 50 modules, all
attached to the SXT through a grating integrating structure. The number
of grating modules is chosen to satisfy the system-level effective area
requirements. In this initial design each reflection grating is ruled
with a line density of 1/d = 5800 l/mm and a groove blaze angle of
16.9^o^. To minimize aberrations caused when focused rays intercept the
grating at different incident angles, the grooves are fanned slightly to
match the convergence of the telescope beam. The gratings are nominally
illuminated at =16.9^o^ with respect to the grating normal, and =2.0^o^
with respect to the grating groove. The RFC/SRC consists of CCDs, which
cover a dispersion arc of 43^o^, or roughly 254 mm (from 0.25 keV to 2.0
keV). The CCDs are nominally formatted in a 1600 by 1600 pixel array
where each pixel is 15 m wide. The RFC/ZOC utilizes an identical CCD
design. In this simple configuration, the RGS satisfies the resolution
requirements for the mission. Techniques are under investigation to
achieve the mission goals by aligning the gratings to make
two-dimensional scalloped or C-shaped images^8,9,10^ and correcting for
the path-length difference between the grating arrays at opposite sides
of the mirror.

# Grating Development

The performance requirements for the RGS present technological
challenges for grating development. The high effective area of the RGS
requires a large number of reflection gratings each with higher spectral
reflectivity than traditional methods of fabrication by mechanical
ruling can provide. While off-plane gratings have good reflectivity,
they must be ruled with line densities and blaze angles that are an
order of magnitude higher than in-plane gratings for the same spectral
resolution. The mass constraints for the system, combined with the large
number of gratings, mean that each grating must be at least a factor of
three lighter than the reflection gratings flown on XMM-Newton. The
gratings must therefore be thin. In order to achieve spectral resolution
approaching the Constellation-X goals, these thin gratings must be flat
and aligned to the same \~2" precision as the much heavier and thicker
XMM-Newton gratings. With four RGA arrays, each containing roughly 700
gratings, mass production presents an additional challenge. The
Constellation-X project is currently pursuing two parallel paths for
grating development. Recent developments from both an MIT team and a
University of Colorado team are described in the sections below.

## Grating Development at MIT

The MIT grating development team is fabricating diffraction gratings
using specialized techniques, which can be used for both in-plane and
off-plane gratings. Heilmann et al. ^11,12^ describes the process.
Gratings are patterned using a "Scanning Beam Interference Lithography"
(SBIL) system, which allows for high speed patterning with extremely
good groove control (period: p/p \< 10^-5^, phase: \~p/100)^13,14^. The
patterned gratings then undergo anisotropic etching to form the grooves.
Their technique exploits the lattice planes of single-crystal silicon to
make extremely smooth, high efficiency gratings. They have fabricated
gratings that demonstrate each of the parameters that are critical for
the Constellation-X RGA. They routinely make gratings with line
densities of 5000 to 10000 l/mm. They have fabricated a grating that is
300 mm in diameter^12^ (see Figure 3). They have made gratings that meet
the mass requirements and that demonstrate the groove form over a range
of blaze angles.^15,16,17^ They have also successfully applied two
low-stress nanoimprint lithography techniques for replicating the
grating, a thermal process and an UV-cure process, that are described in
Heilmann et al^11^. And they have demonstrated flattening of 100
mm-diameter, 0.5 mm-thin grating substrates to better than 100 nm
P-V^18^. The Constellation-X gratings will be ruled with fanned grooves
to match the convergence of the telescope beam. Fabricating these
"radial gratings" requires upgrading the MIT SBIL system to a
"Variable-Period" SBIL. This upgrade is well under way. MIT and their
technology partners are in the process of final assembly and alignment.
A flight prototype grating will be fabricated as soon as the new system
is online.

![](media/image3.pct){width="3.454861111111111in" height="1.71875in"}

![](media/image4.pct){width="2.45in" height="1.836111111111111in"}

Fig 3: (Left) A large MIT grating. This 300mm diameter grating, larger
than required for Constellation-X, was patterned on the SBIL system
(Image reproduced from Heilmann et al^12^.) Gratings of this size have
been patterned in as little as 20 minutes. (Right) Atomic Force
Micrograph (AFM) of a replicated grating before metal coating. Sharp
features are very well replicated, and the microroughness on the blaze
facets is \< 0.2 nm. The thin, low-stress nanoimprint lithography layers
lead to distortions smaller than 1 arcsec on 100 mm-diameter, 0.5
mm-thin substrates^17^.

