The hard X-ray sky now being studied by INTEGRAL and Swift and soon by NuSTAR is rich with energetic phenomena
and highly variable non-thermal phenomena on a broad range of timescales. The High Energy Telescope (HET) on the
proposed Energetic X-ray Imaging Survey Telescope (EXIST) mission will repeatedly survey the full sky for rare and
luminous hard X-ray phenomena at unprecedented sensitivities. It will detect and localize (<20", at 5σ threshold) X-ray
sources quickly for immediate followup identification by two other onboard telescopes - the Soft X-ray imager (SXI)
and Optical/Infrared Telescope (IRT). The large array (4.5 m2) of imaging (0.6 mm pixel) CZT detectors in the HET, a
coded-aperture telescope, will provide unprecedented high sensitivity (~0.06 mCrab Full Sky in a 2 year continuous
scanning survey) in the 5 - 600 keV band. The large field of view (90° × 70°) and zenith scanning with alternating-orbital
nodding motion planned for the first 2 years of the mission will enable nearly continuous monitoring of the full
sky. A 3y followup pointed mission phase provides deep UV-Optical-IR-Soft X-ray and Hard X-ray imaging and
spectroscopy for thousands of sources discovered in the Survey. We review the HET design concept and report the
recent progress of the CZT detector development, which is underway through a series of balloon-borne wide-field hard
X-ray telescope experiments, ProtoEXIST. We carried out a successful flight of the first generation of fine pixel large
area CZT detectors (ProtoEXIST1) on Oct 9, 2009. We also summarize our future plan (ProtoEXIST2 & 3) for the
technology development needed for the HET.
The Energetic X-ray Imaging Survey Telescope (EXIST) is a proposed next generation multi-wavelength survey
mission. The primary instrument is a High Energy telescope (HET) that conducts the deepest survey for Gamma-ray
Bursts (GRBs), obscured-accreting and dormant Supermassive Black Holes and Transients of all varieties for immediate
followup studies by the two secondary instruments: a Soft X-ray Imager (SXI) and an Optical/Infrared Telescope (IRT).
EXIST will explore the early Universe using high redshift GRBs as cosmic probes and survey black holes on all scales.
The HET is a coded aperture telescope employing a large array of imaging CZT detectors (4.5 m2, 0.6 mm pixel) and a
hybrid Tungsten mask. We review the current HET concept which follows an intensive design revision by the HET
imaging working group and the recent engineering studies in the Instrument and Mission Design Lab at the Goddard
Space Flight Center. The HET will locate GRBs and transients quickly (<10-30 sec) and accurately (< 20") for rapid
(< 1-3 min) onboard followup soft X-ray and optical/IR (0.3-2.2 μm) imaging and spectroscopy. The broad energy
band (5-600 keV) and the wide field of view (~90º × 70º at 10% coding fraction) are optimal for capturing GRBs,
obscured AGNs and rare transients. The continuous scan of the entire sky every 3 hours will establish a finely-sampled
long-term history of many X-ray sources, opening up new possibilities for variability studies.
X-ray Phase Fresnel lenses (PFLs) can be considered as diffraction gratings with rotational (axial) symmetry
and radially-varying pitch. The achromatic combinations of refractive and diffractive lenses that have been
proposed for applications in X-ray and gamma-ray astronomy may then be regarded as grisms, again with
variable pitch and axial symmetry. This way of looking at optics for very high angular resolution high-energy
astronomy leads to the consideration of systems that bridge the gap between focusing and interferometry. X-ray
diffractive Axicons and PFLs are shown to be limiting cases of a family of designs that are the X-ray equivalents
of "Axilenses", offering different combinations of effective area and bandpass. It is shown that linear gratings
can be used as diffractive alternatives to the grazing incidence mirror "periscopes" that have been investigated
as beam combiners in an interferometer. The gratings form achromatic fringes in a process related to the Talbot
effect. The results of simulations and of a laboratory demonstration-of-principle experiment are presented.
