NuSTAR (Nuclear Spectroscopic Telescope Array)
NuSTAR is a NASA-funded SMEX (Small Explorer) mission that carries the first focusing hard X-ray (5 - 80 keV) telescope to orbit. NuSTAR will offer a factor 50 - 100 sensitivity improvement compared to previous collimated or coded mask imagers that have operated in this energy band. In addition, NuSTAR provides sub-arcminute imaging with good spectral resolution over a 12 arcminute FOV (Field of View).
NuSTAR will carry out a two-year primary science mission that focuses on four key programs: studying the evolution of massive black holes through surveys carried out in fields with excellent multiwavelength coverage, understanding the population of compact objects and the nature of the massive black hole in the center of the Milky Way, constraining explosion dynamics and nucleosynthesis in supernovae, and probing the nature of particle acceleration in relativistic jets in active galactic nuclei. 1) 2) 3) 4)
The NuSTAR program is managed for NASA by the JPL (Jet Propulsion Laboratory) in Pasadena, CA. The PI (Principal Investigatior) of the mission is Fiona A. Harrison of Caltech (California Institute of Technology). The NuSTAR spacecraft and its payload feature three key technologies to accomplish the mission: 5) 6)
• Hard X-ray optics
• Deployable mast (of SRTM heritage)
• CdZnTe (Cadmium Zinc Telluride) detector technology.
The NuSTAR Small Explorer mission will be the first astronomical telescope on-orbit to utilize the new generation of hard X-ray optics and detector technologies to carry out high-sensitivity observations at X-ray energies significantly greater than 10 keV. Figure 19 shows the total effective area for both telescopes as a function of energy, with a comparison to Chandra (NASA) and XMM-Newton (ESA).
In addition to its core science program, NuSTAR will offer opportunities for a broad range of science investigations, ranging from probing cosmic ray origins to studying the extreme physics around collapsed stars to mapping microflares on the surface of the sun.
Background: NuSTAR is a part of NASA's Explorer Program. While the mission was selected for a Phase A study in 2003 and for a further study in 2005, it was ultimately cancelled by NASA in February 2006 due to agency budget limitations. On September 21, 2007 it was announced that the program had been restarted, with an expected launch in August 2011, though this was later delayed to Q1 2012. 7)
Figure 1: Artist's rendition of the NuSTAR spacecraft in orbit (image credit: NASA/JPL)
Legend to Figure 1: The spacecraft features a mast 10 m in length that deploys after launch to separate the optics modules (right) from the detectors in the focal plane (left).
The NuSTAR minisatellite is based on OSC's (Orbital Sciences Corporation) proven LEOStar-2 bus design of Dulles, VA. The spacecraft is three-axis stabilized with a single articulating solar panel and relies predominantly upon a multi-head star camera (µASC) of DTU (Danish Technical University) for attitude sensing. This enables 80% of the sky to be accessed at any given time, which allows ToO (Target of Opportunity) viewing with few restrictions as well as aids in mission planning. 8)
RF communications: There will be daily downlinks via TDRSS, and uplinks will not be routinely required. The pointing strategy will emphasize long observations of survey fields, specific pointed observations and targets of opportunity.
Table 1: Overview of spacecraft parameters
Figure 2: Diagram of the observatory in the stowed (bottom) and deployed (top) configurations (image credit: NuSTAR collaboration)
NuSTAR metrology system:
The NuSTAR X-ray telescope consists of two co-aligned grazing angle incidence x-ray mirrors, coated with depth-graded multilayers, focusing onto two cadmium-zinc-telluride pixel detectors that are separated from the mirrors by 10 m. The two telescopes are operated independently and the sensitivity of the mission is achieved by combining exposures from the two telescopes. The long focal length required by the hard X-ray optics demands the use of a 10 m extendable mast (Figure 3), manufactured by ATK Space Systems, Goleta, CA. 9)
Figure 3: Rendering of the NuSTAR Observatory (image credit: NuSTAR collaboration)
The observatory must determine the origin (in celestial coordinates) of all detected X-ray photons during post processing, in order to produce sharp images. This task is complicated by distortions due to thermal bending and external forces acting on the mast during orbit.
To track the motion of this mast, the observatory carries two metrology laser subsystems, which are mounted on the bench with the X-ray optics. The metrology lasers are focused on two metrology detectors mounted on the bench with the X-ray detectors. These metrology detectors are based on PSD (Position Sensitive Detectors). To generate a unique aspect solution, the observatory also carries a star tracker camera on the optics (outboard) bench.
