SRG (Spectrum Roentgen Gamma)
Spektr-RG / SRG (Spectrum Roentgen Gamma) astrophysical observatory
Spektr-RG/SRG is an international cooperative space research and technology demonstration mission of Roskosmos (Russia), ESA (Europen Space Agency), IKI (Space Research Institute), Moscow; MPE (Max-Planck-Institute for extraterrestrial Physics), Garching, Germany, and NASA/MSFC, Huntsville, AL, USA. A MOU (Memorandum of Understanding) was signed in March 2007 between DLR and Roskosmos. On Aug. 18, 2009, DLR and Roskosmos signed a detailed agreement during the MAKS International Aviation and Space Salon in Moscow, specifying all the organizational and technical boundary conditions for the eROSITA project. 1) 2)
The overall objective of the mission is to conduct the first all-sky survey with an imaging telescope in the 0.5-11 keV band to discover the hidden population of several hundred thousand obscured supermassive black holes and the first all-sky imaging X-ray time variability survey. In addition to the all-sky surveys it is foreseen to observe dedicated sky regions with high sensitivity to detect ten thousands of clusters of galaxies and thereafter to do follow-up pointed observations of selected sources, in order to investigate the nature of Dark Matter and Dark Energy. The proposed orbit provides an order of magnitude lower particle background than those of Chandra and XMM-Newton, which will allow the detailed study of low-surface-brightness diffuse objects.
The newly defined SRG mission represents a highly significant scientific and technological step beyond Chandra/XMM-Newton and is expected to provide important and timely inputs for the next generation of giant X-ray observatories like IXO (International X-ray Observatory) collaborative mission of ESA along with NASA and JAXA for the timeframe 2021.
• Roskosmos is the provider of the spacecraft bus and the launch of the satellite
• The ART-XC instrument and gamma-ray burst detector are being provided by RosKosmos (an IKI-led consortium)
• ESA will provide the communication subsystem and ground station support.
Initially (early 1990s), the high-energy international mission was called SXG (Spectrum-X Gamma), planned to be developed under the auspices of the Russian Space Agency, with instruments contributed by research groups in a number of European countries and the US. The payload consisted of a number of imaging instruments for cosmic photon observations in the energy range of 0.03 - 100 keV. A launch was planned for 1999. - However, due to repeated delays of the SXG project, caused by the economic situation in post-Soviet Russia, a new approach was taken in the timeframe 2004.
In the early SRG definition phase of the mission (2005-2007), there was a third payload in planning for SRG, namely the LWFT (Lobster-Eye Wide Field Telescope), a new technology introduction designed and developed by the Space Research Center of the University of Leicester, UK. However, the LWFT payload was removed from the SRG mission in 2008 due to budgetary problems in the UK.
The LWFT instrument is an all-sky X-ray monitor comprised of 5 telescope modules, each consisting of approximately 60 MCP (Microchannel Plate) optics, tiled to produce the required field of view and geometrical area. Each telescope module has a so-called "microwell array proportional counter" detector in the focal plane. 8) 9) 10)
The ‘lobster-eye' geometry which permits this huge sky coverage as well as high sensitivity, is achieved with square-pore MCPs based on the eyes of the crustaceans themselves (Figure 1) with Micromegas detectors. This configuration rotates to cover the whole sky in ~90 minutes in continuous slew mode with a given source transiting the FOV in 300-1800 s.
Figure 1: Focal plane diagram showing the lobster-eye geometry (image credit: University of Leicester)
The SGR spacecraft is based on the 'Navigator' multi-use bus of the NPO Lavochkin Scientific Production Association, Khimki, Russia. The spacecraft is 3-axis stabilized and has an estimated launch mass of ~ 2650 kg (total payload mass of 1160 kg). 11) 12) 13) 14)
Figure 2: Artist's view of the deployed SRG spacecraft (image credit: IKI)
Figure 3: Illustration of the Navigator bus architecture (image credit: Lavochkin Association)
Table 1: Basic parameters of the Navigator bus
RF communications: Data transmission rate of 2 Mbit/s.