The primary instrument of the proposed EXIST mission is a coded mask high energy telescope (the HET),
that must have a wide field of view and extremely good sensitivity. In order to achieve the performance goals
it will be crucial to minimize systematic errors so that even for very long total integration times the imaging
performance is close to the statistical photon limit. There is also a requirement to be able to reconstruct images
on-board in near real time in order to detect and localize gamma-ray bursts, as is currently being done by the
BAT instrument on Swift. However for EXIST this must be done while the spacecraft is continuously scanning
the sky. The scanning provides all-sky coverage and is also a key part of the strategy to reduce systematic errors.
The on-board computational problem is made even more challenging for EXIST by the very large number of
detector pixels (more than 107, compared with 32768 for BAT). The EXIST HET Imaging Technical Working
Group has investigated and compared numerous alternative designs for the HET. The selected baseline concept
meets all of the scientific requirements, while being compatible with spacecraft and launch constraints and with
those imposed by the infra-red and soft X-ray telescopes that constitute the other key parts of the payload. The
approach adopted depends on a unique coded mask with two spatial scales. Coarse elements in the mask are
effective over the entire energy band of the instrument and are used to initially locate gamma-ray bursts. A finer
mask component provides the good angular resolution needed to refine the burst position and reduces the cosmic
X-ray background; it is optimized for operation at low energies and becomes transparent in the upper part of the
energy band where an open fraction of 50% is optimal. Monte Carlo simulations and analytic analysis techniques
have been used to demonstrate the capabilities of the proposed design and of the two-step burst localization
MASSIM, the Milli-Arc-Second Structure Imager, is a mission that has been proposed for study within the context
of NASA's Astrophysics Strategic Mission Concept Studies program. It uses a set of achromatic diffractive-refractive
Fresnel lenses on an optics spacecraft to focus 5-11 keV X-rays onto detectors on a second spacecraft
flying in formation 1000 km away. It will have a point-source sensitivity comparable with that of the current
generation of major X-ray observatories (Chandra, XMM-Newton) but an angular resolution some three orders of
magnitude better. MASSIM is optimized for the study of jets and other phenomena that occur in the immediate
vicinity of black holes and neutron stars. It can also be used for studying other astrophysical phenomena on the
milli-arc-second scale, such as those involving proto-stars, the surfaces and surroundings of nearby active stars
and interacting winds.
We describe the MASSIM mission concept, scientific objectives and the trade-offs within the X-ray optics
design. The anticipated performance of the mission and possible future developments using the diffractive-refractive
optics approach to imaging at X-ray and gamma-ray energies are discussed.
The angular resolution of Chandra is close to the practical limit of grazing incidence telescopes due to the difficulty of
imparting an accurate figure and smooth surface to mirror substrates whose physical area is over two orders of
magnitude larger than their effective area. However, important scientific objectives lie beyond the reach of Chandra and
all future missions being planned by the space agencies. By transmitting X-rays diffractive and refractive optics are not
subject to the same limitations and have a superior diffraction limit. A Fresnel zone plate can be paired with a refractive
lens such that their intrinsic chromatic aberrations cancel to 1st order at a specific energy. The result is a limited but
significant energy band where the resolution is a milli arc second or better, for example, at 6 keV. Chromatic aberration
can be corrected to 2nd order by separating the diffractive and refractive elements. This configuration allows a resolution
of a few micro arc seconds. The optics are very light weight but have extremely long focal lengths resulting in a
requirement for very long distance formation flying between optics and detector spacecraft, and small fields of view.
Opacity of the refractive element imposes a lower limit upon the X-ray energy of about a few keV.