Figure 4: Simplified diagram of the metrology system (NuSTAR collaboration)
After spacecraft commissioning, a one-time in-flight calibration of the observatory is performed to establish the launch shifts and the deployed position of the mast. This is done by observing a bright X-ray source (with known celestial coordinates), and simultaneously logging the star tracker data, the laser metrology data and the X-ray detector data. A sketch of the metrology system diagrammed with the mast and a single telescope is shown in Figure 4.
During spacecraft operations, the positions of the laser beams on the PSD detectors are continually recorded at a frequency much higher than the mast oscillations. Also, all star tracker updates are recorded and the positions (and times) on the focal plane where the individual x-ray photons impinge are recorded. All this information is used to generate high-resolution images during on-ground post processing of the data.
The X-ray detectors detect single incoming X-ray photons. That is, the X-ray detectors are not integrating. The described metrology system would not work with an integrating detector (such as a CCD chip) that does not register the arrival time of the individual photons, since the unique aspect solution of the observatory at a certain instant could not be applied to a given detector read-out.
The metrology system is required to measure the translation in 2 axes (those directions transverse to the laser beams) and the clocking angle (rotation around the observatory boresight). Only these 3 DOFs (Degree of Freedom) have the potential to introduce errors large enough that they must be measured. Figure 5 illustrates the DOFs that need to be measured. - Regarding the thermal design, it is required that the metrology system survive a non-operating temperature range of -45ºC to 60ºC.
Figure 5: The DOFs that are required to be measured by the metrology system (image credit: NuSTAR collaboration)
Legend to Figure 5: The sketch is utilizing a coordinate system that is fixed on the optical bench (where the star tracker is mounted).
The science requirement for the NuSTAR observatory that governs the metrology system performance is that the celestial coordinates of a detected bright X-ray source be determined to an accuracy of ~10 arcseconds.
The laser metrology system is implemented as two laser pointers mounted on a bench with the optics. The laser pointers are illuminating 2 PSD detectors mounted ~10 m away on a bench with the X-ray detectors. 10)
Figure 6: Block diagram of the metrology system (image credit: NuSTAR collaboration)
A block diagram of the electronics is shown in Figure 7. The lasers are driven by a processor, through analog laser drivers. The PSD detector is mounted on a PCB (Printed Circuit Board) along with 4 operational amplifiers. The 4 signals for each channel are routed to the main instrument processor where the signal is low pass filtered and multiplexed into a 14 bit A/D converter. The electronics components are space qualified. The electronics power cycle the lasers and make a background measurement 4 times a second to subtract out the background signal produced by dark current, the moon or the Earth in the FOV, sun stray light.
Figure 7: Block diagram of the metrology system (image credit: NuSTAR collaboration)
Figure 8: Schematic view of the metrology laser and its components (image credit: NuSTAR collaboration)
The requirements on the metrology laser call for a beam accuracy of ≤ 130 µm (at ~10 m) over all observatory orientations and orbits following the initial onetime calibration (this implies: “under all sun illumination circumstances”). Thermal stability is the key to achieving this requirement. To minimize the effect of the sun, the structure is primarily made of Invar 36, which has a low CTE (Coefficient of Thermal Expansion). The laser is mounted behind the optics in an invar barrel. This barrel is placed inside another invar barrel to mechanically hold it, to serve as a thermal shield, and to spread the thermal variation from the sun.
Table 2: Metrology system specifications
The NuSTAR metrology system has space qualified two different laser diodes for its environment. Both brands of lasers have demonstrated their capability to be utilized as flight lasers for NuSTAR. It is expected that if one laser diode fails but the other laser diode survives the mission, it will be possible to recover scientific data. Therefore, since, 1) the laser are interchangeable, 2) since no laser diode illustrates any signs of superiority over the other and 3) it is more likely that one laser will survive if 2 different equally reliable lasers are chosen, a project decision was made to fly one laser diode from each vendor on the NuSTAR metrology system (Ref. 10).
Figure 9: Photo of the NuSTAR spacecraft integration at the OSC facility, Dulles, VA (image credit: OSC, NASA/JPL)
Figure 10: Photo of the stowed mast of NuSTAR (image credit: OSC, Caltech)
Launch: The NuSTAR spacecraft was launched on June 13, 2012 on a Pegasus-XL vehicle of OSC (air-launch on the Stargazer L-1011 aircraft). The launch site was the Kwajalein Atoll in the Marshall Islands (Pacific Ocean). 11)
The June launch came almost three months after a planned early March launch date. 12)
Figure 11: Photo of a Pegasus rocket which launches from underneath the L-1011 "Stargazer" aircraft (image credit: NASA, Orbital)
Orbit: Near equatorial circular orbit, altitude ~650 km x 610 km, inclination = 6º.
