r bulletin 111 — august 2002

The Integral Payload

G. Sarri, P. Garé, C. Scharmberg & R. CarliIntegral Project Team, ESA Directorate for Scientific Programmes, ESTEC, Noordwijk, The Netherlands

Introduction Gamma-ray astronomy is more complicatedthan other branches of astronomy because athigh energies matter is transparent, andtherefore source detection and imaging cannotbe accomplished with standard optical-liketechnologies. In addition, the weakness of thesource fluxes calls for instruments with largedetector areas, which tends to make themlarge and heavy.

The first problem to be solved is how to makean image of a source when the incoming lightcannot be focussed, i.e. it passes through mostmaterials without being deviated. The solutionadopted for Integral is to use a method knownas the ‘coded-mask technique’ (Fig. 1). Theconcept is simple, even if the associatedmathematics is far from trivial. A coded maskcan be seen as a chessboard where the blacksquares are made of very thick and heavymaterial with high atomic number, and thewhite squares are either empty or made of verylight and thin material. The incoming gamma-ray radiation is stopped by the black elements,but is unaffected by the white elements. Anysource will cast a shadow on the detector,placed a few metres below the mask. From thekind of shadow produced and knowing thegeometric characteristics of the mask (the so-called ‘code’), the position and shape of thesource in the sky can be reconstructed on theground. Of course, the presence of severalsources in the sky, the effect of the external andinternal backgrounds, and the fact that thegeometry of the mask is much more complexthan a simple chessboard, make the imagereconstruction a complex mathematical exercise.

The second problem is how to image only whatit is in the field of view of the instrument, and toexclude photons or high-energy particlesreaching the detector from other directions.This could be achieved by surrounding thedetector with a structure of material so thickand heavy that it is able to stop any undesiredradiation. Unfortunately, this is not practical fortwo reasons: firstly, the structure will weighseveral tons, and secondly too much materialaround the detector will create even moredisturbance due to emissions of secondaryradiation. The technique actually used is called‘active anti-coincidence’ and the Anti-Coincidence System (ACS) is a key element ofmost gamma-ray instruments. The ACSconsists of several blocks of thick crystalsurrounding the detector, with the exception ofthe field of view. When the crystal is hit by a

Gamma-rays represent one of the most energetic forms of radiation innature. They carry large quantities of energy radiated from some ofthe cataclysmic of all astronomical events, including exploding stars,colliding neutron stars, particles trapped in magnetic fields, andmatter being swallowed by black holes. Integral’s payload instruments– IBIS, OMC, JEM-X and SPI – will study these gamma-rays throughdetailed imaging and high-resolution spectroscopy, providingastronomers around the World with their clearest views yet of themost extreme environments in our Universe.

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Figure 1. Basic principles of agamma-ray instrument

Blocked photon: stopped bythe closed element of the maskwhich shadows the detector

Valid photon: passes through theopen element of the mask and isrecorded by the detector

Vetoed photon or particle:coming from outside thefield of view of theinstrument, it is detectedat the same time in theanticoincidence crystaland in the detector and isrejected

MASK

ANTI-COINCIDENCE

DETECTOR

Figure 2. The four Integralinstruments are integratedon the upper structure ofthe satellite, known as thePayload Module (PLM)(illustration Medialab)

Integral’s instrumentsThe Integral payload consists of two maingamma-ray instruments, and two monitoringinstruments operating in the X-ray and opticalbands (Fig. 2). The two high-energy instrumentsare the SPI (SPectrometer on Integral), and theIBIS (Imager on-Board the Integral Satellite).The X-ray monitor is the JEM-X (Joint EuropeanMonitor for X-rays), and the optical instrumentis the OMC (Optical Monitoring Camera). All fourinstruments have been designed with goodscientific complementarity in mind in terms ofenergy range, energy resolution and imagingcapability. Each of the two main instrumentshas imaging and energy-resolution capabilities,but whilst the IBIS is best for imaging, the SPIis optimised for spectroscopy. The twomonitors will provide complementary, but stillfundamental, observations of the high-energysources at X-ray and optical wavelengths.