Figure 4: Illustration of the SRG spacecraft (image credit: NPO Lavochkin)
• On January 20, 2017, the completed eROSITA X-ray telescope boarded a cargo plane and was transported from Munich, where it had been built at the MPE (Max Planck Institute for Extraterrestrial Physics), to Moscow. At NPO Lavochkin Scientific Production Association, Khimki near Moscow, eROSITA will be further tested and integrated with the SRG (Spectrum Roentgen Gamma) spacecraft in preparation for launch in spring 2018. 15)
- "With its much higher sensitivity than previous survey missions, eROSITA will discover a multitude of new X-ray sources," expects Dr. Andrea Merloni, eROSITA project scientist. "We will be able to study not only the distribution of clusters of galaxies – eROSITA will detect more than 100,000 of these most massive bound objects in the Universe – but also millions of active black holes at the centers of galaxies, as well as rare objects in the Milky Way, such as isolated neutron stars. The survey will thus provide new insights into a wide range of high-energy astrophysical phenomena – maybe even reveal some completely new phenomena – and give us new clues about the mysterious "Dark Energy", the force behind the accelerated expansion of the Universe."
- SRG also carries the Russian telescope "ART-XC". Both instruments will be launched with a Proton rocket from the Russian launch site Baikonur in Kazakhstan after another 2600 km transport.
Figure 5: After about ten years of development and integration the eROSITA X-ray telescope is complete: with 7 mirror modules and 54 mirror shells each combined with 7 specially built X-ray cameras. You see the telescope here after final integration at MPE, shortly before transport to further testing (image credit: MPE)
• December 2016: The Russian-built ART-XC telescope and the Navigator platform (which would carry both instruments into space), were reported to be in a high degree of readiness for flight. The ART-AC instrument was finally delivered from RKTs Progress in Samara to NPO Lavochkin by Dec. 27, 2016, according to Anatoly Zak (Ref. 18).
- The final assembly and the integrated testing of the Spektr-RG / SRG spacecraft is expected to take around nine months, making the spacecraft available for the delivery to Baikonur around October 2017.
• December 2016: The final test campaign on the full telescope (vibration, mass properties and EMC) took place at IAGB (Ottobrunn), prior to the Moscow flight of eROSITA.
• November 2016: The eROSITA FM (Flight Model) telescope has been fully integrated in the MPE integration Lab. After completion of the work in September 2016, the telescope left MPE to be transported to the PANTER test facility, where it underwent an extensive test campaign. 16)
• April 2016: The final calibration of eROSITA's 8 MAs (Mirror Assemblies) is still ongoing in the PANTER facility of MPE, while the calibration of all CAs (Camera Assemblies) is underway in the smaller PUMA facility at MPE. 17)
- In parallel, the project started the preparation for the complete telescope integration (Figure 6), each of the 7 MA-CA pairs will be mounted first, thereby precisely adjusting the distance between mirror and camera to the individual focal lengths which has been measured during the mirror calibration.
- The SRG spacecraft is assembled in proto-flight configuration, waiting for the integration of the radio complex FM, expected for May 2016. The development for the GCS (Ground Control Segment) is ongoing, with no delays. The SRG GCC (Ground Control Center) will be operative in Q2 of 2017. A compatibility test with the MPE GCS is scheduled for early 2017. The launch vehicle are available and the SRG launch is confirmed for September 2017.
Figure 6: The integration of the MAs and MCs (shown here) requires an extreme accuracy: the individual focal length (1600 mm) of each pair has to be within 50 µm. The metrology system has been developed at MPE (image credit: MPE)
Launch: A launch of the SRG spacecraft is scheduled for 2018 on a Proton/Block-DM-03 vehicle from Baikonur, Kazakhstan. 18)
Orbit: The orbit of SRG has been selected to be at L2 (Lagrangian Point 2). The spacecraft will be in a Lissajous (or halo) orbit about the Lagrangian point L2. In the Sun‐Earth system the L2 point is on the rotating Sun-Earth axis about the same distance away as L1 (1.5 million km, representing 1/100 the distance from Earth to the Sun) but at the opposite side of the Earth. The L1 location is inside the Earth orbit while the L2 location is outside the Earth orbit. 19)
Note: The initial LEO circular orbit of the mission of 580 km altitude was revised in 2008 to a Lagrangian orbit at L2.
Figure 7: Illustration of the Lagrangian points in the Sun-Earth system (image credit: IKI)
• The 3 months flight to L2 will be used in the verification and calibration of the payload.