• February 19, 2014: NuSTAR has created the first map of radioactive material in a supernova remnant. The results, from a remnant named Cassiopeia A (Cas A), reveal how shock waves likely rip apart massive dying stars. 13) 14) 15)
Figure 12: This is the first map (false color image) of radioactivity in a supernova remnant, the blown-out bits and pieces of a massive star that exploded. The blue color shows radioactive material mapped in high-energy X-rays using NuSTAR (image credit: NASA/JPL-Caltech, CXC, SAO)
Legend to Figure 12: The mystery of how Cassiopeia A exploded is unraveling thanks to new data from NASA's NuSTAR. In this image, NuSTAR data, which show high-energy X-rays from radioactive material, are colored blue. Lower-energy X-rays from non-radioactive material, imaged previously with NASA's Chandra X-ray Observatory, are shown in red, yellow and green. The new view shows a more complete picture of Cassiopeia A, the remains of a star that blew up in a supernova event whose light reached Earth about 350 years ago, when it could have appeared to observers as a star that suddenly brightened. The remnant is located 11,000 light-years away from Earth.
NuSTAR is the first telescope capable of taking detailed pictures of the radioactive material in the Cassiopeia A supernova remnant. While other telescopes have detected radioactivity in these objects before, NuSTAR is the first capable of pinpointing the location of the radioactivity, creating maps. When massive star explode, they create many elements: non-radioactive ones like iron and calcium found in your blood and bones; and radioactive elements like titanium-44, the decay of which sends out high-energy X-ray light that NuSTAR can see.
By mapping titanium-44 in Cassiopeia A, astronomers get a direct look at what happened in the core of the star when it was blasted to smithereens. These NuSTAR data complement previous observations made by Chandra, which show elements, such as iron, that were heated by shock waves farther out from the remnant's center.
In this image, the red, yellow and green data were collected by Chandra at energies ranging from 1 to 7 keV. The red color shows heated iron, and green represents heated silicon and magnesium. The yellow is what astronomers call continuum emission, and represents a range of X-ray energies. The titanium-44, shown in blue, was detected by NuSTAR at energies ranging between 68 and 78 keV (Ref. 13).
• January 2014: NuSTAR is now executing its primary science mission, and with an expected orbit lifetime of 10 years, the project anticipates proposing a guest investigator program, to begin in late 2014 (Ref. 35).
• January 2014: NuSTAR's unique viewpoint, in seeing the highest-energy X-rays, is showing the project well-studied objects and regions in a whole new light. Figure 13of NuSTAR shows the energized remains of a dead star, a structure nicknamed the "Hand of God" after its resemblance to a hand. 16) 17)
- The new "Hand of God" image shows a nebula 17,000 light years away, powered by a dead, spinning star called PSR B1509-58, or B1509 for short. The dead star, called a pulsar, is the leftover core of a star that exploded in a supernova. The pulsar is only about 19 km in diameter but packs a big punch: it is spinning around nearly seven times every second, spewing particles into material that was upheaved during the star's violent death. These particles are interacting with magnetic fields around the ejected material, causing it to glow with X-rays. The result is a cloud that, in previous images, looked like an open hand. - One of the big mysteries of this object, called a pulsar wind nebula, is whether the pulsar's particles are interacting with the material in a specific way to make it appear as a hand, or if the material is in fact shaped like a hand.
- With approximately 10 times greater spatial resolution and more than 100 times greater sensitivity than previous missions in this energy band, NuSTAR has opened the high-energy sky to sensitive study. Over the first year of the mission, NuSTAR has undertaken a range of studies, from observations of energetic events towards the center of the Milky Way galaxy to detailed studies of distant supermassive black holes. 18)
Figure 13: High-energy X-ray view of the 'Hand of God' (image credit: NASA/JPL, Caltech, McGill University)
Legend to Figure 13: NuSTAR has imaged the structure in high-energy X-rays for the first time, shown in blue. Lower-energy X-ray light, previously detected by NASA's Chandra X-ray Observatory, is shown in green and red. NuSTAR's view is providing new clues to the puzzle. The hand actually shrinks in the NuSTAR image, looking more like a fist, as indicated by the blue color. The northern region, where the fingers are located, shrinks more than the southern part, where a jet lies, implying the two areas are physically different. The red cloud at the end of the finger region is a different structure, called RCW 89. Astronomers think the pulsar's wind is heating the cloud, causing it to glow with lower-energy X-ray light.
In this image, X-ray light seen by Chandra with energy ranges of 0.5 to 2 keV and 2 to 4 keV is shown in red and green, respectively, while X-ray light detected by NuSTAR in the higher-energy range of 7 to 25 keV is blue.