Also forming part of the payload is a smallradiation monitor, which will continuouslymeasure the charged-particle environment ofthe spacecraft, particularly the electrons andprotons. This will provide essential information

particle or a photon, it generates a flash of light,which can be recorded in time. If the timingcoincides with an event in the detector, therelevant particle or photon is considered ascoming from outside the instrument’s field ofview and it is disregarded, or ‘vetoed’.

The detector is the core element of thetelescope where the interactions with thephotons take place and the relevant signals aregenerated. Three interaction mechanisms areinvolved: the photoelectric effect, Comptonscattering and pair production, depending onthe energy of the incoming photon. The higherthe energy that has to be measured, the greatermust be the stopping power of the detector,and therefore its thickness. The quality of theimage generated by the telescope, i.e. itsspatial resolution, depends on the number ofelements constituting each detector, which inIntegral’s case ranges from a few tens toseveral thousands of crystals. As the goal ofthe Integral mission is precise imaging coupledwith fine spectroscopy, the instrumentdetectors will also provide high-resolution linespectroscopy.

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Table 1. Summary of the Integral payload’s main scientific performances

Parameter SPI IBIS JEM-X OMC

Energy Range 20 keV – 8 MeV 15 keV – 10 MeV 3 keV – 35 keV V-filter (at 550 nm)

Detector Area 500 cm2 2700 cm2 1000 cm2 (two units) 1.33 x 1.33 cm2

1024 x 1024 pixels 50 mm aperture diameter

Energy Resolution 2.4 keV @ 1.33 MeV 8% @ 100 keV 16% E > 6 keV 12 bits, 4095 digital levels9% @ 1 MeV

Fully Coded Field of View 16° 9° x 9° 4.8° 5° x 5°

Partially Coded Field of View 35° 29° x 29° 13.2°(zero response) (zero response)

Angular Resolution 2.8° (FWHM) 12 arcmin (FWHM) 3 arcmin 17.6 arcsec x 17.6 arcsec

Point Source Location 30 arcmin 30 arcsec (ISGRI) 30 arcsec

Continuum Sensitivity 3 x 10-7 ph cm-2 s-1 keV-1 5 x 10–7 ph cm-2 s-1 keV-1 1.3 x 10-5 ph cm-2 s-1 keV-1

3 σ in 106 sec @ 1MeV, δE=1 MeV @ 100 keV, δE=E/2 @ 6 keV1.6 x 10–7 ph cm-2 s-1 keV-1 8 x 10-6 ph cm-2 s-1 keV-1

@ 1 MeV, δE=E/2 @ 30 keV

Line Sensitivity 2 x 10-5 ph cm-2 s-1 1.1 x 10–5 ph cm-2 s-1 1.7 x 10-5 ph cm-2 s-1

3 σ in 106 sec @ 1MeV @ 100 keV @ 6 keV5 x 10-5 ph cm-2 s-1 5 x 10-5 ph cm-2 s-1

@ 1 MeV @ 30 keV

Timing Accuracy 3 σ 160 µs 120 µs – 30 min 122 µs 2 ms (frame transfer time)3 s (time resolution)

Limiting Magnitude 17.6 (10 x 100 sec, 3 σ)18.2 (50 x 100 sec, 3 σ)

Sensitivity to Variations mV < 0.1, for mV < 16 (15 x 100 sec, 3 σ)

Figure 3. The Integral instrument consortia

Figure 4. Exploded view of the Spectrometer (SPI), with the core of the instrument,the detector element, housed in a cryostat (courtesy of CNES)

to the payload in cases where high particlebackgrounds (radiation belts, solar flares) arebeing encountered, allowing instrument highvoltages to be switched off and on asappropriate, and will also provide backgroundinformation for sensitivity estimates.

