AstroSat (Astronomy Satellite) of India
AstroSat is India's first dedicated astronomy mission, a broad spectral band Indian national space observatory. AstroSat will provide an opportunity for the Indian astronomers to carry out cutting-edge research in the frontier areas of X-ray and ultraviolet astronomy and allow them to address some of the outstanding problems in modern astrophysics.
• Multi-wavelength observations: for a wide variety of both Galactic and extra-galactic source types [AGN (Active Galactic Nuclei), binaries, flaring stars, SNRs, clusters...]. Use of five co-aligned telescopes simultaneously cover the hard X-ray to visible bands
• Broadband X-ray spectral measurements: Emission and absorption features with medium energy resolution capability in the 0.3 – 100 keV spectral band with 3 co-aligned X-ray instruments.
• High time-resolution studies: Periodic, aperiodic and chaotic X-ray variability in X-ray binaries. Detect new accreting milli-sec binaries and AXPs. Study evolution of pulse and orbital periods.
AstroSat is expected to focus on high-resolution UV imaging for morphological studies of galactic and extragalactic objects,broad-band studies of X-ray sources and other multiwavelength targets ranging from nearby stars to the very distant active galactic nuclei.
AstroSat is a collaborative project of the following institutions:
- TIFR (Tata Institute of Fundamental Research), Mumbai
- ISRO (Indian Space Research Organization), Bangalore
- IIA (Tata Institute of Fundamental Research), Bangalore
- IUCAA (Inter-University Centre for Astronomy & Astrophysics), Pune
- RRI (Raman Research Institute), Bangalore
- Physical Research Laboratory, Ahmedabad
- CSA (Canadian Space Agency), Canada
- Leicester University, U.K.
- Participation of many Indian Universities and research centers.
The IUCAA (Inter-University Centre for Astronomy and Astrophysics) is an autonomous institution set up by the University Grants Commission to promote nucleation and growth of active groups in astronomy and astrophysics in Indian universities. IUCAA is located in the University of Pune campus next to the National Centre for Radio Astrophysics, which operates the Giant Meter-wave Radio Telescope. The PI (Principal Investigator) of AstroSat is P. C. Agrawal of TIFR, Mumbai.
Figure 1: Illustration of the AstroSat spacecraft and its instrument complement (image credit: ISRO)
The spacecraft bus configuration and design have heritage and are similar to the ones earlier used IRS bus. A BMU (Bus Management Unit), similar to the one used in CartoSat-2, is selected for the integrated main bus functions including AOCS, command processing, house keeping telemetry, sensor processing and antenna position processing.
EPS (Electrical Power Subsystem): Two deployable solar panels with single axis rotation are used for power generation. During the full orbit, except for the eclipse period, the panels are always oriented normal to the sun in order to generate maximum power. Whenever the stellar orientation is changed the panels are reoriented. The EPS provides a power of 1250 W, the required payload power is 488 W.
AOCS (Attitude and Orbit Control Subsystem): The spacecraft is 3-axis stabilized. The attitude is sensed with two star sensors and three gyros to provide 1 arcsec pointing capability. Actuation is provided by reaction wheels and magnetic torquers for momentum dumping. The pointing accuracy is < 0.05º and 0.2 arcsec/s drift.
RF communications: X-band downlink of payload data at a rate of 105 Mbit/s (real-time) and 210 Mbit/s (recorder dump). A solid state recorder with 160 Gbit storage capacity is used for onboard storage of data.
The AstroSat spacecraft has a launch mass of ~ 1650 kg, including 868 kg of payload mass. The expected operating life time of the satellite is five years.
Figure 2: Artist's rendition of the deployed AstroSat spacecraft configuration (image credit: ISRO)
Figure 3: Integration of AstroSat in a clean room at the ISRO Satellite Center (image credit: ISRO) 11)
Launch: The AstroSat spacecraft was launched on September 28, 2015 (04:30:00 UTC) on the PSLV-C30 vehicle in the PSLV-XL configuration of ISRO from SDSC (Satish Dhawan Space Center) SHAR, ISRO's launch site on the south-east coast of India, Sriharikota. 12) 13)
AstroSat was successfully placed into orbit and separated from the fourth stage of PSLV-C30. The separation of all the six co-passenger satellites was completed in the subsequent three minutes. The seven satellites carried by PSLV-C30 had a total mass of 1631 kg at lift-off.
Orbit: Near-equatorial orbit, altitude = 650 km, inclination = 8º, period = 97 minutes. The orbit is selected in such a way to obtain a minimum of SAA (South Atlantic Anomaly) transits.