The concept of a gamma-ray telescope based on a Laue lens offers the possibility to increase the sensitivity by more
than an order of magnitude with respect to existing instruments. Laue lenses have been developed by our
collaboration for several years : the main achievement of this R&D program was the CLAIRE lens prototype, which
has successfully demonstrated the feasibility of the concept in astrophysical conditions. Since then, the endeavour
has been oriented towards the development of efficient diffracting elements (crystal slabs) in order to increase both
the effective area and the width of the energy bandpass focused, the aim being to step from a technological Laue lens
to a scientifically exploitable lens. The latest mission concept featuring a gamma-ray lens is the European Gamma-
Ray Imager (GRI) which intends to make use of the Laue lens to cover energies from 200 keV to 1300 keV.
Investigations of two promising materials, low mosaicity copper and gradient concentration silicongermanium
are presented in this paper. The measurements have been performed during three runs: 6 + 4 days at the
European Synchrotron Radiation Facility (Grenoble, France), on beamline ID15A, using a 500 keV monochromatic
beam, and 14 days on the GAMS 4 instrument of the Institute Laue Langevin (Grenoble, France) featuring a highly
monochromatic beam of 517 keV. Despite it was not perfectly homogeneous, the presented copper crystal has
exhibited peak reflectivity of 25 % in accordance with theoretical predictions, and a mosaicity around 26 arcsec, the
ideal range for the realization of a Laue lens such as GRI. Silicon-germanium featuring a constant gradient have
been measured for the very first time at 500 keV. Two samples showed a quite homogeneous reflectivity reaching
26%, which is far from the 48 % already observed in experimental crystals but a very encouraging beginning. The
measured results have been used to estimate the performance of the GRI Laue lens design.
With its large collecting area XEUS will be ideally suited to probe strong gravity fields around collapsed objects and to constrain the equation of state of dense matter in neutron stars. For these studies, detectors are needed which can measure 106 events/sec with high time resolution (10 μsec) and good energy resolution (ΔE = 200 - 300 eV FWHM) combined with an energy and flux independent dead time. The current baseline for a dedicated fast timing detector on XEUS is an array of 19 silicon drift detectors (SDD) operated as single photon detectors. Optionally we have studied an array of 40 x 20 SDD/DEPFET macro pixel detectors read out at a constant frame rate of 105/sec. Alternatively to these two dedicated detectors, a high time resolution mode of the Wide Field Imager (1024 x 1024 DEPFET array with 78μm x 78μm pixels) is considered here. We have simulated the expected timing performance of these detector options based on results from laboratory measurements. We have performed Monte Carlo simulations using the latest available XEUS mirror response files for Crab like sources and intensities ranging from 102 up to 4x106 events/sec. Our results are discussed in the light of the scientific requirements for fast timing as expressed in the ESA Cosmic Vision 2015-2025 plan.
EXIST is being studied as the Black Hole Finder Probe, one of the 3 Einstein Probe missions under NASA's Beyond Einstein program. The major science goals for EXIST include highly sensitive full-sky
hard X-ray survey in a very wide energy band of 5 - 600 keV. The scientific requirements of wide energy band (10-600 keV for the High Energy Telescope considered for EXIST) and large field of view (approximately 130° × 60° in the current design, incorporating an array of 18 contiguous very large area coded aperture telescopes) presents significant imaging challenges. The requirement of achieving high imaging sensitivity puts stringent limits on the uniformity and knowledge of systematics for the detector plane. In order to accomplish the ambitious scientific requirements of EXIST, it is necessary to implement many novel techniques. Here we present the initial results of our extensive Monte-Carlo simulations of coded mask imaging for EXIST to estimate the performance degradation due to various factors affecting the imaging such as the non-ideal detector plane and bright partially coded sources.
The Rossi X-ray Timing Explorer (RXTE) has demonstrated the power
of observations in the time domain in the study of X-ray binaries.