MIT is also developing technology and methodology for assembling and
aligning the gratings. To mount gratings to the desired flatness and
alignment tolerances they are exploring techniques that use a vacuum
chuck to constrain a grating and hold it flat while it is epoxied to
thin ribs on a support structure. Precision ribs are then attached to
the first grating and a second grating is mounted. A grating module is
assembled in alternating layers of ribs and gratings. This process
improves grating flatness by a factor of 2 to 3. The process and the
measured flatness of gratings in a test module are described in Akilian
et al^19^.

![](media/image5.png){width="3.875in" height="2.1819444444444445in"}The
MIT team continues modeling the grating efficiency and the spectral
resolution. Efficiency data for the MIT fabricated gratings has been
acquired in collaboration with the University of Colorado team at a test
facility maintained by that group^20,12^. Efficiency as a function of
wavelength was acquired at LBNL's Advanced Light Source (ALS)
facility^9^, and at the Brookhaven NSLS facility^21,18^. Initial
modeling of the measured efficiency is described in Seely et al.^21^.
The MIT team is using the PCGrate program to investigate non-ideal
effects. A comparison between the efficiency modeled using an AFM groove
profile of a grating fabricated at MIT and the efficiency for an ideal
sawtoothed grating with the same design parameters is shown in Figure 4.
This is critical input both for system-level planning and for
optimization of the grating design.

> Fig 4: Efficiency modeled using the AFM grove profile for a grating
> fabricated at MIT and for the ideal sawtooth groove of the same design
> parameters. The incident light is linearly polarized. Efficiency in
> the spectral orders is significantly reduced for the non-ideal groove.

## Grating Development at University of Colorado

The University of Colorado grating development team has concentrated on
demonstrating the x-ray performance of off-plane gratings. They maintain
a test facility that is used to measure grating efficiency at discrete
energies^20^. Resolution measurements are much more difficult because
they require flight-representative illumination of the gratings. The
team is preparing for a test at the Max Planck Institute for
Extraterrestrial Physics (MPE) Panter facility in Munich in
collaboration with the MPE group. The long 120 meter beam line
approximates illumination in-flight. The test setup is illustrated in
Figure 5. For these tests a spare XMM mirror will be used. It has a
\~15" psf, very similar to the projected SXT performance. The grating
will be placed 50 cm from the mirror at graze angles ranging from 0.5 to
3 degrees. With a combination of the Panter monochromator, which has a
lower wavelength limit of 50 Å, and discrete sources, which extend to
the Be line at 114 Å, the Constellation-X RGS range can be sampled.
These tests are expected to take place in the coming months.

Fig 5: Diagram of the MPE Panter test configuration. The beam line is
120 m long. An XMM flight spare mirror, with a focal length of 7.5 m
will be used to illuminate the grating. A holographic grating will
fabricated at Horiba Jobin Yvon for these resolution tests.

The University of Colorado team is working with the Horiba Jobin Yvon
company to fabricate a test grating for the Panter campaign. The grating
master is holographically ruled and then replicated onto a 16 mm thick
pyrex substrate. The groove parameters are designed to optimize the
throughput at 1 keV in 3^rd^ order with a quasi-radial fan pattern
designed to focus at the \~7.5 focal length of the PANTER test mirror.
The average groove density is greater than 5000 l/mm with a 15^o^ blaze.

![](media/image6.png){width="2.375in" height="2.375in"}The University of
Colorado team is also addressing the challenges posed by thin
substrates. Closely packed, thin grating arrays offer maximum throughput
with minimal structural blockage. However, it is difficult to meet the
surface quality requirements with thin substrates. They have studied a
variety of fabrication methods. Pyrex, silicon carbide, and silicon have
been used as substrates with thicknesses ranging from 0.5 to 2 mm.
Details of these studies are presented in Shipley et al.^22^ They find
that groove profiles replicated onto a thin substrate with polymers
share a tendency to bend and cause optical surface error. Distortion is
induced during the polymer curing process and is directly proportional
to the replication layer thickness. Gratings made of stiff substrates
with very thin or no replication layer will result in optimal surface
figure. The effects of gravity on thin substrates and its release after
launch need to be included in both surface figure and alignment
tolerance budgets, as must temperature changes and CTE difference
between a substrate and reflection layer that may cause bimetallic
bending. Moreover, thin substrate gratings used in flight applications
must be robust in a vibration environment and resist fracture.