The four scientific instruments have beenprovided by separate scientific consortia, eachled by a Principal Investigator, or PI (Fig. 3).They are nationally funded, with ESA contributingthe Data Processing Electronics (the dedicatedonboard computers), the cryo-cooler, and high-reliability electronic components. The pre-processing and distribution of the scientific datato the science community will be the responsibilityof the Integral Science Data Centre (ISDC),which is also nationally funded via a PI-ledconsortium. A summary of the Integral payload’smain scientific performances and spacecraftresource allocations is given in Tables 1 and 2.

SPI (Spectrometer on Integral)PIs: J.P. Roques (formerly G. Vedrenne) (CESR,Toulouse) and V. Schönfelder (MPE, Garching)

The SPI is a spaceborne spectrometerdesigned to perform high-resolution gamma-ray spectroscopy (Fig. 4). The instrument willexplore the most energetic phenomena thatoccur in the Universe, such as neutron stars,black holes, supernovae and the mostfundamental problems in physics and astro-physics, such as nuclear de-excitation, positronannihilation, and synchrotron emission. To meetits scientific objectives, the instrument has tosatisfy very challenging measurement require-ments. It has thus been designed to providegood angular resolution and an excellentenergy resolution in the range 20 keV – 8 MeV,with imaging and accurate positioning of pointsources (~2.8°) or extended celestial gamma-ray emissions. The SPI’s detection, shieldingand imaging capabilities for high-energyphotons rely on three main features: a 19-detector focal plane, an active shieldingtelescope and a passive coded mask.

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Table 2. Summary of spacecraft resources allocated to the Integral payloads (including DPE)

Instrument SPI IBIS JEM-X OMC Total % of spacecraftresources

Mass 1273 kg 731 kg 76 kg 23 kg 2103 53 %

Peak power 384 W 234 W 68 W 26 W 712 W 45 %in sunlight

Average power 159 W 8 W 8 W 17 W 192 W 26 %in eclipse

Data rate Up to 20 kbps Up to 60 kbps Up to 8 kbps Up to 2 kbps Up to 86 kbps 95 %

Figure 5. The SPI detectors,consisting of 19 encapsu-lated germanium crystalsmounted on a cold plate,which is nominally main-tained at 90 K by fourStirling mechanical coolers (courtesy of CESR)

Figure 6. The SPI duringcalibration activities at the

Centre d’Etudes Atomiquesin Bruyères le Chatel (F)

(courtesy ofCESR/CEA/CNES)

The mask, located on the top of the SPI, holdsa coded motif of tungsten blocks (63 opaqueelements and 64 transparent elements). Thetungsten stops gamma-rays in the energy range20 keV – 8 MeV with an effective blockingpower greater than 95% at 1 MeV. Similarly, the‘holes’ have a 60% transparency at 20 keV and80% at 50 keV. The Plastic Scintillator Anti-Coincidence (PSAC) sub-assembly locatedbelow the mask reduces the background due

to the mask’s 511 keV secondary radiationemission. It is composed of a plastic scintillatorenclosed in a light-tight box with photo-multiplier tubes that convert the light flashesinto electrical pulses. The signals are processedby the PSAC electronics, which send asynchronous veto signal associated with thedetected events to the active shielding controlelectronics (Veto Control Unit).

