The LSST Camera for the Vera C. Rubin Observatory has been constructed at SLAC National Accelerator Laboratory. The Camera covers a 3.5-degree field of view with 3.2 gigapixels. The goal of the LSST survey is to provide a well-understood astronomical source catalog to the community. The LSST Camera’s focal plane is populated by 189 sensors on the science focal plane that are a combination of E2V CCD250 and ITL STA3800 deep-depletion, back-illuminated devices, accompanying eight guide sensors, and four wavefront sensors. Nine science sensors are grouped as a ”Raft” with three identical electronics boards (REBs), each operating three sensors. The REB can change the operating voltages and CCD clock, allowing operation of sensors from two different vendors in the same focal plane. We conducted phased electro-optical testing campaigns to characterize and optimize the sensor performance in the construction phase. We collected images with the focal plane illuminated by flat illuminators and some specialty projectors to produce structured images. During these tests, we found some performance issues in noise, bias stability, gain stability, image persistence, and distortion in flat images, including ”tearing”. To mitigate those non-idealities, we attempted different clocking and operation voltages and switching from unipolar voltages to bipolar voltages in parallel clock rails for E2V devices. We describe the details and the results of the optimizations.
The LSST Camera is the sole instrument for the Vera C. Rubin Observatory and consists of a 3.2 gigapixel focal plane mosaic with in-vacuum controllers, dedicated guider and wavefront CCDs, a three-element corrector whose largest lens is 1.55m in diameter, six optical interference filters covering a 320–1050 nm bandpass with an out-of-plane filter exchange mechanism, and camera slow control and data acquisition systems capable of digitizing each image in 2 seconds. In this paper, we describe the verification testing program performed throughout the Camera integration and results from characterization of the Camera’s performance. These include an electro-optical testing program, measurement of the focal plane height and optical alignment, and integrated functional testing of the Camera’s major mechanisms: shutter, filter exchange system and refrigeration systems. The Camera is due to be shipped to the Rubin Observatory in 2024, and plans for its commissioning on Cerro Pachon are briefly described.
The LSST Camera is a complex, highly integrated instrument for the Vera C. Rubin Observatory. Now that the assembly is complete, we present the highlights of the LSST Camera assembly: successful installation of all Raft Tower Modules (RTM) into the cryostat, integration of the world’s largest lens with the camera body, and successful integration and testing of the shutter and filter exchange systems. While the integration of the LSST Camera is a story of success, there were challenges faced along the way which we present: component failures, late design changes, and facility infrastructure issues.
Electro-optical testing and characterization of the Vera C. Rubin Observatory Legacy Survey of Space and Time (LSST) Camera focal plane, consisting of 205 charge-coupled devices (CCDs) arranged into 21 stand-alone Raft Tower Modules (RTMs) and 4 Corner Raft Tower Modules (CRTMs), is currently being performed at the SLAC National Accelerator Laboratory. Testing of the camera sensors is performed using a set of custom-built optical projectors, designed to illuminate the full focal plane or specific regions of the focal plane with a series of light illumination patterns: the crosstalk projector, the flat illuminator projector, and the spot grid projector. In addition to measurements of crosstalk, linearity and full well, the ability to project realistically-sized sources, using the spot grid projector, makes possible unique measurements of instrumental signatures such as deferred charge distortions, astrometric shifts due to sensor effects, and the brighter-fatter effect, prior to camera first light. Here we present the optical projector designs and usage, the electro-optical measurements and how these results have been used in testing and improving the LSST Camera instrumental signature removal algorithms.
The Integration and Verification Testing and characterization of the expected performance of the Large Synoptic Survey Telescope (LSST) Camera is described. The LSST Camera will be the largest astronomical camera ever constructed, featuring a 3.2 Gpixel focal plane mosaic of 189 CCDs. In this paper, we describe the verification testing program developed in parallel with the integration of the Camera, and the results from our performance characterization of the Camera. Our testing program includes electro-optical characterization and CCD height measurements of the focal plane, at several steps during integration, as well as a complete functional and characterization program for the finished focal plane. It also includes a suite of functional tests of the major Camera mechanisms: shutter, filter exchange system and thermal control. Finally, we expect to test the fully assembled Camera prior to its scheduled completion and delivery to the LSST observatory in early calendar 2021.