Secondary payloads on this flight are:
• LAPAN-A2, a microsatellite (68 kg) of LAPAN, Indonesia
• ExactView-9 (EV9), a nanosatellite (5.5 kg) of exactEarth Inc., Canada. EV9 incorporates a next generation AIS (Automatic Identification System) receiver.
• Four Lemur nanosatellites (Lemur-2 through Lemur-5, 3U CubeSats, 4 kg each) of Spire Global, Inc. (formerly NanoSatisfi Inc.) of San Francisco, CA.
Figure 4: Alternate view of the deployed AstroSat spacecraft (image credit: ISRO)
• On Sept 28, 2016 AstroSat completed one year in orbit. In this time, the spacecraft orbited the earth more than 5400 times and executed 343 individual pointings to 141 different cosmic sources. The observed data are being routinely downloaded at the Indian Space Science Data Center (ISSDC) at every visible pass and are being processed at the Payload Operation Centers. The archive and dissemination system at the ISSDC will soon be operational to serve this data to the world community. 14)
• The CZT imager has proved to be a capable all-sky detector of GRBs (Gamma Ray Bursts). In the case of one bright GRB, hard X-ray polarization has been detected by the CZTI. Polarization of the Crab Nebula, too, has now been measured by the CZTI at energies above 100 keV. 15) 16)
• April 2016: The AstroSat mission has completed its performance verification and started Science Operations since April 15, 2016. A phase of guaranteed time observations for the instrument teams is currently ongoing. From October 2016 onwards, observatory access will be open to Guest Observers from the Indian science community. Observing time will be awarded on the basis of peer-reviewed proposals. 17)
- ISRO in collaboration with IUAA in Pune has set up an AstroSat Support Cell (ASC) to facilitate the proposal making process and the use of AstroSat data. The Cell will operate out of IUCAA and will provide resource material, tools, training and help to guest observers. A website has been set up (http://astrosat-ssc.iucaa.in) containing a portal to the APPS (AstroSat Proposal Processing System), Exposure Time and Visibility calculators.
- Activities of the AstroSat Support Cell will include providing long-term support and maintenance of the APPS, running a help desk for proposal and data related queries, and organizing workshops to familiarize users with proposal preparation and data analysis techniques. It is planned to hold every year two long duration workshops at IUCAA and several short duration workshops at different parts of the country.
- A Memorandum of Understanding between ISRO and IUCAA for the operation of the ASC (AstroSat Support Cell) has been signed on May 05, 2016.
• January 2016: ASTROSAT, the multi-wavelength spaceborne observatory of India, has completed more than 100 days in orbit and has experiments covering the UV and X-ray wavebands to conduct observations in the area of astronomy. The PV (Performance Verification) phase of AstroSat is now half way through and it was decided to review the operations so far in order to assess the gain in understanding of the spacecraft operations and the scientific outcome envisaged. Based on this review it is certain that ISRO and the Science community stands vindicated that the outcomes are as expected and in a short while the learning from this mission is enormous. 18)
- As far as payloads are concerned, each of them was switched ON one at a time and the preliminary performance was checked before going further for calibrations and observations of certain target sources. Few calibrations are still pending and further observations are planned before the end of performance verification phase of this satellite in March 2016. Some highlights of the observations obtained in the first 100 days of observations with the payloads are present here:
1) UVIT (UV Imaging Telescope): UVIT started observing the sky on 62nd day after the launch. Preliminary analysis of the initial observations indicate that the payload meets the requirements of sensitivity in FUV (130-180 nm) of maximum effective area as ~12 cm2 and spatial resolution of <1.8" FWHM in ultraviolet.
2) SXT (Soft X-ray Telescope): The initial operations of venting of the camera body, switching ON the electronics and the temperature control and stability were achieved in October 2015. The instrument was assessed first with the onboard calibration sources. The camera door was opened to the sky on October 26 , 2015 with first light image of the blazar PKS 2155-304. Figure 5 is of the Tycho Supernova remnant (also called SN 1572 or 3C 10), one of the bright supernovae visible to the naked eye as found in historical records. This remnant is located in the constellation Cassiopeia. An X-ray spectrum provides both the continuum spectrum which is indicative of the temperature of the plasma and the lines of the elements which are expected to be formed during the final evolution before the supernova explosion. The X-ray spectrum provides observational proof of these elements.
Figure 5: X-ray Spectrum of the Tycho supernova remnant using SXT; the emission lines from ionized Mg, Si, S, Ar, Ca in the millions of degrees hot plasma can be seen clearly, the most prominent line being that of ionized Silicon (image credit: ISRO, AstroSat Team)
3) LAXPCs (Large Area X-ray Proportional Counters): There are three LAXPCs covering the energy range of 3-80 keV. These are currently the largest area proportional counters operating in space. These counters were switched ON in October 2015, with the high voltages turned ON gradually. The gas in the counters was purified using an onboard purifier. Figure 6 gives the continuum spectrum of GRS 1915+105, an X-ray binary with a black hole. This source also emits jets and is termed as a micro-quasar. The continuum spectrum changes as the source goes through different spectral states.