The detection plane is made of 19encapsulated, hexagonal, high-puritygermanium detectors mounted on a‘cold plate’ cooled to 90 K (Fig. 5). Thedetection plane is itself placed in athermally insulated ‘cold box’ (CBX),whose temperature is maintained atapproximately 210 K by passive cooling.Wherever possible, the box’s structurehas been manufactured from beryllium(limiting the generation of secondaryradiation background noise in thedetectors). The cooling of the detectorsis achieved via the Cold Bus Bar linkingthe cold plate to the Active Cooling(ACC) sub-assembly composed of twopairs of cryocoolers (four compressorsand four displacers). The cryocoolersare fixed onto a radiator, itself mountedon the structure (LSA) supporting theinstrument active shielding and other

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Figure 7. Cut-away view ofthe IBIS detector unit. The two crystal layers, theISGRI (low-energy detection)and the PICSIT (high-energydetection), are surroundedby the veto modules of theanticoincidence system. Thecollimator (hopper) is madeof CFRP covered by a layerof tungsten(courtesy of Laben)

various events detected by the Spectrometer. Itfirst compensates for the differential delays thatmay occur in the different electronics chains.After this ‘time alignment’, the AFEE and PSDevents observed in one or more detectors arerecorded, with their time of occurrence, relativetime, AFEE energy, and PSD identifier (single-or multiple-site interaction). Whenever the ACSactivates the veto signal, events that occurredin that time period are marked as vetoed andhandled separately. The DFEE delivers theannotated lists of AFEE or PSD non-vetoedevents to the Digital Processing Electronics(DPE) for inclusion in the telemetry packets.

The DPE, located on the Payload Module closeto the instrument, acts as the functionalinterface between the instrument and theService Module. It performs the instrumentcommanding (operating modes) and mon-itoring (house-keeping management), as wellas the scientific data management (i.e. buildingthe spectra and transmitting the data packetsto the spacecraft’s data-handling system fortransmission to ground).

IBIS (Imager on-Board the Integral Satellite)PI: P. Ubertini (CNR-IAS, Rome) Co-PIs: F. Lebrun (CEA, Saclay) & G. Di Cocco(CNR-ITESRE, Bologna)

The IBIS will provide high-resolution images ofcelestial objects of all classes, ranging from themost compact galactic systems to extra-galactic objects (Fig. 7). Better than anyprevious imaging instrument operating in thegamma-ray range between 15 keV and 10 MeV, IBIS will achieve an angular resolutionof 12 arcmin and a spectroscopic resolution of 8–9 % over the range 0.1–1 MeV.

electronics. A radiator (PAC) mounted on thestructure of the active shielding performspassive cooling of the cold box via twoammonia heat pipes. Each pair of cryocoolersis controlled by a separate Cooler DriveElectronics (CDE) unit located on the PayloadModule platform near the SPI (Fig. 6).

The bias voltage of each germanium detector isprogrammable up to 5000 V. Dedicated high-voltage filters (located on the cold plate) andcharge-sensitive amplifiers (operating at 200 K)amplify the signal produced by each detector. Apulse-shape amplifier and pulse-heightanalyser located in the Analogue Front-EndElectronics (AFEE 1) process this signal further.The low-voltage supplies for the 19 amplificationchains as well as the germanium-detector high-voltage power supplies are located in AFEE 2.

The Anti-Coincidence System is made up ofscintillator crystals (bismuth germanate oxide),photomultipliers tubes (182 PMTs) with theassociated electronics (91 FEE) and a shieldingVeto Control Unit (VCU). The key function of theactive anti-coincidence sub-assembly is toprotect the detection plane against thebackground (photons and charged particles)from sources located outside the field of view.To that end, the scintillator crystals encircle60% of the telescope’s height, as well asenclosing the back of the detection plane. Thecrystals convert all incoming events intophotons of approximately 480 nm wavelength.The light flashes produced are converted by thephotomultiplier tubes into electrical pulses.These, in turn, are sorted, normalised andsummed by the ACS electronics. The VCU isresponsible for the proper functioning andhealth of the ACS, performing the overallmonitoring and control.