They have developed closely packed thin substrate gratings as part of a
diffuse X-ray spectrometer sounding rocket^23,24^. This provides an
excellent test bed for Constellation-X grating technology. Electroformed
nickel is used for the grating substrate and electroless nickel is used
for the reflection layer. To achieve spectral resolution of \> 100 the
rocket gratings must be flat and parallel to within one part in 2000
along their length. To achieve this, a grating mount was designed which
pulls the gratings from each end and holds them in tension. A full
complement of gratings in their fixture is shown in Figure 6.

Fig 6: Illustration of the thin substrate grating mount for the
University of Colorado sounding rocket.

# Detector Development

There are several technological challenges for the RFC detectors. The
RGS effective area requirements at low energies mean that the detectors
must combine high quantum efficiencies with high transmission of the
optical blocking filters. The Chandra ACIS back-illuminated (BI) CCDs
had a combined efficiency (quantum efficiency times filter transmission)
of 0.73 \* 0.20 = 0.15 at 0.25 keV. The Suzaku XIS BI CCDs had combined
efficiency as measured on the ground of 0.80 \* 0.31 = 0.25 at the same
energy. For Constellation-X, the RFC must have a combined efficiency of
\~0.78 at 0.25 keV. With four systems, each requiring roughly 13
detectors, the production yield must be substantially higher than the 1%
yield typical for past generations of BI x-ray CCDs. System-level power
constraints and the radiation environment at L2 provide additional
challenges for the technology development.

## CCD Development at MIT 

The MIT Kavli Institute in collaboration with MIT/Lincoln Laboratory, is
developing CCDs for the Constellation-X RGS. They are building special
CCDs that are operated in an "Event-Driven" mode. This is described in
Doty et al.^25^ and Ricker et al.^26^ Briefly, the CCD is scanned for
photon events and then only the pixels with photon-induced charge are
digitized. Since typically less than 1% of the pixels on a CCD contain
signal charge in a given frame, ED-CCDs can be operated at high readout
rates and with power reductions of a factor of 100 over conventional
readout modes. The MIT team has manufactured two lots of ED-CCDs. The
most recent is formatted as 512 by 512 pixels. These are currently
undergoing X-ray tests. They have now been successfully readout at a
frame rate of 1 Mpix/sec in conventional CCD readout mode. This is 10
times faster than for any other x-ray low-noise MOS CCD, which makes
them much less sensitive to optical straylight than previous CCDS. With
a 1Mpix/sec readout rate an optical blocking filter of only 150Å
aluminum is sufficient to limit the electronic noise due to optical
contamination in the 3000-11000 Å band to 1 photon/frame/superpixel.

Back-side processing of the CCDs involves a new Molecular Beam Epitaxy
(MBE) technique that has recently been developed at Lincoln Laboratory.
Back-side processing thins the silicon deadlayer to optimize the charge
collection of low energy photons. The new processes attempt to generate
a steep potential gradient at the illuminated surface of the device to
minimize electron losses near the surface. Trapping and recombination of
electrons at the silicon-silicon dioxide interface is the usual reason
for poor energy resolution of back-side illuminated CCDs. The presence
of electric field there improves the energy resolution and quantum
efficiency of the device. First tests of these MBE-produced CCDS show
exciting results. These will be reported in Ricker et al.^27^. At 0.25
keV, near the low energy end of the Con-X bandpass, the energy
resolution of roughly 50 eV is more than sufficient to separate the
spatially overlapping spectral orders. The quantum efficiency of these
CCDs has not yet been tested. However, the excellent energy resolution
and the nearly symmetric profile show excellent charge collection
capabilities, and strongly suggest that the quantum efficiency will be
high.

# Summary

The Constellation-X RGS will provide high-resolution, high-throughput
soft x-ray spectra with potentially orders of magnitude improvements in
performance over the soft x-ray spectrometers that are currently
operating. This requires significant technological advances in the
development of both the gratings and the detectors. The high-throughput
requirement is perhaps the most challenging both for the development of
high-efficiency, closely-packed gratings, and for CCDs with high
quantum-efficiency and ultra-thin blocking filters. The technology
development teams are making significant progress towards realizing each
of these requirements.

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