• August 29, 2013: NuSTAR is giving the wider astronomical community a first look at its unique X-ray images of the cosmos. The first batch of data from the black-hole hunting telescope is publicly available today, Aug. 29, via NASA's HEASARC (High Energy Astrophysics Science Archive Research Center). 19)
- The images, taken from July to August 2012, shortly after the spacecraft launched, comprise an assortment of extreme objects, including black holes near and far. The more distant black holes are some of the most luminous objects in the universe, radiating X-rays as they ferociously consume surrounding gas. One type of black hole in the new batch of data is a blazar, which is an active, supermassive black hole pointing a jet toward Earth. Systems known as X-ray binaries, in which a compact object such as a neutron star or black hole feeds off a stellar companion, are also in the mix, along with the remnants of stellar blasts called supernovas.
• Feb. 27, 2013: Two X-ray space observatories, NASA's NuSTAR and the ESA's XMM-Newton missions, have teamed up to measure definitively, for the first time, the spin rate of a black hole with a mass 2 million times that of our sun. 20)
The supermassive black hole lies at the dust- and gas-filled heart of a galaxy called NGC 1365, and it is spinning almost as fast as Einstein's theory of gravity will allow. The observations also are a powerful test of Einstein's theory of general relativity, which says gravity can bend space-time, the fabric that shapes our universe, and the light that travels through it.
NuSTAR is designed to detect the highest-energy X-ray light in great detail. It complements telescopes that observe lower-energy X-ray light, such as XMM-Newton and NASA's Chandra X-ray Observatory. Scientists use these and other telescopes to estimate the rates at which black holes spin (Figure 15).
Figure 14: The artist's concept illustrates a supermassive black hole with millions to billions times the mass of our sun. Supermassive black holes are enormously dense objects buried at the hearts of galaxies (image credit: NASA/JPL)
Until now, these measurements were not certain because clouds of gas could have been obscuring the black holes and confusing the results. With help from XMM-Newton, NuSTAR was able to see a broader range of X-ray energies and penetrate deeper into the region around the black hole. The new data demonstrate that X-rays are not being warped by the clouds, but by the tremendous gravity of the black hole. This proves that spin rates of supermassive black holes can be determined conclusively. Measuring the spin of a supermassive black hole is fundamental to understanding its past history and that of its host galaxy.
Supermassive black holes are surrounded by pancake-like accretion disks, formed as their gravity pulls matter inward. Einstein's theory predicts the faster a black hole spins, the closer the accretion disk lies to the black hole. The closer the accretion disk is, the more gravity from the black hole will warp X-ray light streaming off the disk (Ref. 20).
Figure 15: Artist's view of two models of black hole spin (image credit: NASA/JPL, Caltech)
Legend to Figure 15: Scientists measure the spin rates of supermassive black holes by spreading the X-ray light into different colors. The light comes from accretion disks that swirl around black holes, as shown in both of the artist's concepts. They use X-ray space telescopes to study these colors, and, in particular, look for a "fingerprint" of iron — the peak shown in both graphs, or spectra — to see how sharp it is. Prior to observations with NASA's NuSTAR (Spectroscopic Telescope Array), and the European Space Agency's XMM-Newton telescope, there were two competing models to explain why this peak might not appear to be sharp.
The "rotation" model shown at top of Figure 15 held that the iron feature was being spread out by distorting effects caused by the immense gravity of the black hole. If this model were correct, then the amount of distortion seen in the iron feature should reveal the spin rate of the black hole.
The alternate model held (bottom of Figure 15) that obscuring clouds lying near the black hole were making the iron line appear artificially distorted. If this model were correct, the data could not be used to measure black hole spin.
NuSTAR helped to solve the case, ruling out the alternate "obscuring cloud" model. Its high-energy X-ray data — shown at top as green bump to the right of the peak — revealed that features in the X-ray spectrum are in fact coming from the accretion disk and not from the obscuring clouds. Together with XMM-Newton, the space observatories were able to make the first conclusive measurement of a black hole's spin rate, and more generally, confirm that the "gravitational distortion" model is accurate (Ref. 20). 21)
The solid lines of Figure 16 show two theoretical models that explain low-energy X-ray emission seen previously from the spiral galaxy NGC 1365 by XMM-Newton. The red line explains the emission using a model where clouds of dust and gas partially block the X-ray light, and the green line represents a model in which the emission is reflected off the inner edge of the accretion disk, very close to the black hole. 22)
The blue circles show the latest measurements from XMM-Newton, and the yellow circles show the data from NuSTAR. While both models fit the XMM-Newton data equally well, only the disk reflection model fits the NuSTAR data.
The results show that the iron feature, the sharp peak at left, is being affected black hole's immense gravity and not intervening clouds. The degree to which the iron feature is spread out reveals the spin rate of the black hole.