The task of the Pulse Shape Discriminator(PSD) is to actively reduce the instrumentalbackground in the 200 keV – 2 MeV energyrange by pulse-shape discrimination. It digitisesthe current pulses from single detector eventsthat correspond to photon energies in theabove-specified range. The observed currentpulses are then compared on-board with alibrary of single-site current-pulse templatesstored in the sub-assembly, to distinguishsingle-site from multiple-site interactions withinthe germanium detectors. By rejecting single-site and retaining only multiple-site events,active background reduction is achieved,leading to a sensitivity improvement of about afactor two within the PSD energy range.

The Digital Front-End Electronics (DFEE)performs the real-time acquisition, assembly,time-stamping and intermediate storage of the

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Hopper

Cd Te layer(ISGRI)

CsI layer(PICSIT)

Photomultiplier (PMT)Veto Module

Figure 8. The IBIS detectorunit. The eight modules ofthe upper detector layer canbe seen at the bottom of thecollimator. Each module ismade up of 2048 sensingelements. (courtesy of CNR-IAS/CEAand Laben)

Figure 9. The IBIS mask ontop of the Integral PayloadModule. The two JEM-Xmasks are covered by thethermal blanket. The baffleof the OMC is visible in theupper part of the picture,and the two star trackers inthe lower part. On the left isthe SPI

The imaging capability of the instrument isprovided by casting a ‘shadowgram’ of acoded-mask aperture onto a two-layerposition-sensitive detector (Fig. 8). The energyof the incoming gamma-ray photon, transferredto charged particles, is dissipated andmeasured with two parallel pixellised detectorplanes surrounded by an active anti-coincidence scintillation system. A total of 16 384 independent semiconductor crystals,made of cadmium telluride, constitute theupper layer of the detection unit, designed tocover the lower gamma-ray energy range from15 keV to 0.5 MeV. High-energy photons aredetected by the lower layer, made with 4096

caesium-iodide crystal scintillators. Time-coincident events in both planes resulting fromCompton scattering are also analysed.

In order to reject background events asenergetic particles, the sides of both planes aswell as the back face are shielded by an activeanti-coincidence system made of bismutegermanate oxide (BGO) crystal scintillatorblocks coupled with photomultipliers. Atruncated carbon-fibre pyramid covered with athin tungsten layer is placed on top of thedetector as a collimating system to reduce thelow-energy X-ray background. An on-boardCalibration Unit (CU), consisting of a 22Naradioactive source in combination with ascintillator-based coincidence strobe generator,is included within the instrument as a knownreference for in-flight calibration.

The coded mask is obtained from a pattern of 95 x 95 elements, covering a total area of 1064 x1064 mm2. Half of the elements aremade of 16 mm-thick tungsten, offering 70%opacity at 1.5 MeV; the remaining elements areopen and consequently transparent to photons(Fig. 9).

The upper detection layer, the Integral SoftGamma-Ray Imager (ISGRI), is made of eightidentical, modularly organised detection units,providing a total sensitive area of 2621 cm2.Each module contains 2048 independentlyoperated CdTe detectors, 4 x 4 x 2 mm3 in size.Due to the high number of pixels, speciallydeveloped Application-Specific IntegratedCircuits (ASICs) convert the electrical chargesinto proportional voltage signals. Each ASIC isdesigned to support four pixels, and so 4096ASICs have been manufactured for the IBISflight model. Final processing of the analogueparameters is done by eight electronics unitsaccommodated in two ISGRI Electronics Boxes(IEB 1 and 2).

The upper energy range of 200 keV – 10 MeVis covered by the Pixellated Imager Caesium-Iodide Telescope (PICsIT), which is located 90 mm below ISGRI. Like the ISGRI, the layeris modularly organised with eight rectangulardetector units, each consisting of 512 tallium-doped caesium-iodide crystal scintillator barsof 8.35 x 8.35 mm2 with a thickness of 30 mm.The bars are optically coupled and readout bycustom-made low-leakage silicon photodiodes.32 ASICs inside each module, containing pre-amplification, pulse shaping, amplification,signal discrimination, peak detection, andanalogue storage, analyse the signals from thephoto-diodes. Two separate PICsIT ElectronicsBoxes (PEB 1 and 2) process the detectorsignals and convert them into digital event-