• 4 years - duration of an all sky survey; 8 all sky surveys (scanning mode: 6 rotations/day, 1 degree advance per day)
• 3 years - follow-up period (goal) used for pointed observations of a selection of the most interesting galaxy clusters and AGNs (Active Galactic Nuclei).
Sensor complement: (eROSITA, ART-XC)
The scientific payload consists of two independent telescopes — a soft-X-ray survey instrument, eROSITA, being provided by Germany, and a medium-X-ray-energy survey instrument, ART-XC (Astronomical Roentgen Telescope- X-ray Concentrator), being developed by Russia.
eROSITA (extended ROentgen Survey with an Imaging Telescope Array):
eROSITA is an instrument designed and developed at the MPE (Max-Planck-Institut für Extraterrestrik), Garching, Germany (lead institute and project management) with DLR funding. Further cooperating institutions and companies in the eROSITA project are: Institut für Astronomie und Astrophysik der Universität Tübingen (IAAT); Astrophysikalisches Institut Potsdam (AIP); Dr.-Remeis-Sternwarte, Bamberg; Hamburger Sternwarte; Space Research Institute (IKI), Moscow; Roskosmos, Moscow; Kayser-Threde GmbH, Munich; Carl Zeiss AG, Oberkochen; MLT (Media Lario Technologies), Italy.
The general design of the eROSITA X-ray telescope is derived from that of ABRIXAS (A Broadband Imaging X-ray All-Sky Survey). A bundle of 7 mirror modules with short focal lengths make up a compact telescope which is ideal for survey observations. Similar designs had been proposed for the missions DUO (Dark Universe Observatory) and ROSITA but were not realized due to programmatic shortfall. Compared to those, however, the effective area in the soft X-ray band has now much increased by adding 27 additional outer mirror shells to the original 27 ones of each mirror module. The requirement on the on-axis resolution has also been confined, namely to 15 arc seconds HEW (Half Energy Width). For these reasons the prefix "extended" was added to the original name "ROSITA". The scientific motivation for this extension is founded in the ambitious goal to detect about 100,000 clusters of galaxies which trace the large scale structure of the Universe in space and time. 20) 21)
- An extension of the ROSAT imaging all-sky survey to higher X-ray energies up to 10 keV with unprecedented spectral and angular resolution. Thereby, eROSITA will act as a pathfinder for the next X-ray observatories, particularly the SIMBOL-X and the IXO (International X-ray Observatory) missions.
- With the detection of a sufficiently large number of clusters of galaxies, precise measurements will be feasible of the equation of state of Dark Energy and of "baryonic acoustic oscillations" in the power density spectrum of galaxy clusters.
• To detect systematically all obscured accreting Black Holes in nearby galaxies, as well as many (> 170000) new, distant active galactic nuclei
• To detect the hot intergalactic medium of 50-100 thousand galaxy clusters and groups and hot gas in filaments between clusters, so as to map out the large-scale structure in the Universe for the study of cosmic structure evolution
• To study in detail the physics of galactic X-ray source populations, like pre-main sequence stars, supernova remnants and X-ray binaries.
eROSITA will perform a deep survey of the entire X-ray sky. In the soft band (0.5-2 keV), it will be about 30 times more sensitive than ROSAT, while in the hard band (2-8 keV) it will provide the first ever true imaging survey of the sky. The design driving science is the detection of large samples of galaxy clusters to redshifts z > 1 in order to study the large scale structure in the Universe and test cosmological models including Dark Energy. In addition, eROSITA is expected to yield a sample of around 3 million AGN (Active Galactic Nuclei), including obscured objects, revolutionizing our view of the evolution of supermassive black holes and of their role in the structure formation process. The survey will also provide new insights into a wide range of astrophysical phenomena, including X-ray binaries, active stars and diffuse emission within the Galaxy.
The X-ray telescope of eROSITA consists of 7 identical and co-aligned mirror modules, each with 54 nested Wolter-1 type mirror shells. The mirror shells are glued onto a spider wheel which is screwed to the mirror interface structure making a rigid mechanical unit. The assembly of 7 modules forms a compact hexagonal configuration with 1300 mm diameter (Figure 9) and will be attached to the telescope structure which connects to the 7 separate CCD cameras in the focal planes. The co-alignment of the mirror module enables eROSITA to perform also pointed observations.