Figure 16: Two X-ray observatories are better than one (image credit: NASA/JPL, Caltech)
• Feb. 21, 2013: NuSTAR has been in orbit around Earth for more than eight months since its launch in June 2012, studying black holes and probing the nature of the high-energy X-ray universe. Mission and science operations have settled down into a mostly predictable daily routine, and the science team is making good progress toward achieving the primary, or "Level 1," science goals. Examples of some of the key NuSTAR observations performed to date include mapping of the central regions of our Milky Way galaxy, studying the remnants of exploded stars in our galaxy, and surveys of several well-studied extragalactic fields. 23)
• January 2013: Since launch, the NuSTAR team has been fine-tuning the telescope. The mission has looked at a range of extreme, high-energy objects already, including black holes near and far, and the incredibly dense cores of dead stars. In addition, NuSTAR has begun black-hole searches in the inner region of the Milky Way galaxy and in distant galaxies in the universe. 24) 25)
Figure 17: This new view of the spiral galaxy IC 342, seven million light years away, includes data from NuSTAR (image credit: NASA/JPL,Caltech/DSS)
Legend to Figure 17: This new view of spiral galaxy IC 342, also known as Caldwell 5, includes data from NASA's NuSTAR spacecraft. High-energy X-ray data from NuSTAR have been translated to the color magenta, and superimposed on a visible-light view highlighting the galaxy and its star-studded arms. NuSTAR is the first orbiting telescope to take focused pictures of the cosmos in high-energy X-ray light; previous observations of this same galaxy taken at similar wavelengths blurred the entire object into one pixel.
The two magenta spots are blazing black holes first detected at lower-energy X-ray wavelengths by NASA's Chandra X-ray Observatory. With NuSTAR's complementary data, astronomers can start to home in on the black holes' mysterious properties. The black holes appear much brighter than typical stellar-mass black holes, such as those that pepper our own galaxy, yet they cannot be supermassive black holes or they would have sunk to the galaxy’s center. Instead, they may be intermediate in mass, or there may be something else going on to explain their extremely energetic state. NuSTAR will help solve this puzzle.
The image (Figure 17) shows NuSTAR X-ray data taken at 10 to 35 keV. The visible-light image is from the Digitized Sky Survey.
• November 2012: The first three months of science, or "Phase E," operations have been a busy time for NASA's NuSTAR. Since the science operations began on August 1, 2012, the NuSTAR team has wrestled with learning to point the telescope's flexible system of optics, mast, spacecraft bus and solar array. Characterization of the behavior of NuSTAR in space took an additional six weeks, but has now been completed. Several adjustments to attitude control parameters — factors in pointing the telescope — were necessary to bring the spacecraft into the required specifications, and the satellite's slew rate, or the speed at which it points to new targets, has recently been enhanced by a factor of two. 26)
Calibrating the telescope is now a primary focus for the science team. NuSTAR has made a number of coordinated observations of famous, bright X-ray sources like the Crab nebula, Hercules X-1, 3C 273 and IC 4329A. The first two are neutron stars in our Milky Way galaxy, and the latter two are accreting black holes in the centers of nearby galaxies. These coordinated calibration campaigns have been done with many of the low- and high-energy X-ray astronomical satellites currently in orbit: NASA's Chandra X-ray and Swift observatories, ESA's (European Space Agency's) XMM-Newton and INTEGRAL telescopes, and JAXA's (Japan Aerospace Exploration Agency's) Suzaku.
• On June 28, 2012, NuSTAR has taken its first snapshots of the highest energy X-rays in the cosmos. The first images from the observatory show Cygnus X-1, a black hole in our galaxy that is siphoning gas off a giant-star companion. This particular black hole was chosen as a first target because it is extremely bright in X-rays, allowing the NuSTAR team to easily see where the telescope's focused X-rays are falling on the detectors. 27)
• On June 21, 2012 (9 days after launch), NuSTAR successfully deployed its lengthy mast, giving it the ability to see the highest energy X-rays in the universe. 28)
The NuSTAR team will now begin to verify the pointing and motion capabilities of the satellite, and fine-tune the alignment of the mast. In about five days, the team will instruct NuSTAR to take its "first light" pictures, which are used to calibrate the telescope.