Figure 6: Continuum spectrum of GRS 1915+105 obtained with one of the LAXPC unit. The top panel shows the observed pints with a fit line. Bottom panel shows the residuals of observed points with respect to the fit. The residual figure is checked for goodness of fit (image credit: ISRO, AstroSat Team)
4) SSM (Scanning Sky Monitor): The aim of the SSM is to scan the sky in order to detect and locate X-ray transients in the energy range of 2-10 keV. This payload is now observing portions of half of the sky on the other side of sun for X-ray transients. Stares were performed on 4U0115+63, which is a neutron star binary pulsar on October 26, 2015. During these stares the payload can be operated with a fine time resolution. Following figure shows the detection of the 3.6s rotation period of the neutron star using this payload.
Figure 7: A period folding output of the light curve of the neutron star binary 4U0115+634. The peak indicates the detection of the rotation period (~3.6 s) of the neutron star (image credit: ISRO, AstroSat Team)
5) CZTI (Cadmium Zinc Telluride Imager): The CZTI was the first scientific payload to be switched ON during October 6-11, 2015. It operates in the 20-150keV range and provides observations in the hard X-ray energy range. In addition to capability of extending the hard part of the energy range for studying X-ray binaries and AGNs (Active Galactic Nuclei ), it has capability to detect Gamma ray bursts and is also expected to reveal polarization in the bright X-ray sources in the hard X-ray band. The Crab source has been used to calibrate the timing capability of the instrument. When the Crab observation light curve is divided into two halves and a study of the pulse period (~33 ms) is performed, a spin could be detected of the Crab pulsar.
Figure 8: The image is a data fit significance as a function of the trial period for the Crab pulsar observed by the CZTI instrument during 12 November 2015. The abscissa shows the difference of the trial period from the average period during the 24-hour observation. The first half of the data clearly shows a period shorter than that in the second half of the data. The difference of 18 nanoseconds matches exactly the known rate of spin-down of the Crab Pulsar (image credit: ISRO, AstroSat Team)
• November 20, 2015: Book on AstroSat Released. 19)
• October 27, 2015: The SXT (Soft X-ray Telescope) camera door was opened on 26 October, 2015. The first object viewed was a blazar PKS2155-304. Blazars are a particular type of quasars which are also high energy gamma ray sources. 20)
• October 2015: the SSM (Scanning Sky Monitor) became operational on October 12, 2015, on the 15th day after the successful launch of AstroSat on 28th September 2015. The temperatures of all the packages of the payload have been within the expected limits. Spacecraft was oriented in such a way that the well-known X-ray source "Crab" was positioned at the centre of Field Of View (FOV) of two of the SSM units, SSM1 and SSM2, which have a crossed FOV. 21)
• October 2015: During the first week of CZTI operation, the supernova remnant Crab Nebula and the black hole source Cyg X-1 were monitored. The Crab Nebula can be treated as a standard candle and it was used as a calibrator for timing and imaging, and also to measure the response of the instrument at large off-axis angles. One of the projected objectives of CZTI is wide-angle monitoring of the sky in hard X-ray band to record strange and rare events like GRBs (Gamma-ray Bursts). 22)
- Luckily, on the first day of operation of CZTI, the Swift satellite reported the detection of a Gamma-ray burst, at 09:55:01 UT, named GRB 151006A. We were eager to know whether CZTI was operational at that time (i.e. outside SAA) and if the GRB was in a favorable condition to be observed. A quick calculation showed that this GRB was 60.7 degrees away from the CZTI pointing direction and, at this angle, CZTI should be sensitive to this GRB at energies greater than about 60 keV. The instrument time is yet to be calibrated precisely as the data analysis pipe-line is yet to be streamlined; still a band of youngsters delved into the voluminous data to extract the precious information about this messenger of a blast from the extremities of the universe: GRB 151006A.
- The GRB made its presence felt as an increase in the recorded counts, shown in Figure 9. At higher energies (above 100 keV), the shielding material at the side of the CZTI is designed to be more transparent and one can see a significant and sharp jump in the counts above 100 keV during the GRB time.
- One of the much anticipated properties of CZTI is its ability to identify X-rays depending on the method by which they interact with the detector. If it is by inelastic scattering (called the Compton scattering), they should obey certain scattering principles; and when all the recorded events were subjected to the Compton scattering criteria, there indeed was a significant jump in the count rate. In Figure 10, the so-called `Compton' events (that is, double events satisfying all the requirements of the theoretical expectations of Compton scattering) are plotted as a function of time, the reference time (time zero) being the trigger time of the GRB reported by the Swift satellite.