Figure 8: Schematic view of the eROSITA instrument (image credit: MPE, Ref. 14)
Figure 9: Configuration of the eROSITA mirror modules (image credit: MLT, MPE)
The mirrors manufacturing is based on a replication process from the ultra-smooth polished master. The mirror manufacturing process is divided into the following main steps (Figure 10):
1) The master is created in order to have a shape that is the negative of the final reflective surface of the mirror to be produced.
2) The master is then super-polished to have appropriate roughness and shape accuracy.
3) A layer of gold is deposited onto the master.
4) The master is then mounted on a support frame that holds it during electroforming, allowing also a proper rotation inside the galvanic bath. The master is then placed in the electroforming bath containing a proprietary chemistry, where the metal layer is deposited up to the desired thickness forming directly the mirror.
5) After the electroforming process is completed the mirror and the master are separated by thermal separation. The particular properties of the separation layer ensure a clean interface separation at the original master's outer surface, thus reproducing the master's optical surface quality onto the mirror.
6) Integration in VOB (Vertical Optical Bench).
7) X-ray test.
Figure 10: Overview of the mirror manufacturing process (image credit: MLT, MPE)
In the frame of the eROSITA mission Media Lario Technologies is in charge of the production of the new mandrels from n. 27 to n. 1, which are the bigger ones. The remaining mandrels from n. 54 to n. 28 are provided by the Max Planck Institute and are the ones used for the ABRIXAS mission (Ref. 20).
Table 2: Basic data of the eROSITA telescope 34)
Figure 11: Illustration of the eROSITA instrument (image credit: MPE)
Each telescope consists of a highly nested mirror system with 54 paraboloid-hyperboloid mirror shells and a frame store pnCCD (pn-junction CCD detectors) camera with a 3 cm x 3 cm large image area (corresponding to a field of view of 1º in diameter on the satellite) with a pixel size of 75 µm by 75 µm. The imaging area consists of 384 x 384 pixels.
The camera is equipped with seven dedicated focal plane pnCCD detectors which are located at the focal point of each of the mirror modules. The pnCCD detector concept permits accurate spectroscopy of X-rays as well as imaging with high time resolution. The pnCCD detector is based on the successful XMM-Newton pnCCD detector concept but was further improved in terms of design and technology. 35)
In particular, a frame store section is added to the image area for the purpose of simultaneous imaging and a readout capability in separate CCD areas. The thickness of the whole pnCCD chip of 450 µm is uniformly sensitive to X-rays from very low up to very high energies. The X-ray photon detection efficiency is at least 90% in the energy band from 0.3 keV to 10 keV. Frame store operation allows very high frame rates of up to 200 X-ray images per second without smearing of the image (Figure 12).
Operating the CCD in frame store (i.e. frame transfer) mode is accomplished by transferring rapidly the image into the frame store area within 200 µs after the 50 ms exposure time. Since the frame store area is shielded against X-rays, the stored image can be read out without interference row by row, while the photons of the next image accumulate in the image area. The readout time is less than 10 ms for the complete image, in the remaining time the CAMEX (CMOS Amplifier and MultiplEXer) readout chip is switched into standby mode to minimize the average heat dissipation to the focal plane CCD.
Figure 12: Prototype of the eROSITA pnCCD detector module with 2 cm x 2 cm large pnCCD (image credit: MPI-HLL)
The pnCCD shows best energy resolution at temperatures below -60º C. For the satellite mission a lower temperature is required due to the proton environment which causes radiation damage by creating defects in the silicon lattice. The eROSITA pnCCD detector offers in addition the possibility to select the optimum gain out of a choice of 16 discrete levels. This option allows spectroscopy and imaging of higher energetic particles, e.g. electrons or protons, on ground as well as in space.
Detector module: The pnCCD is a column-parallel device. Each of the 384 CCD channels is equipped for this purpose with an anode, which is connected to the gate of the on-chip JFET (Junction Field-Effect Transistor). The CCD on-chip transistors are operated in source follower mode and are read out in parallel by the CAMEX chip. This mixed signal ASIC comprises 128 parallel readout channels; each of them includes an eightfold correlated double sampling filter. The specific settings, which determine one of the various possible operating modes, are stored in digital registers of the CAMEX. Programming is done via a serial interface. The ASIC features the following functional blocks:
- Current source to bias the pnCCD JFET
- JFET preamplifier for pnCCD signal amplification
- Programmable RC low pass for bandwidth limitation
- Eight-fold correlated double sampling filter
- Sample and hold stage
- 128 to 1 multiplexer
- Output buffer
- On-chip digital sequencer (16 x 64 bit)
- Serial programming interface with LVDS
- Option of integrated bias current DACs (digital analog converters).