• Confirmation of the successful deployment of NuSTAR's solar arrays has been received. All systems are nominal. 29)
Sensor complement: (NuSTAR)
The instrument has the same name as the spacecraft. NuSTAR is based in large part on the technologies developed for the HEFT (High-Energy Focusing Telescope) balloon experiment. The NuSTAR instrument team is from the following institutions: Caltech, Columbia University, DTU (Danish Technical University), LLNL (Lawrence Livermore National Laboratory),NASA/GSFC (Goddard Space Flight Center), and UCB (University of California, Berkeley). 30) 31) 32) 33) 34)
NuSTAR operates in the band from 3 to 79 keV, extending the sensitivity of focusing far beyond the ~10 keV high-energy cutoff achieved by all previous X-ray satellites. The inherently low background associated with concentrating the X-ray light enables NuSTAR to probe the hard X-ray sky with a more than 100-fold improvement in sensitivity over the collimated or coded mask instruments that have operated in this bandpass. Using its unprecedented combination of sensitivity and spatial and spectral resolution, NuSTAR will pursue five primary scientific objectives: 35)
1) probe the obscured AGN (Active Galactic Ncleus) activity out to the peak epoch of galaxy assembly in the universe (at z <~ 2) by surveying selected regions of the sky
2) study the population of hard X-ray-emitting compact objects in the Galaxy by mapping the central regions of the Milky Way
3) study the non-thermal radiation in young supernova remnants, both the hard X-ray continuum and the emission from the radioactive element 44 Ti
4) observe blazars contemporaneously with ground-based radio, optical, and TeV telescopes, as well as with Fermi and Swift, to constrain the structure of AGN jets
5) observe line and continuum emission from core-collapse supernovae in the Local Group, and from nearby Type Ia events, to constrain explosion models. During its baseline two-year mission, NuSTAR will also undertake a broad program of targeted observations.
The NuSTAR instrument
Instrument: The NuSTAR instrument is the first astronomical telescope in orbit to utilize the new generation of hard X-ray optics and solid-state detector technologies to carry out high-sensitivity observations at X-ray energies significantly greater than 10 keV. It consists of two co-aligned depth-graded multilayer coated grazing incidence optics focused onto solid state CdZnTe pixel detectors with a focal length of 10.15 m. Figure 19 shows the total effective area for both telescopes as a function of energy, with a comparison to Chandra and XMM. The energy band extends from about 5 - 79 keV, being limited at the low-energy end by the optics thermal cover and shield entrance window, and at the high energy end by the K-edge (at 78.4 keV) in the Platinum mirror coatings.
For (focusing) X-ray telescopes, the standard metric for specifying angular resolution is the HPD (Half-Power Diameter), which is also called the HEW (Half-Energy Width). This is the angular diameter of the image of a point source, which contains half the flux (at a given energy) focused by the telescope. From the standpoint of detecting and measuring sources with an X-ray telescope, the HPD proves more useful than other imaging metrics—e.g., full width at FWHM (Full Width Half maximum) and RMS (Root-Mean-of Squares) image blur. However, the RMS is useful in formulating imaging error budgets for the geometric-optics terms that govern the PSF (Point Spread Function) core. 36)
Table 3: Key instrument performance parameters (Ref. 1)
Figure 18: Two X-ray telescopes and the electromagnetic spectrum in energy and wavelength presentation (image credit: NASA/JPL, Caltech)
The instrument FOV is energy-dependent due to changes in multilayer reflectance as a function of energy and optics shell radius, which results in overall loss of reflectance and more vignetting at high energy (Figure 20). The spectral resolution is 500 eV at energies below ~30 keV, and increases to 1.2 keV at the upper end of the energy range. The 2 µs temporal resolution, determined by the bit rate allocated in the telemetry stream for time tags, is more than adequate to meet the scientific requirements. The intrinsic temporal resolution of the detector is better than 1 µs. The target of opportunity (ToO) response time is required to be less than 24 hours; however, on average the turnaround will be faster, with targets typically acquired within 6 hours.
Figure 19: Effective area for two telescopes as a function of energy compared with the Chandra and XMM focusing telescopes (image credit: NuSTAR collaboration)
Table 4: Baseline mission science plan
Figure 20: Effective area as a function of off-axis angle, as a fraction of on-axis area, for several energies (image credit: NuSTAR collaboration)
The NuSTAR instrument is launched in a compact, stowed configuration, and after launch a 10 m mast is deployed to achieve a focal length of 10.15 m. Since the absolute deployment location of the mast is difficult to measure on the ground, due to complications associated with complete gravity off-loading, an adjustment mechanism is built into the last section of the mast to enable a one-time alignment to optimize the location of the optical axes on the focal plane. This mechanism provides two angular adjustments as well as rotation. The mast is not perfectly rigid, it is being subjected to thermal distortions particularly when going in and out of Earth's shadow which translate into changes in telescope alignment of 1 -2 arcmin. These mast alignment changes are measured by the combination of an optics bench-mounted star tracker and a laser metrology system.
The same combination of sensors also provides the absolute instrument aspect. In order to limit the FOV open to the detectors, and therefore the diffuse cosmic background, an aperture stop consisting of three rings deploys with the mast. The aperture stop is shown in the deployed configuration in Figure 21. In the stowed configuration, the top will be 0.83 m above the focal plane surface.