The Large Synoptic Survey Telescope, under construction in Chile, is an 8.4 m optical survey telescope with a dedicated 3.2 Giga-pixel camera. The design and construction of the camera is spearheaded at SLAC National Accelerator Laboratory and here we present a general overview of the camera integration and test activities. An overview of the methodologies used for the planning and management of this subsystem will be given, along with a high-level summary of the status of the major pieces of I&T hardware. Finally a brief update will be given on the current state of the LSST Camera integration and testing program.
The Integration and Verification Testing of the Large Synoptic Survey Telescope (LSST) Camera is described. The LSST Camera will be the largest astronomical camera ever constructed, featuring a 3.2 giga-pixel focal plane mosaic of 189 CCDs with in-vacuum controllers and readout, dedicated guider and wavefront CCDs, a three element corrector with a 1.6-meter diameter initial optic, six optical filters covering wavelengths from 320 to 1000 nm with a novel filter exchange mechanism, and camera-control and data acquisition capable of digitizing each image in two seconds. In this paper, we describe the integration processes under way to assemble the Camera and the associated verification testing program. The Camera assembly proceeds along two parallel paths: one for the focal plane and cryostat and the other for the Camera structure itself. A range of verification tests will be performed interspersed with assembly to verify design requirements with a test-as-you-build methodology. Ultimately, the cryostat will be installed into the Camera structure as the two assembly paths merge, and a suite of final Camera system tests performed. The LSST Camera is scheduled for completion and delivery to the LSST observatory in 2020.
We present the mechanical device used to install the Raft Tower Modules (RTMs) into the cryosat of the camera for the Large Synoptic Survey Telescope (LSST). In an RTM, the charge-coupled devices (CCDs) are packaged into a 3 x 3 Raft Sensor Assembly (RSA) and coupled to a Raft Electronics Crate (REC). An RTM weighs ~10 kg, is roughly 500 mm tall, and has a 126.5 mm-square footprint at the CCDs. The grid array which supports the RTM in the cryostat has a center-to-center distance of 127 mm. One of the key challenges for installing the RTMs in the 500 μm gap between CCDs of adjacent modules - contact between adjacent CCDs is strictly forbidden.
The Bench for Optical Testing (BOT) is a test stand that will be used for metrology and optical testing of the Large Synoptic Survey Telescope (LSST) Camera CCD sensors, immediately after the integration step where the sensors are installed into the Cryostat to form the LSST’s 3.2 gigapixel, 640mm diameter focal plane. The BOT uses existing methods to economically verify sensor performance, including measurement of focal plane flatness, CCD sensor spacing, gain stability, cross-talk, flat field images, response in each filter band, and dark level. This paper describes the requirements, design, and preliminary test results for the BOT test equipment.
The LSST Camera focal plane comprises twenty-one raft tower modules (RTMs), each with nine CCD sensors and their associated electronics. RTMs are assembled at Brookhaven National Lab and shipped to SLAC National Lab, where they must be re-verified before being assembled into the full focal plane. The process for accepting an RTM at SLAC has been thoroughly documented, including unpacking a raft from its shipping container, verifying aliveness of the electrical connections, performing metrology and electro-optical testing in an environment similar to the full Camera, and finally storing the RTM until it can be installed into the LSST Camera
We present an overview of the Integration and Verification Testing activities of the Large Synoptic Survey Telescope (LSST) Camera at the SLAC National Accelerator Lab (SLAC). The LSST Camera, the sole instrument for LSST and under construction now, is comprised of a 3.2 Giga-pixel imager and a three element corrector with a 3.5 degree diameter field of view. LSST Camera Integration and Test will be taking place over the next four years, with final delivery to the LSST observatory anticipated in early 2020. We outline the planning for Integration and Test, describe some of the key verification hardware systems being developed, and identify some of the more complicated assembly/integration activities. Specific details of integration and verification hardware systems will be discussed, highlighting some of the technical challenges anticipated.