Figure 13: Schematic drawing and geometry of the eROSITA flight detector (image credit: MPI-HLL)
eROSITA camera: The entire eROSITA camera including control and readout electronics is developed in a close cooperation by MPE Garching and MPI-HLL (Max-Planck-Institut Halbleiterlabor), Munich.
The electronics of each eROSITA camera can be subdivided into frontend electronics nearby the detector and the other electronics, which is located in a particular box per camera system. That way the seven eROSITA cameras are independent of each other. Their arrangement is shown in Figure 14.
The camera housing provides also a thermal interface and protects the detector against light, dust and radiation (Figure 15). A cylindrical titanium block, which is glued on top of the Al2O3 detector PCB (Printed Circuit Board), is fitted into the CCD-module casing and fixed with a single screw (Figure 16). The aluminum CCD module casing is cooled to -80ºC using heat pipes; it is thermally insulated with MLI (Multilayer Insulation). The module is mounted with thin glass fiber brackets to the surrounding proton shield providing a temperature of approximately +20ºC.
Figure 14: Arrangement of the seven eROSITA camera modules (without electronics boxes), image credit: MPE
Figure 15: Cross section of the eROSITA camera module (image credit: MPE)
Legend to Figure 15: X-rays enter the pnCCD image area top left through the cut-out in the proton shield. While the copper proton shield is not cooled, the camera casing, which surrounds the CCD detector, is connected with a heat pipe to the central cooling system for all seven cameras. The electronics box with the DAQ (Data Acquisition) system belonging to the camera is not shown.
Figure 16: Illustration of the eROSITA detector module (image credit: MPE)
Figure 17: Photo of the eROSITA QM (Qualification Model) camera (image credit: MPE) 36)
Data acquisition system electronics: Apart from the detector electronics, each camera comprises a DAQ system and electrical interfaces (Figure 18). The main interfaces are a synchronous bidirectional bus for telecommands and telemetry and a power line (+27 V nominal and redundant from power distribution unit).
The DAQ electronics provides the following subsystems:
- Power converters and control units. Some voltages are switchable and/or commandable by telecommands
- Sequencer with reprogrammable FPGA, which generates all necessary timing signals for the detector and the ADCs (Analog Digital Converters)
- ADCs, which digitize the differential analog CAMEX signals into 14 bit data for each CCD pixel per frame. The digital data are delivered to a DSP via a FIFO implemented in the FPGA using DMA transfers.
- DSP event processor: A digital signal processor (DSP) calculates offset as well as noise tables and is able to transmit unprocessed raw data for testing purposes. The DSP applies offset and common mode corrections, event thresholds (low and high), and optionally performs split event recognition. The processed data (with energy, position and time information) are transmitted to the interface controller. The event processor executes and distributes telecommands, collects housekeeping and science data and is able to upload new software to itself and the FPGA. Memory uploads can be stored temporarily in volatile memory or permanently in reprogrammable FLASH memory. The initial program will be stored in higher qualified boot-PROMs. The DSP uses its fast internal memory for calculations. An external memory is available for additional data tables and testing.
- Onboard calibration wheel control: Movement and position readout of the calibration mechanism with its selectable positions: open (nominal), closed (testing) and X-ray source (calibration).
The event processor must complete the digital signal processing of a frame, i.e. of 147,456 pixels, during the cycle time of 50 ms. This includes in particular for each pixel offset subtraction as well as common mode determination and subtraction per row. Finally, events are discriminated from noise by comparison with a low threshold and they are rejected, if a high threshold is exceeded, i.e. they are recognized as particles.
Figure 18: Block diagram of the eROSITA camera electronics (image credit: MPI)
Figure 19: eROSITA performance (image credit: MPE)
The point source sensitivity of eROSITA: ~30 times better than ROSAT (soft band 0.5-2 keV) and ~100 times better than HEAO/RXTE (hard band 2-10 keV).