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Figure 10. The JEM-Xdetector assembly. Theupper element is thedetector vessel, whilst thelower, black element is theDigital Front-End Electronics(DFEE). The square patternof the collimator can beseen on the top of thedetector vessel(courtesy of DSRI)

Figure 11. The JEM-X mask is made from a thintungsten plate. The codedpattern is provided by aregular combination of smallhexagonal holes. Supportingribs are provided to carrythe launch loads(courtesy of Univ. ofValencia/Sener)

position and photon-energyinformation.

16 Veto Detector Modules (VDMs),each made of BGO scintillatorcrystal and analysed by twophotomultipliers, function as anti-coincidence detectors for ISGRI andPICsIT events to reduce the detectorbackground. Eight BGO modulesare located below the two detectorlayers, and eight are placed aroundthem. The signals from the photo-multipliers are routed to the VetoElectronics Box (VEB), in which theanalogue signals are discriminatedby programmable thresholds togenerate binary veto signals. TheVEB also controls the onboardCalibration Unit. The veto eventsignals are used directly by theISGRI and PICsIT in order to rejectcoincident events.

During times of high photon fluxes,the IBIS may generate more eventsthan can be transferred to groundwith the allocated telemetry rate. Aspecial pre-processing unit hastherefore been built to enhance thespacecraft DPE’s capabilities. It willaccumulate data into onboardhistograms, reconstruct multiple orCompton events, and calculateenergy amplitude corrections usingdedicated calibration tables.

JEM-X (Joint European X-Ray Monitor)PI: N. Lund (DSRI, Copenhagen)

In order to have a comprehensive under-standing of physical phenomena in the celestialsources observed by the SPI and IBIS, it isimportant to extend the energy range of theobservations to the lower X-ray energy band.Therefore an X-ray monitor was needed inIntegral’s payload complement. JEM-X willprovide images with an angular resolution aslow as 3 arcsec in the 3 – 35 keV energy band(Fig. 10).

The instrument consists of two identical high-pressure micro-strip gas chambers, which viewthe sky through two identical coded masks(Fig. 11). For each of the two JEM-Xs, the X-rayphotons entering the instrument’s field of viewpass through the holes in the coded mask,which is located about 3.4 m above thedetector entrance window. Inside the detector,which is filled with a mixture of 90% xenon and10% methane, the photons are absorbed bythe xenon gas. This photon absorption process

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is mainly photoelectric and the emittedelectrons ionise the other atoms, therebygenerating an ionisation cloud. The cloud isamplified whilst drifting towards the detectionplane, thanks to an electric field appliedbetween the detector entrance window and thedetection plane itself. The avalanche ofionisation is eventually detected by the micro-strip plate, which constitutes the detector focalplane (Fig. 12). The resulting electrical signal isproportional to the energy of the incomingphoton. The position is determined by knowingon which anode/cathode group the charge iscollected.

The imaging principle is the same as for the twoother main instruments and is based on thecoded-mask technique. The main hardwareelements of the JEM-X are therefore the mask,the detector (with its key constituents thecollimator, the pressure vessel with entrancewindow, and the micro-strip plate), the DigitalFront-End Electronics for signal processing,and the dedicated onboard computer (DPE).

orthogonal coordinate is given by a set ofpickup electrodes on the rear surface of theglass plate. The micro-strip plate is surroundedby a veto electrode, which is used to suppressevents caused by charged particles enteringthe detector from the side.

The instrument electronics are physicallydistributed in three locations. The detectorvessel hosts the first level, whose purpose isanalogue signal processing and high-voltagedistribution to the micro-strip sensors. Thesecond level is a dedicated box called theDigital Front-End Electronics, which performsthe digital elaboration of the signals. It alsogenerates, controls and distributes the highvoltage and the secondary voltages to theinstrument. The last element is the Data-Processing Electronics, which is also aseparate unit provided and controlled by thesatellite. Its functions are data reception fromthe front end, data compression, and themanagement of all telemetry and telecommandinterfaces with the satellite.