We employ electrostatic conversion drift calculations to match CCD pixel signal covariances observed in at field exposures acquired using candidate sensor devices for the LSST Camera.1, 2 We thus constrain pixel geometry distortions present at the end of integration, based on signal images recorded. We use available data from several operational voltage parameter settings to validate our understanding. Our primary goal is to optimize flux point spread function (FPSF) estimation quantitatively, and thereby minimize sensor-induced errors which may limit performance in precision astronomy applications. We consider alternative compensation scenarios that will take maximum advantage of our understanding of this underlying mechanism in data processing pipelines currently under development. To quantitatively capture the pixel response in high-contrast/high dynamic range operational extrema, we propose herein some straightforward laboratory tests that involve altering the time order of source illumination on sensors, within individual test exposures. Hence the word hysteretic in the title of this paper.
The LSST Camera science sensor array will incorporate 189 large format Charge Coupled Device (CCD) image sensors.
Each CCD will include over 16 million pixels and will be divided into 16 equally sized segments and each segment will
be read through a separate output amplifier.
The science goals of the project require CCD sensors with state of the art performance in many aspects. The broad
survey wavelength coverage requires fully depleted, 100 micrometer thick, high resistivity, bulk silicon as the imager
substrate. Image quality requirements place strict limits on the image degradation that may be caused by sensor effects:
optical, electronic, and mechanical.
In this paper we discuss the design of the prototype sensors, the hardware and software that has been used to perform
electro-optic testing of the sensors, and a selection of the results of the testing to date. The architectural features that lead
to internal electrostatic fields, the various effects on charge collection and transport that are caused by them, including
charge diffusion and redistribution, effects on delivered PSF, and potential impacts on delivered science data quality are
addressed.
Near-future astronomical survey experiments, such as LSST, possess system requirements of unprecedented
fidelity that span photometry, astrometry and shape transfer. Some of these requirements flow directly to the
array of science imaging sensors at the focal plane. Availability of high quality characterization data acquired
in the course of our sensor development program has given us an opportunity to develop and test a framework
for simulation and modeling that is based on a limited set of physical and geometric effects. In this paper we
describe those models, provide quantitative comparisons between data and modeled response, and extrapolate
the response model to predict imaging array response to astronomical exposure. The emergent picture departs
from the notion of a fixed, rectilinear grid that maps photo-conversions to the potential well of the channel.
In place of that, we have a situation where structures from device fabrication, local silicon bulk resistivity
variations and photo-converted carrier patterns still accumulating at the channel, together influence and distort
positions within the photosensitive volume that map to pixel boundaries. Strategies for efficient extraction of
modeling parameters from routinely acquired characterization data are described. Methods for high fidelity
illumination/image distribution parameter retrieval, in the presence of such distortions, are also discussed.
KEYWORDS: Large Synoptic Survey Telescope, Device simulation, Stars, Galactic astronomy, Data modeling, Photons, Systems modeling, Solar system, Sensors, Atmospheric modeling
The LSST will, over a 10-year period, produce a multi-color, multi-epoch survey of more than
18000 square degrees of the southern sky. It will generate a multi-petabyte archive of images and
catalogs of astrophysical sources from which a wide variety of high-precision statistical studies can
be undertaken. To accomplish these goals, the LSST project has developed a suite of modeling and
simulation tools for use in validating that the design and the as-delivered components of the LSST
system will yield data products with the required statistical properties. In this paper we describe the
development, and use of the LSST simulation framework, including the generation of simulated
catalogs and images for targeted trade studies, simulations of the observing cadence of the LSST, the
creation of large-scale simulations that test the procedures for data calibration, and use of end-to-end
image simulations to evaluate the performance of the system as a whole.
The design of the Large Synoptic Survey Telescope (LSST) requires a camera system of unprecedented size and complexity. Achieving the science goals of the LSST project, through design, fabrication, integration, and operation, requires a thorough understanding of the camera performance. Essential to this effort is the camera modeling which defines the effects of a large number of potential mechanical, optical, electronic or sensor variations which can only be captured with sophisticated instrument modeling that incorporates all of the crucial parameters. This paper presents the ongoing development of LSST camera instrument modeling and details the parametric issues and attendant analysis involved with this modeling.