The dynamical variation of the X-ray emission from accreting
neutron stars and stellar mass black holes is a powerful probe of
their strong gravitational fields. At the same time, oscillations
at the neutron star spin frequency during X-ray bursts have been
used to set important constraints on the mass and radius of
neutron stars. The X-ray Evolving Universe Spectroscopy mission
(XEUS), the potential follow-on mission to XMM-Newton, will have
a mirror aperture more than ten times larger than the effective
area of the RXTE proportional counter array (RXTE/PCA). Combined
with a small dedicated fast X-ray timing detector in the focal
plane (XTRA: XEUS Timing for Relativistic Astrophysics), this
collecting area will provide a leap in timing sensitivity of more
than one order of magnitude over the RXTE/PCA for bright sources,
and will open a brand new window on faint X-ray sources, owing to
the negligible detector background. Furthermore, the use of
advanced Silicon drift chambers will improve the energy resolution by a factor of ~ 6 over the RXTE/PCA. The spectroscopic diagnostics of the strong field region, such as the gravitational redshift and the relativistic broadening of the Iron line, will become exploitable simultaneous with the fast timing. XTRA will provide unique opportunities, enabling for example the testing of general relativity in the strong gravity field regime and investigating with unprecedented accuracy the equation of state of matter at supranuclear density. XTRA when observing bright X-ray sources will record typically several hundred thousand counts per second and careful design will be necessary to ensure that the full potential of these data can be exploited. We describe the proposed implementation of XTRA in the XEUS focal plane using Silicon drift detectors and present the current performance of these devices.
CLAIRE is a balloon-borne experiment dedicated to validating the concept of a diffraction gamma-ray lens. This new concept for high energy telescopes is very promising and could significantly increase sensitivity and angular resolution in nuclear astrophysics. CLAIRE's lens consists of 556 Ge-Si crystals, focusing 170 keV gamma-ray photons onto a 3x3 matrix of HPGe detectors, each detector element being only 1.4x1.4x4 cm3. On June 14 2001, CLAIRE was launched by the French Space Agency (CNES)from its balloon base at Gap in the French Alps and was recovered near the Atlantic ocean (500 km to the west) after about 5 hours at float altitude. Pointing accuracy and gondola stabilization allowed us to select 1h12' of "good time intervals" for the data analysis. During this time, 33 diffracted photons have been detected leading to a 3σ detection of the source. Additional measurements made on a ground based 205 meters long test range are also presented. The results of this latter experiment confirm those of the stratospheric flight.
Phase Fresnel lenses have the same imaging properties as zone
plates, but with the possibility of concentrating all of the
incident power into the primary focus, increasing the maximum
theoretical efficiency from 11% to close to 100%. For X-rays,
and in particular for gamma-rays, large, diffraction-limited phase
Fresnel lenses can be made relatively easily. The focal length is
very long - for example up to a million kms. However, the
correspondingly high 'plate-scale' of the image means that the
ultra-high (sub-micro-arc-second) angular resolution possible with
a diffraction limited gamma-ray lens a few meters in diameter can
be exploited with detectors having mm spatial resolution. The potential of such systems for ultra-high angular resolution
astronomy, and for attaining the sensitivity improvements
desperately needed for certain other studies, are reviewed and the
advantages and disadvantages vis-a-vis alternative approaches
are discussed. We report on reduced-scale 'proof-of-principle tests' which are planned and on mission studies of the implementation of a Fresnel telescope on a space mission with lens and detector on two
spacecraft separated by one million km. Such a telescope would be
capable of resolving emission from super-massive black holes on
the scale of their event horizons and would have the sensitivity
necessary to detect gamma-ray lines from distant supernovae.
We show how diffractive/refractive optics leads to a continuum of
possible system designs between filled aperture lenses and
wideband interferometric arrays.