Figure 20: Photo of the eROSITA telescope structure during the integration phase at MPE, in spring 2012 (image credit: MPE) 37)
Data share policy of the SRG project:
Data Rights and Policies (MPE):
• German eROSITA data are made public after a 2 year proprietary period.
• Periodic data releases envisaged (e.g. 6, 24, 48 months)
• Proprietary data via eROSITA_DE collaboration (consortium)
• Projects/Papers regulated by Working Groups
• Individual External Collaborations
• Group External Collaborations.
Figure 21: The data of the SRG project are evenly shared between IKI and MPE (image credit: MPE)
ART-XC (Astronomical Roentgen Telescope - X-ray Concentrator)
ART-XC is a Russian-led complementary instrument to eROSITA. It is also a 7-module X-ray telescope system that provides higher energy coverage, up to 30 keV (with limited sensitivity above 12 keV).
Table 3: ART-XC collaboration (Ref. 14)
• All-sky X-ray survey in the 6-11 keV energy region with a sensitivity of 3 x 10-13 erg s-1 cm-2 keV-1; discovery in the course of survey at local Universe several thousand new AGNs (Active Galactic Nuclei)
• Study of intrinsically heavily absorbed/Compton thick AGNs (NH ≥ 3 x 1023 cm-2)
• Study of massive nearby galaxie clusters with T ≥ 4 keV in pointing observation mode
• Study heavily obscured galactic X-ray binary systems
• Study broadband spectra of Galactic objects (including binary systems, anomalous pulsars, supernova remnants) up to30 keV, spectroscopy and timing of point sources
• Study non-thermal component in the Galaxy diffuse emission
• Search for cyclotron line features X-ray pulsar spectra.
Instrument: ART-XC will consist of seven independent, but co-aligned, telescope modules with seven corresponding cadmium telluride (CdTe) focal plane detectors. Each will operate over the approximate energy range of 6-30 keV, with an angular resolution of 1 arcmin, a field of view of ~30 arcmin and an energy resolution about 10% at 14 keV. NASA/MSFC (Marshall Space Flight Center) fabricated 4 of the 7 mirror modules, to complement those fabricated by VNIIEF (All-Russian Federal Nuclear Center) in Russia. 40) 41) 42) 43)
Table 4: Performance characteristics of the ART-XC and eROSITA instruments aboard the SRG mission
ART-XC is the smaller one of the two telescopes and has a worse resolution compared to eROSITA. But it works in the range of 5-30 keV and therefore is used for the higher-energetic X-rays. The combination of both telescopes will result in an extremely detailed broadband all sky survey.
Figure 22: The ART-XC Instrument with seven mirror modules and seven focal-plane detectors (image credit: ART-XC collaboration)
Figure 23: A cross section of an ART X-ray mirror module. The inner baffle and the heaters are not shown (image credit: ART-XC collaboration)
Table 5: Overview of ART-XC instrument design parameters
• The X-ray modules are fabricated by the VNIIEF (Russia) and MSFC (USA)
• The NASA-IKI reimbursable agreement has been signed on February 7, 2011 to build 4 flight units
• NASA is to deliver 4 flight modules for the ART-XC instruments by the summer of 2014
• The work at the MSFC has been started on March 24, 2011.
X-ray optics designs:
• Both the VNIIEF and the MSFC designs are based on the single spider scheme
• The shell diameters vary from 50 to 150 mm
• The VNIIEF X-ray module design calls for the mirror shell thickness of 250 µm. The vibration tests for the qualification unit are in progress
• MSFC is exploring the variable thickness option. The design of the ART spider and housing is in progress.
Figure 24: Schematic view of the CdTe detector in the vacuum tight internal box (image credit: ART-XC consortium)
Figure 25: Two views of an ART-XC teleccope (image credit: ART-XC consortium)
Figure 26: Illustration of the ART-XC instrument (image credit: ART-XC consortium)
Figure 27: Optical performance requirements of ART-XC and eROSITA (image credit: ART-XC consortium)
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The information compiled and edited in this article was provided by Herbert J. Kramer from his documentation of: "Observation of the Earth and Its Environment: Survey of Missions and Sensors" (Springer Verlag) as well as many other sources after the publication of the 4th edition in 2002. - Comments and corrections to this article are always welcome for further updates (firstname.lastname@example.org).