The Large Synoptic Survey Telescope (LSST) uses a novel, three-mirror, modified Paul-Baker design,
with an 8.4-meter primary mirror, a 3.4-m secondary, and a 5.0-m tertiary feeding a refractive camera design with 3
lenses (0.69-1.55m) and a set of broadband filters/corrector lenses. Performance is excellent over a 9.6 square
degree field and ultraviolet to near infrared wavelengths.
We describe the image quality error budget analysis methodology which includes effects from optical and
optomechanical considerations such as index inhomogeneity, fabrication and null-testing error, temperature
gradients, gravity, pressure, stress, birefringence, and vibration.
Extracting science from the LSST data stream requires a detailed knowledge of the properties of the LSST catalogs and
images (from their detection limits to the accuracy of the calibration to how well galaxy shapes can be characterized).
These properties will depend on many of the LSST components including the design of the telescope, the conditions
under which the data are taken and the overall survey strategy. To understand how these components impact the nature
of the LSST data the simulations group is developing a framework for high fidelity simulations that scale to the volume
of data expected from the LSST. This framework comprises galaxy, stellar and solar system catalogs designed to match
the depths and properties of the LSST (to r=28), transient and moving sources, and image simulations that ray-trace the
photons from above the atmosphere through the optics and to the camera. We describe here the state of the current
simulation framework and its computational challenges.
How structures of various scales formed and evolved from the early Universe up to present time is a fundamental
question of astrophysics. EDGE will trace the cosmic history of the baryons from the early generations of massive
stars by Gamma-Ray Burst (GRB) explosions, through the period of galaxy cluster formation, down to the very low
redshift Universe, when between a third and one half of the baryons are expected to reside in cosmic filaments undergoing
gravitational collapse by dark matter (the so-called warm hot intragalactic medium). In addition EDGE, with its
unprecedented capabilities, will provide key results in many important fields. These scientific goals are feasible with a
medium class mission using existing technology combined with innovative instrumental and observational capabilities
by: (a) observing with fast reaction Gamma-Ray Bursts with a high spectral resolution (R ~ 500). This enables the study
of their (star-forming) environment and the use of GRBs as back lights of large scale cosmological structures; (b)
observing and surveying extended sources (galaxy clusters, WHIM) with high sensitivity using two wide field of view
X-ray telescopes (one with a high angular resolution and the other with a high spectral resolution). The mission concept
includes four main instruments: a Wide-field Spectrometer with excellent energy resolution (3 eV at 0.6 keV), a Wide-
Field Imager with high angular resolution (HPD 15") constant over the full 1.4 degree field of view, and a Wide Field
Monitor with a FOV of 1/4 of the sky, which will trigger the fast repointing to the GRB. Extension of its energy response
up to 1 MeV will be achieved with a GRB detector with no imaging capability. This mission is proposed to ESA as part
of the Cosmic Vision call. We will briefly review the science drivers and describe in more detail the payload of this
mission.
We present a spectrometer design based on a novel nanofabricated blazed X-ray transmission grating which is modeled
to have superior efficiency. Here we outline a full instrument design proposed for Constellation-X which is expected to
give resolving powers ~2000 (HEW). The spectrometer advantages include lower mass budget and smaller diffractor
area, as well as order-of-magnitude more relaxed alignment tolerances for crucial degrees of freedom than reflection
grating schemes considered in the past1,2,3. The spectrometer readout is based on conventional CCD technology adapted
to operate with very high speed and low power. This instrument will enable high resolution absorption and emission line
spectroscopy in the critical band between 0.2 and 1.5 keV.
KEYWORDS: Sensors, Metrology, Large Synoptic Survey Telescope, Cameras, Distortion, Temperature metrology, Data acquisition, Active optics, Kinematics, Actuators
Meeting the science goals for the Large Synoptic Survey Telescope (LSST) translates into a demanding set of
imaging performance requirements for the optical system over a wide (3.5°) field of view. In turn, meeting those
imaging requirements necessitates maintaining precise control of the focal plane surface (10 μm P-V) over the
entire field of view (640 mm diameter) at the operating temperature (T ~ -100°C) and over the operational
elevation angle range. We briefly describe the heirarchical design approach for the LSST Camera focal plane and
the baseline design for assembling the flat focal plane at room temperature. Preliminary results of gravity load
and thermal distortion calculations are provided, and early metrological verification of candidate materials under
cold thermal conditions are presented. A detailed, generalized method for stitching together sparse metrology
data originating from differential, non-contact metrological data acquisition spanning multiple (non-continuous)
sensor surfaces making up the focal plane, is described and demonstrated. Finally, we describe some in situ
alignment verification alternatives, some of which may be integrated into the camera's focal plane.