The mission concept MAX is a space borne crystal diffraction telescope, featuring a broad-band Laue lens optimized for the observation of compact sources in two wide energy bands of high astrophysical relevance. For the first time in this domain, gamma-rays will be focused from the large collecting area of a crystal diffraction lens onto a very small detector volume. As a consequence, the background noise is extremely low, making possible unprecedented sensitivities. The primary scientific objective of MAX is the study of type Ia supernovae by measuring intensities, shifts and shapes of their nuclear gamma-ray lines. When finally understood and calibrated, these profoundly radioactive events will be crucial in measuring the size, shape, and age of the Universe. Observing the radioactivities from a substantial sample of supernovae and novae will significantly improve our understanding of explosive nucleosynthesis. Moreover, the sensitive gamma-ray line spectroscopy performed with MAX is expected to clarify the nature of galactic microquasars (e+e- annihilation radiation from the jets), neutrons stars and pulsars, X-ray Binaries, AGN, solar flares and, last but not least, gamma-ray afterglow from gamma-burst counterparts.
SPI, the Spectrometer on board the ESA INTEGRAL satellite, to be launched in October 2002, will study the gamma-ray sky in the 20 keV to 8 MeV energy band with a spectral resolution of 2 keV for photons of 1 MeV, thanks to its 19 germanium detectors spanning an active area of 500 cm2. A coded mask imaging technique provides a 2° angular resolution. The 16° field of view is defined by an active BGO veto shield, furthermore used for background rejection. In April 2001 the flight model of SPI underwent a one-month calibration campaign at CEA in Bruyères le Châtel using low intensity radioactive sources and the CEA accelerator for homogeneity measurements and high intensity radioactive sources for imaging performance measurements. After integration of all scientific payloads (the spectrometer SPI, the imager IBIS and the monitors JEM-X and OMC) on the INTEGRAL satellite, a cross-calibration campaign has been performed at the ESA center in Noordwijk. A set of sources has been placed in the field of view of the different instruments in order to compare their performances and determine their mutual influence. We report on the scientific goals of this calibration activity, and present the measurements performed as well as some preliminary results.
ECLAIRs is a microsatellite devoted to the multi-wavelength observation of the prompt emission of GRBs. For about 100 GRBs per year, ECLAIRs will provide high time resolution high sensitivity spectral coverage from a few eV up to a few hundred keV and localization to 10' in near real time. This capability is achieved by combining wide field optical and X-ray cameras sharing a common field of view with a coded-mask gamma camera providing the trigger and the localization of the bursts. ECLAIRs relies upon an international collaboration involving theoretical and hardware groups from Europe and the United States. In particular, it builds upon the extensive knowledge and expertise that is currently being gained with missions such as HETE-2 and INTEGRAL.
We describe a proposal for an added capability of fast timing to the European x-ray astronomy mission XEUS. The scientific value of fast timing observations for the investigation of compact objects is recognized and has been demonstrated through observations by the Rossi x-ray Timing Explorer. We propose to make use of the huge collecting area of XEUS for timing studies with unparalleled photon statistics and time resolution. We describe the sceintifc motivation, e.g. to probe strong gravity fields around collapsed objects and to constrain the equation of state of dense matter in neutron stars. We discuss options for the implementation of detectors which coudl be small silicon drift detectors out of focus.
Fresnel lenses can focus gamma-rays by using a combination of diffraction and refraction. Such lenses (and variations on them in which the chromatic aberration is much reduced) have the potential for revolutionizing gamma-ray astronomy. Diffraction-limited lenses of several meters in size are feasible and do not require high technology for their manufacture. Focal lengths are long - up to a million kilometers - but developments in formation flying of spacecraft make possible a mission in which the lens and detector are on two separate spacecraft separated by this distance. A telescope based on these principles can have angular resolution better than a micro second of arc - sufficient to resolve the event horizon of black holes in the nucleii of AGNs. At the same time, the sensitivity can be three orders of magnitude better than that of current instrumentation.