Figure 21: Diagram of the NuSTAR instrument showing the principal elements (image credit: NuSTAR collaboration)
A blowup of an individual optics module is also shown in Figure 23. Each layer of the optic has an upper and lower conic shell (equivalent to the parabola-hyperbola sections of a Wolter-I optic). Each shell is composed of multiple glass segments formed by thermal slumping. Each piece of glass is coated with a depth-graded multilayer to enhance reflectivity. The enhanced reflectivity provided by the multilayers, along with the shallow graze angles afforded by the long focal length of the optics (10.15 m) provide high effective area over the NuSTAR energy band of 6-79 keV, and a FOV of 12 arcminutes by 12 arcminutes. There are 133 concentric layers which together form each optic. 37) 38)
The glass layers (a Titanium-glass-epoxy-graphite composite structure) are built up on a Titanium mandrel. Titanium support spiders located on the top and bottom of each optic connect it to the optical bench. The compliant, radially-symmetric spiders accommodate thermal expansion effects. Thermal covers (5 µm polyimide) on the entrance and exit apertures of the optic reduce thermal gradients by blocking direct view of the sun and deep space. Three flight modules (called FM0, FM1 and FM2 respectively) are being fabricated. The first two modules are the flight units, and the third module is to provide for more extensive X-ray characterization than is permitted for either of the flight modules, given the compressed delivery schedule of the optics..
Figure 22: Schematic view of a focusing near-grazing X-ray telescope (image credit: NuSTAR collaboration)
Figure 23: NuSTAR instrument showing a blowup of an optics module (image credit: NuSTAR collaboration)
The layers of glass are physically separated from and attached to each other by means of precisely-machined graphite spacers which run lengthwise down the glass segments, and which constrain the glass to the proper conical shape of the Wolter 1 geometry. The spacers are 1.2 mm wide (except in the inner 5 layers where they are 1.6 mm wide to provide more bonding area in this high stress region) in order to minimize X-ray shadowing. The spacers are bonded to the Titanium mandrel and to the glass layers by means of a low outgassing epoxy (Henkel F131). The inner 65 layers of glass form integral shells by means of ~60º sectors, and the outer 65 layers by means of ~30º sectors (dodecants), see Figure 24 (left). The number of graphite spacers is fixed at 5 per glass segment for fabrication simplicity. The transition from sextants to dodecants provides for a reduction in the spacer span between glass segments as the radius of the layers grows, thus maintaining good control of the glass figure. Figure 24 (right) shows the details of the 3 sextant layers of glass that serve as an “intermediate mandrel” for transitioning from sextant to dodecant layers.
Figure 24: Optic drawing showing glass segmentation, support spiders and polyimide thermal shield (left); on the right is the end view of optics showing intermediate mandrel (IM) details; IM glass (blue); IM spacers (light blue); spacers (gray); sextants and dodecants above/below the IM (image credit: NuSTAR collaboration)
The right side image of Figure 24 shows the details of the 3 sextant layers of glass that serve as an “intermediate mandrel” for transitioning from sextant to dodecant layers. The mandrel, sextant and dodecant glass layers and spacers are visible. Double width spacers azimuthally tie the dodecants to the sextant glass at the intermediate mandrel, increasing torsional stiffness and providing a path for distribution of launch loads. The glass segments are coated with depth-graded multilayers. All three flight modules use W/Si for the outer layers. FM0 uses Pt/SiC for its inner layers, while FM1 and FM2 use Pt/C for the inner layers. In order to optimize multilayer reflectivity, the optic’s layers are divided into 10 groups, with a different multilayer recipe for each group.
Table 5: Summary of the optics module parameters
Figure 25: One of NuSTAR's two mirrors, or optics, assembled in a clean room at Columbia University's Nevis Laboratory (image credit: NuSTAR collaboration)
Figure 26: NuSTAR FM0 (Flight Module 0) with 106 layers on assembly machine of Columbia University’s Nevis Laboratory (image credit: NuSTAR collaboration)
Focal plane modules: Each focal plane consists of four CdZnTe pixel sensors coupled to a custom low-noise ASIC (Application Specific Integrated Circuit). Each hybrid contains a 32 x 32 array of 600 µm pixels with a resulting plate scale of 12.300/pixel, so that the mirror point spread function is over-sampled. The sensors are placed in a 2 x 2 array with a minimal (~ 500 µm) gap between them to fill a total subtended FOV of 13 arcmin on a side (Figure 27). Table 6 summarizes the primary characteristics of the focal plane.