The most recent observations of the cosmic microwave background (e.g., WMAP) show that baryons contribute about 4% to the total density of the Universe. However at redshift less than or equal to 1, about half of these baryons have not yet been observed. Cosmological simulations predict that these "missing" baryons should be distributed in filaments, have temperatures of 105 to 107 K and overdensities of a few to hundred times the average baryon density, forming the so-called Warm-Hot Intergalactic Medium (WHIM). There is increasing evidence from Chandra and XMM-Newton that the WHIM may indeed exist. However it is clear that to map the morphology of the WHIM and to measure its physical conditions, a completely different class of instruments is required. Measuring the WHIM in emission in the soft X-ray band is a promising option. To detect the relatively weak, extended emission of the WHIM, the instrument should have a large grasp (collecting area times field of view), and an energy resolving power of about 500 at 1 keV is required to separate the emission of these large scale filaments from foreground emission.
We discuss a design that includes X-ray mirrors in combination with a large 2D cryogenic detector, which will allow us to map a significant fraction of this gas. Such detector and its read-out based on Frequency Domain Multiplexing, are currently under development at SRON. It seems feasible to build an array of 24 x 24 pixels of TES microcalorimeters with good energy resolution (few eV). This detector will be combined with a mirror design which is based on 2 and 4 reflections and gives a large area (> 500 cm2) over a relatively large field of view. A preliminary study of the mission concept indicates that this can be implemented in a relatively small satellite (total weight 650 kg). While the main goal of this satellite will be to map and study the physical properties of the missing baryons, the instrument's large area and large field of view will also result in major progress in related fields.
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 angstrom. 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 (Aeff > 1000 cm2 below 0.6 keV) and resolution (Δλ/λ > 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.
Efficiency measurements of a grazing-incidence diffraction grating, planned for the Constellation-X Reflection Grating Spectrometer (RGS), were performed using polarized synchrotron radiation at the NRL Brookhaven beamline X24C. The off-plane TM and TE efficiencies of the 5000 groove/mm MIT test grating, patterned on a silicon wafer, were measured and compared to the efficiencies calculated using the PCGRATE-SX code. The calculated and measured efficiencies are in agreement when using groove profiles derived from AFM measurements. The TM and TE efficiencies differ, offering the possibility of performing unique astrophysical science studies by exploiting the polarization sensitivity of the off-plane gratings. The grating calibrations demonstrate the importance of using polarized synchrotron radiation and code calculations for the understanding of the Constellation-X grating performance, in particular the effects of the groove profile and microroughness on the efficiency. The optimization of grazing incidence gratings, for both the off-plane and in-plane mounts, planned for the RGS and x-ray spectrometers on other missions will require detailed synchrotron measurements and code calculations.
A very significant fraction of the baryonic matter in the local universe is predicted to form a Warm Hot Intergalactic Medium (WHIM) of very low density, moderately hot gas, tracing the cosmic web. Its X-ray emission is dominated by metal features, but is weak (< 0.01 photons/cm2/s/sr) and potentially hard to separate from the galactic component. However, a mission capable of directly mapping this component of the large scale structure of the universe, via a small number of well chosen emission lines, is now within reach due to recent improvements in cryogenic X-ray detector energy resolution. To map the WHIM, the energy resolution and grasp are optimized. A number of missions have been proposed to map the missing baryons including MBE (US/SMEX program) and DIOS (Japan). The design of the mirror and detector have still room for improvements which will be discussed. With these improvements it is feasible to map a 10 x 10 degree area of the sky in 2 years out to z = 0.2 with sufficient sensitivity to directly detect WHIM structure, such as filaments connecting clusters of galaxies. This structure is predicted by the current Cold Dark Matter paradigm which thus far appears to provide a good description of the distribution of matter as traced by galaxies.