We present the design and performance of the gamma-ray lens telescope CLAIRE, which flew on a stratospheric balloon on June 14, 2001. The objective of this project is to validate the concept of a Laue diffraction lens for nuclear astrophysics. Instruments of this type, benefiting from the dramatic improvement of the signal/noise ratio brought about by focusing, will combine unprecedented sensitivities with high angular resolution. CLAIRE's lens consists of Ge-Si mosaic crystals, focusing gamma-ray photons from its 505 cm2 area onto a small solid state detector, with only 7.2 cm3 volume for background noise. The diffracted energy of 170 keV results in a focal length of 279 cm, yet the entire payload weighed under 500 kg. CLAIRE was launched by the French Space Agency (CNES) from its balloon base at Gap in the French Alps (Southeast of France) and was recovered near Bordeaux in the Southwest of France after roughly 5 hours at float altitude. After presenting the principle of a diffraction lens, the CLAIRE 2001 flight is analyzed in terms of pointing accuracy, background noise and diffraction efficiency of the lens.
ESA's INTErnational Gamma-Ray Astrophysics Laboratory (INTEGRAL) will be launched in October 2002. Its two main instruments are the imager IBIS and the spectrometer SPI. Both emply coded apertures to obtain directional information on the incoming radiation. SPI's detection plane consists of 19 hexagonal Ge detectors, its coded aperture has 63 tungsten-alloy elements of 30 mm thickness.
INTEGRAL is ESA's high-energy astrophysics mission to be launched into a high eccentric orbit early in the next decade. One of the two mission's main telescopes is the gamma-ray spectrometer SPI. This instrument features a compact array of 19 high-purity germanium detectors shielded by a massive anticoincidence system. A coded aperture of the HURA type modulates the astrophysical signal. We present the spectrometer system and its characteristics and discuss the choices that led to the present design. The instrument properties like imaging capability, energy resolution and sensitivity have been evaluated by extensive Monte-Carlo simulations. The expected performance for narrow-line spectroscopy is characterized by an energy resolution of approximately 1.6 keV at 1 MeV, an angular resolution of approximately 2 degrees within a totally coded field of view of approximately 15 degrees, and a sensitivity of (2 - 5) multiplied by 10-6 gamma/(cm2 s) for 4 multiplied by 106 s observation time in the nominal energy range from approximately 20 keV and approximately 8 MeV. With these characteristic features it will be possible for the first time to explore the gamma-ray sky in greater depth and detail than it was possible with previous gamma- ray telescopes like SIGMA, OSSE and COMPTEL. In particular the field of nuclear astrophysics will be addressed with an unprecedented combination of sensitivity and energy. Especially the high-energy resolution allows for the first time measuring gamma-ray line profiles. Such lines are emitted by the debris of nucleosynthesis processes, by the annihilation process near compact objects and by the nuclear interaction between cosmic rays and interstellar matter. Lines of all these processes have been measured so far, but, owing to the relatively poor energy resolution, details of the emission processes in the source regions could not be studied. With the high-resolution spectroscopy of SPI such detailed investigations will be possible opening a wealth of astrophysical investigations.
We have begun to study a mission to carry out the first high sensitivity imaging survey of the entire sky at hard x-ray energies (5 - 600 keV). The Energetic X-ray Imaging Survey Telescope (EXIST) would include 2 - 4 large area coded aperture telescopes with offset fields of view allowing total exposures of >= 500 ksec and flux sensitivities below 1 mCrab over the full sky in a year with time resolution from msec to months for each source as well as high spatial and spectral resolution for sources, transients and gamma-ray bursts. A pointed observatory phase, with the telescopes co-aligned, would follow and achieve still greater sensitivities and temporal coverage, allowing the detailed study of virtually all classes of accretion sources (cataclysmic variables to quasars) as well as diffuse galactic emission. The baseline concept originally proposed for the detector is a modularized array (4 X 4) of Cd-Zn-Te crystals (6.25 cm2 each, or 100 cm2/module). An array of 5 X 5 modules, or 2500 cm2 total detector area with 1.25 mm spatial resolution, would constitute the focal plane readout of each of the four telescopes. A brief descriptio of the proposed detector and telescopes and predicted backgrounds and sensitivity is given.