Three JEM-X flight units have beenmanufactured, two flight units and one flightspare. The two JEM-X units on the spacecraftwill be operated independently and simul-taneously, thereby making the instrument fullyredundant with respect to any possible failure.

OMC (Optical Monitoring Camera)PI: M. Mas-Hesse (formerly A. Gimenez)(LAEFF/INTA, Madrid)

The Integral model payload was designed tostudy simultaneously high-energy sources in awide field of view over many decades in energy,and thus to make a major contribution to short-time-scale high-energy astrophysics. The OMC(Fig. 13) observes the optical emission from theprime targets of the two gamma-ray instrumentswith the support of the X-ray monitor. Thiscapability provides information on the natureand the physics of the sources over a broadwavelength range. Multi-band observations areparticularly important in high-energy astrophysics,where variability is typically rapid, unpredictableand of large amplitude. The main scientificobjectives with OMC are therefore to:– monitor the optical emission of all high-energy

targets within its field of view, simultaneouslywith the high-energy instruments

– provide simultaneous and calibrated standardV-band photometry of the high-energysources, to allow comparison of their high-energy behaviour with previous or futureground-based optical measurements

– analyse and locate the optical counterpartsof high-energy transients detected by the otherinstruments, especially gamma-ray transients

The coded mask is a plate of tungsten 0.5 mmthick and with a diameter of about 0.5 m. Dueto its thinness, it behaves like a membrane, andthe titanium supporting structure has a pre-tensioning system to help keep the naturalfrequency above the forcing frequencies thatwill be experienced during launch. The maskelements are hexagonal and 3.3 mm in size,and the holes are produced by electricalerosion. The collimator, located on top of thedetector vessel, reduces the background andlimits the instrument’s field of view to about 6 deg, which is also driven by the detector-maskcombination. Its cells are square in shape andit is made of molybdenum. Four small radio-active sources mounted on the collimator willserve as references for the in-flight calibrationof the instrument. The collimator also has astructural function, which is to protect the verythin entrance window against the internaldetector pressure and the launch loads.

The entrance window must be as thin aspossible and is made from a metal with a lowatomic number to allow good transmission ofthe low-energy (3 keV) X-rays. A 0.25 mm-thickberyllium foil has been selected. The detectormicro-strip plate and its basic conditioningelectronics are contained in a stainless-steelvessel. The gas inside the vessel has a nominalpressure of about 1.4 bar. The micro-strip plateis the sensing element of the detector and isthe equivalent of the wires in a classicalmultiwire proportional chamber. It is made up ofa pattern of alternating cathode and anodestrips with a pitch of 1 mm, built on a glass-plate substrate. This pattern gives onecoordinate in the detection plane, and the

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Figure 12. The micro-stripplate and its associatedelectronics are the coreelements of the JEM-X

detector. Due to theirextreme sensitivity to

contamination, all assemblyoperations have been

carried out in an extra-cleanenvironment

(courtesy of DSRI)

Figure 14. The OMC Camera Unit during its final acceptance testing. The squareelement close to the focal-plane region is the radiator for the CCD(courtesy of LAEFF/INTA)

– monitor any other optically variable sourcewithin the OMC field of view which may requirelong periods of continuous observations fortheir physical understanding.

The OMC is also designed to provide data forthe estimation on the ground of the precisepointing of the observatory with an accuracy ofa few arcseconds. This information allows theImager, Spectrometer and X-ray monitorimages to be reconstructed on the ground withmaximum angular resolution.