The Reflection Grating Spectrometer of the Constellation-X mission has
two strong candidate configurations. The first configuration, the
in-plane grating (IPG), is a set of reflection gratings similar to
those flown on XMM-Newton and has grooves perpendicular to the
direction of incident light. In the second configuration, the
off-plane grating (OPG), the grooves are closer to being parallel to
the incident light, and diffract along a cone. It has advantages of
higher packing density, and higher reflectivity. Confinement of these
gratings to sub-apertures of the optic allow high spectral
resolution. We have developed a raytrace model and analysis technique
for the off-plane grating configuration. Initial estimates indicate
that first order resolving powers in excess of 1000 (defined with
half-energy width) are achievable for sufficiently long wavelengths
(λ ≥ 12Å), provided separate accommodation is made
for gratings in the subaperture region farther from the zeroth order
location.
The ESA mission XMM-Newton was launched in 1999. Two of the three X-ray telescopes include reflection grating spectrometers (RGS). These spectrometers consist of a set of reflection gratings and an array of 9 back-illuminated CCDs, optimized for the soft energy response (0.35 - 2 keV). These CCDs can be passively cooled between -80 and -120°C. After a short description of the instrument we compare the performance of these CCD detectors with the pre-flight expectations and discuss the effect of some design choices on the in-flight performance. We concentrate on the effects of radiation damage due to cosmic rays and coronal mass ejections of the Sun, including flickering pixels and the effects of cooling the detector to -110°C. We also address the stability of the detector response including the assessment of possible contamination of these cooled detectors.
Cosmic soft X-ray spectroscopy exploits principal transitions of astrophysically abundant elements to infer physical properties of objects in the sky. Most of these transitions, however, fall well below 2 keV, or 6 Angstroms. Consquently, grating spectrometers offer the current, best means by which to analyze soft X-rays from such sources, where throughput and resolving power must be maximized together. We describe grating spectrometer design candidates for the future mission Constellation-X, and how the grating array on board (~1000 gratings in a 1600mm diameter, each for 4 instruments) may be implemented. Grating fabrication and grating alignment approaches require special consideration (over the XMM-Newton RGS experience), because of grating replication fidelity and instrument mass constraints.
XMM-Newton was launched in December 1999 and science operations started in March 2000. Following two years of very successful operations, a report on the instrument performance and a selection of exciting new results are presented. Behind two of the three telescopes of XMM-Newton Reflection Grating Spectrometers (RGS) are placed. Each spectrometer consists of an array of reflection gratings and a set of back illuminated CCDs. They cover the wavelength band between 6 and 38 Angstromwith a resolution varying between 100 and 600 (E/DE) and a maximum effective area of 140 cm2 for the two spectrometers combined. The selected wavelength band covers the K-shell transitions of C, N, O, Ne, Mg and Si as well as the L- and M-shell transitions of Fe. After a short introduction to the instrument design, the in-orbit performance is given. This includes the line spread function, the wavelength scale and the effective area including their stability during the more than 2 years of operations. Following this a number of key scientific results are briefly addressed, illustrating the power of the RGS instrument in combination with the other instruments on-board of XMM-Newton as well as the wealth of information which is obtained as the RGS instruments operate continuously.
The activities during the instrument calibrations are summarized and first data are presented. The main instrument features, the line-spread function and the effective area, are discussed and the status of the in-flight calibrations is summarized.
The ESA X-ray Multi Mirror mission, XMM-Newton, carries two identical Reflection Grating Spectrometers behind two of its three nested sets of Wolter I type mirrors. The instrument allows high-resolution (E/(Delta) E equals 100 to 500) measurements in the soft X-ray range (6 to 38 A or 2.1 to 0.3 keV) with a maximum effective area of about 150 cm2 at 15 A. The satellite was successfully launched on December 10, 1999, from Guyana Space Center. Following the launch the instrument commissioning was started early in 2000. First results for the Reflection Grating Spectrometers are presented concentrating on instrumental parameters such as resolution, instrument background and CCD performance. The instrument performance is illustrated by first results from HR 1099, a non-eclipsing RS CVn binary.