Figure 27: The NuSTAR focal plane configuration (left) and photo of an engineering test module (right), image credit: NuSTAR collaboration)
Table 6: Summary of the focal plane configuration
To achieve a low energy threshold and good spectral performance, the detector readout is designed for very low noise. The electronic noise contribution (including detector leakage current) to the energy resolution is 400 eV, and the low-energy threshold is 2.5 keV for an event registering in a single pixel. Over most of the energy range the detector spectral resolution is limited by charge collection uniformity in the CdZnTe crystal. At low energies, between 5 and 30 keV, the average spectral resolution for a typical flight detector is 500 eV FWHM (Full Width Half Maximum), while at 60 keV it is 1.0 keV, and at 86 keV it is 1.2 keV.
The focal plane will be passively cooled in flight to between 0-5ºC. The passive cooling is enabled by the low-power dissipation of the detector readout chip (50 µW/pixel). At in-flight operating temperatures, the detector leakage current is a negligible contributor to the resolution. In addition to measuring the deposited energy and arrival time for each event, the readout architecture enables a depth of interaction measurement which can be used both to maximize photo peak efficiency at high energy, where charge trapping effects can lead to a low-energy `tail' on the energy resolution, and in addition reject background from the back portion of the detector.
The readout of each focal plane module is controlled by an FPGA-embedded microprocessor. Because each pixel triggers independently, and the electronics shaping time is short, there are no pile-up issues equivalent to the CCD focal planes on XMM and Chandra. The maximum rate that events can be processed is 400 cps (cycles per second) in each telescope; however, pulse pileup does not occur until substantially higher rates (~ 105 cps). The readout system is designed so that source fluxes can be measured up to count rates of 104 cps. At the nominal faint-source count rates, the readout dead time is < 2%.
The focal plane is surrounded by an active 2 cm thick CsI(Na) shield and incorporates a deployable aperture stop. The CsI shield extends 20 cm above the detector, and has an opening angle of 16º, while the passive aperture stop defines a much narrower opening of 4º diameter. Figure 28 shows the expected background counts per unit detector area. At low energies the background is dominated by diffuse leakage through the portion of the aperture stop FOV not blocked by the optics bench. The spectral features between 25 and 35 keV are fluorescence from the CsI shield. The background level shown in Figure 28 assumes the use of the depth-of-interaction measurement to reject interactions in the back of the detector, which results in about a factor two background reduction at 60 keV (Ref. 37).
Figure 28: Predicted detector background count rate per unit area as a function of energy (image credit: NuSTAR collaboration)
Figure 29: Photo of one of two NuSTAR focal plane modules (image credit: NuSTAR collaboration) 39)
Data will be publicly available at HEASARC (High Energy Astrophysics Science Archive Research Center) of NASA/GSFC following validation at the science operations center located at Caltech.
The MOC (Mission Operations Center) is located at UCB/SSL. Command uplinks and data downlinks will be through a ground station, operated by ASI (Agenzia Spaziale Italiana), located in Malindi, Kenya. Most science targets will be viewed for a week or more, so that after a 30-day in-orbit checkout and commissioning period, commanding will be rare. The turnaround time for ToO (Target of Opportunity) observations depends largely on timing relative to the ground station passes. The Malindi station is visible once per 90 minute orbit, but commands can take up to 12 hours to prepare given that the MOC is not staffed 24 hours/day.
The science data will be transferred from the MOC to the SOC (Science Operations Center). The SOC is in charge to process and validate the data, and distribute products to the science team. All science data will be converted to FITS (Flexible Image Transport System) format conforming to OGIP (Office of Guest Investigator Programs) standards, and analysis software will adopt the FTOOLS approach and environment. The NuSTAR science data has no proprietary period, and after a six-month interval during which the instrument calibration will be understood and the performance verified, data will enter the public science archive, located at the HEASARC.
NuSTAR Mission System: 40)
• Core components:
- MOC (Mission Operations Center)
- FDC (Flight Dynamics Center)
- SOC (Science Operations Center)
- Remote ground stations
- Secure network links to remote ground stations
- Acquisition data upload via secure circuits
- Post-pass telemetry file delivery via Open Internet
- Standardized data flows between MOC, FDC, SOC
Figure 30: Core components and interfaces of the NuSTAR mission at USB/SSL (image credit: USB/SSL)
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4) Kristin Kruse Madsen, Brian Grefenstette, Fiona A. Harrison, “The Nuclear Spectroscopic Telescope Array,” IACHEC (International Astronomical Consortium for High Energy Calibration), Woods, Hole, MA, USA, April 12-15, 2010, URL: http://web.mit.edu/iachec/meetings/2010/Presentations/Madsen_NuSTAR.pdf
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20) Whitney Clavin, J. D. Harrington, “NASA's NuSTAR Helps Solve Riddle of Black Hole Spin,” NASA/JPL, Feb. 27, 2013, URL: http://www.nasa.gov/mission_pages/nustar/news/nustar20130227.html
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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.