The OMC consists of a charge-coupled-devicecamera unit (Fig. 14) connected to a singleelectronics unit. The core of the unit is a large-format CCD (2048 x 1024 pixels) working inframe-transfer mode (1024 x 1024 image area)to avoid the need for a mechanical shutter. TheCCD resides in the focal plane of a refractivesystem (Fig. 15) with an entrance pupil of ~50 mm and a field of view of 5.0 x 5.0 deg2. It will be passively cooled by means of aradiator to an operational temperature range of-100 to -70°C. The complete system coversthe wavelength range between 500 and 850 nm.A V-filter is included to allow for photometriccalibration in a standard system. An opticalbaffle guarantees the necessary stray-lightreduction for diffuse background. A once-onlydeployable cover mounted on a speciallydesigned fore-baffle will protect the optics fromcontamination during ground and early in-orbitoperations. The fore-baffle, besides accom-modating the cover mechanism, will alsoprotect the main baffle entrance from directsolar irradiation.

The CCD read-out electronics, residing in theOMC Electronics Unit, has multiple functions. On

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COVER SYSTEM

MAIN BAFFLE

FOCAL PLANE ASSEMBLY

I/F SUPPORT

OPTICS

CCD PASSIVECOOLING

Figure 13. Schematic of theOptical Monitoring Camera(OMC). The CCD sensingelement is passively cooledto – 80ºC(courtesy of LAEFF/INTA)

the one hand it will control and command theCCD, and on the other it will digitise the read-outs and process the data for transmission toground via the DPE and the spacecraft telemetrysystem. Within the CCD cavity of the camera,there are two LEDs for calibration purposes.When activated they illuminate the image area ofthe CCD. The differential response of each pixelto this known illumination pattern is used to builda flat-field correction matrix, which is needed forthe photometric calibration of the images(ground processing). This system is intended todetermine the relative quantum efficiency ofeach CCD pixel to an accuracy of better than1% in a period of less than 1000 sec. New flat-field images will be taken with a periodicity of theorder of CCD degradation once Integral is in orbit. For comparison, dark-currentmeasurements will be performed using masked CCD pixels. Periodic dark-currentmeasurements and flat-field images will alsocontribute to the absolute photometriccalibration performed on the ground using thestandard photometry stars imaged in ‘ScienceMode’.

During the nominal mission phase, scientificdata will be acquired repetitively when thesatellite is in a stable pointing mode. The OMCis then in its standard operating mode, calledthe ‘Science’ mode. The instrument does notprovide images whilst the spacecraft is slewing,but is ready to accept new imaging commandsfor the next acquisition. In Science mode, the

OMC will take images of the full fieldof view every 1 to 255 sec,depending on the integration timesfor the different targets. Thebaseline is to follow a givensequence of different integrationtimes within these limits in order tomonitor both bright and faintsources in the field of view. Thissequence is configured just beforethe Science mode is entered, usinga dedicated imaging command. Thetarget stars are selected from a skycatalogue (the OMC Catalogue)using a software tool provided bythe OMC team to the IntegralScience Operations Centre.

Other operating modes for specificinvestigations are the FastMonitoring sub-mode, which willallow the monitoring of rapidlyvariable sources down to 1 secperiodicity, and the Trigger sub-mode, which will allow themonitoring of new sources with ashort response time.

ConclusionsSeven years after the kick-off of its designphase (Phase-B), the Integral satellite is nowready to fly. As confirmed by the recentlyconcluded Instrument Flight AcceptanceReview, the performances of the four payloadsare as good or even better than those foreseenduring the Phase-A study.

AcknowledgementsThe authors wish to acknowledge the hugeefforts of the Principal Investigators, theInstrument Project Managers, and theirscientific and industrial teams. Thanks to theirdedication and to the work of Alenia Spazio assatellite Prime Contractor, a new ship has beenbuilt to sail towards the new frontiers ofastronomy. r

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Figure 15. The OMC focal-plane assembly, showing

the CCD at the top(courtesy of LAEFF/INTA)