The optical chain of the spectroscopic x-ray telescopes aboard the Constellation-X spacecraft employs a reflective grating spectrometer to provide high resolution spectra for multiple spectra as a slitless spectrometer in the spectral feature rich, soft x-ray band. As a part of the spectroscopic readout array, we provide a zero-order camera that images the sky in the soft band inaccessible to the microcalorimeters. Technological enhancements required for producing the RGS instruments are described, along with prototype development progress, fabrication and testing results.
We have developed a comprehensive model of the response of a CCD to soft x-ray illumination. The model is based on the Monte Carlo technique and follows the interactions with the device material of individual photons thrown into the structure, calculating device reaction to each of them. It incorporates a very detailed description of the CCD gate structure, as well as accurately measured absorption coefficients. The fluorescent and escape peak model takes into account interactions inside the gate structure, which dramatically improves the agreement with the experimental data at energies close to the Si absorption edge. The shape of the low energy tail is simulated according to our new model of electron cloud charge splitting at the interface between Si and SiO2. An origin of the tail in the horizontally split events is explained as coming from the p+ area in the channel stop region and is modeled accordingly.
In the frame of the XMM project, several test campaigns are accomplished to qualify the optical elements of the mission. The test described in this paper are performed on a XMM flight model mirror module added with a reflection grating assembly (RGA). The mirror module contains 58 x-ray optical quality shells, an x-ray baffle (XRB) to reduce the straylight. This complete XMM flight model mirror assembly (MA) is tested in a vertical configuration at CSL, in a full aperture or partial EUV collimated beam illumination, and with an x-ray pencil beam. One of the advantages of the EUV collimated beam is to verify the correct position of the RGA when integrated in flight configuration on the mirror module structure. This is not possible in x-ray with a finite source distance. The partial EUV illumination is performed to verify the correct integration of the RGA grating stacks. The pencil beam allows to make an accurate metrology of the XRB position, and to verify the positions of the 0, 1 and 2 diffraction order foci. In this paper, the tested module is first exposed, and the approach to qualify the instrument is described. The analysis of the results achieved over the different test configurations is presented. The impact of the environmental test on the reflection grating box is also diagnosed.
The x-ray multi-mirror (XMM) mission is the second of four cornerstone projects of the ESA long-term program for space science, Horizon 2000. The payload comprises three co- aligned high-throughput, imaging telescopes with a FOV of 30 arcmin and spatial resolution less than 20 arcsec. Imaging CCD-detectors (EPIC) are placed in the focus of each telescope. Behind two of the three telescopes, about half the x-ray light is utilized by the reflection grating spectrometer (RGS). The x-ray instruments are co-aligned and measure simultaneously with an optical monitor (OM). The RGS instruments achieve high spectral resolution and high efficiency in the combined first and second order of diffraction in the wavelength range between 5 and 35 angstrom. The design incorporates an array of reflection gratings placed in the converging beam at the exit from the x-ray telescope. The grating stack diffracts the x-rays to an array of dedicated charge-coupled device (CCD) detectors offset from the telescope focal plane. The cooling of the CCDs is provided through a passive radiator. The design and performance of the instrument are described below.
The reflection grating spectrometer (RGS) on-board the x-ray multi-mirror (XMM) mission incorporates an array of reflection gratings oriented at grazing incidence in the x- ray optical path immediately behind a grazing incidence telescope. Dispersed light is imaged on a strip of CCD- detectors slightly offset from the telescope focal plane. The grating array picks off roughly half the light emanating from the telescope; the other half passes undeflected through the array where it is imaged by the European photon imaging camera (EPIC) experiment. XMM carries two such identical units, plus a third telescope with an EPIC detector, but no RGS. The basic elements of the RGA include: 202 identical reflection gratings, a set of precision rails with bosses that determine the position and alignment of each grating, a monolithic beryllium integrating structure on which the rails are mounted, and a set of three, kinematic support mounts which fix the array to the telescope. In this paper, we review our progress on the fabrication and testing of the RGA hardware, with particular attention to the components comprising the engineering qualification model, a flight-representative prototype which will be completely assembled in September of this year.
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