Coded mask X-ray and gamma-ray telescopes are the only way of obtaining true images in the photon energy range from approximately 10 keV to a few MeV. The detectors used must be position sensitive, and the types employed in gamma-ray coded mask telescopes up to now have had limited energy resolution. With a view to developing position sensitive detectors which have the energy resolution attainable with Germanium we have procured and characterized in the laboratory a detector comprising a small array of high purity Germanium elements each 15 X 15 X 50 mm. Although having only nine elements, its construction is such that is should later be possible to build larger modules in the same way and finally to assemble modules into a large detector plane array. The nine element array is being incorporated into a coded mask telescope which will be tested in a balloon flight. Laboratory tests on the array detector and comparisons with simulations are reported and the anticipated performance of the small array telescope considered. The feasibility of a large instrument based on this approach, which is under study for a space mission, and its expected capabilities are discussed.
The International Gamma-Ray Astrophysics Laboratory (INTEGRAL) is a proposed joint ESA/NASA/Russia gamma-ray astronomy mission which will provide both imaging and spectroscopy. It is currently at the final stages of an ESA phase-A study which it is hoped will lead to it being adopted during 1993 as the second 'medium-class' mission within ESA's Horizon 2000 plan. Launched in less than 10 years time it will be the successor to the current generation of gamma-ray spacecraft, NASA's Compton Observatory (GRO) and the Soviet- French Granat/Sigma mission. The baseline is to have two main instruments covering the photon energy range 50 keV to 10 MeV, one concentrating on high-resolution spectroscopy, the other emphasizing imaging. In addition there will be two monitors--an X-ray monitor which will extend the photon energy range continuously covered down to a few keV, and an Optical Transient Camera which will search for optical emission from gamma-ray bursts.
The pointing system described in this paper was originally developed as part of the Hard X-ray Imaging Telescope (XRT) built by the University of Birmingham for flight on the Spacelab-2 mission in 1985. The primary scientific objective of the XRT was the imaging of extended celestial X-ray sources in the energy band 2.5 - 30 kev using the coded-aperture technique. In order to maximize the observing time available to the XRT the instrument was provided with an independent pointing mount. The performance parameters of the pointing system were determined by the requirements of the XRT and resulted in the development of a two- axis gimbal system capable of supporting a moving mass of 280 kg and providing an inertial pointing stability of 20'. The mechanical configuration of a balanced payload with gimbal support bearings rated to withstand the launch environment without off-loading was chosen to enhance reliability and minimize development costs. The electrical configuration is based around duel redundant torque motors and synchros on each axis. The control loop is closed via redundant Intersil IM6100 microprocessors. The control software uses a novel algorithm to estimate gimbal rates from timing transition data from the synchros.
The Energetic X-ray Observatory on Space Station (EXOSS) is a mission concept for high-sensitivity coded-aperture sky surveys and studies of the spectral and temporal behavior of astrophysical sources from approximately 3 keV to 1 MeV. The scientific motivation for the mission and the instrument requirements, including the need for high angular resolution to resolve and identify numerous detectable sources, are summarized. Two baseline telescopes are described: one employing a 1.4-sq-m array of Xe gas imaging proportional counters to cover the 3 to 100 keV range with 1 arcmin resolution; the second using a 2.8-sq-m array of NaI/CsI imaging phoswich detectors to span the 20 keV to MeV range with 12 arcmin resolution.
The capabilities of the European Photon Imaging Camera (EPIC), the main instrument of ESA's 'Cornerstone' mission in X-ray astronomy with multiple mirrors (XMM), are discussed. The CCD characteristics, spatial resolution, energy bandpass and faint source sensitivity, spectral resolution and sensitivity, and timing capability are addressed, and the scientific rationale of the EPIC is summarized. The EPIC instrument system concept is briefly described.