- This information was flashed to the scientific community through GCN (Gamma-ray Coordinates Network) maintained by NASA.
- Gamma-ray bursts - blasts from the past: Gamma-ray bursts are, as the name suggests, bursts of gamma-rays, coming from apparently random directions in the sky. They were discovered serendipitously in the sixties by the American Vela satellites designed to detect possible surreptitious nuclear weapon tests by the then Soviet Union. For long, they remained a mystery, but in the late 1990s the Italian-Dutch satellite Beppo-SAX managed to measure longer wavelength lingering radiations from them in soft X-rays (called the after-glows) and identify them with far away galaxies. Currently, there are two dedicated satellites measuring their properties: the Swift and the Fermi satellites. Thousands of GRBs have been detected and some of them are identified to be so far away that they originated when the universe was less than a billion years old (the current age of the universe is 13 billion years).
- So, what is the big deal of CZTI detecting one more GRB ? — In spite of the vast amount of data available, GRBs still remain a mystery. One class of GRBs called the long GRBs are associated with newly formed black holes while another class, called the short GRBs, are believed to be the tell-tale signs of the merger of two compact objects. There is also an emerging school of thought which postulates that GRBs originate from neutron stars with extremely high magnetic field, called the magnetars. The current debate about the origin of GRBs is accentuated by the fact that the characteristics of the burst of gamma-rays are ill understood and the radiation mechanisms responsible for the emission is not quantified.
Figure 9: Observed count profile of GRB 151006A (image credit: ISRO)
Figure 10: Observed count profile of Compton events during GRB 151006A (image credit: ISRO)
• October 12, 2015: In the first week of CZTI (Cadmium Zinc Telluride Imager) operation, the Crab Nebula was stared at continuously and was also viewed at different angles to firm up the imaging ability of the instrument. The Crab Nebula was also made to bombard the instrument at several large off-axis angles so that CZTI characteristics as a hard X-ray wide angle monitor can be quantified. 23)
Figure 11: Image of Crab Nebula in hard X-rays above 25 keV. The bright spot near the center indicates Crab. The effective imaging resolution here is about 10 arcmin. The faint patches outside are `side-lobes' of the imaging process and they will be suppressed significantly when data from all quadrants are analyzed simultaneously, which will also improve the image resolution to better than 8 arcmin (image credit: ISRO, AstroSat Team)
• Soon after its separation from PSLV-C30, the two solar arrays of AstroSat were automatically deployed and the Spacecraft Control Center at the Mission Operations Complex of ISTRAC (ISRO Telemetry, Tracking and Command Network) at Bangalore took control of AstroSat (Ref. 12).
- In the coming days, AstroSat will be brought to the final operational configuration and all its five scientific payloads will be thoroughly tested before the commencement of regular operations.
Sensor complement: (UVIT, SXT, LAXPC, CZTI, SSM)
AstroSat carries four coaligned astronomy payloads for simultaneous multi-band observations and one ultraviolet instrument with two telescopes. In addition, a CPM (Charged Particle Monitor) is installed for the control and operation of the sensor complement. 24) 25) 26) 27) 28)
Table 1: Overview of instrument parameters (Ref. 1)
UVIT (Ultraviolet Imaging Telescopes):
The UVIT instrument is a collaboration between ISRO and the Canadian Space Agency (CSA), a contract was signed in 2004. The NRC-HIA (National Research Council Canada - Herzberg Institute of Astrophysics) provides scientific and technical expertise with funding from CSA. Canada is providing the UV photon counting detector subsystem for UVIT. 29) 30) 31)
The objective of UVIT is to perform imaging simultaneously in three channels: 130-180 nm (FUV), 180-300 nm (NUV),and 320-530 nm (VIS). The FOV (Field of View) is a circle of ~ 28 arcmin diameter, the angular resolution is 1.8 arcsec for the ultraviolet channels and 2.0 arcsec for the VIS channel. In each of the three channels a spectral band can be selected through a set of filters mounted on a wheel; in addition, for the two ultraviolet channels, a grating can be selected in the wheel to do slit-less spectroscopy with a resolution of ~ 100 cm-1.
The instrument comprises two telescopes: one is for the FUV (130-180 nm) channel, and the other is for simultaneous imaging in the NUV (180-300 nm) & VIS (320-530 nm) channels. Each of the two telescopes is a f/12 Ritchey-Chretien combination with a primary mirror of ~375 mm diameter (Figure 12), a focal length of ~ 4750 mm, and a plate scale of ~ 24 µm/arcsec. 32) 33) 34)
The images from VIS channel are also used to find aspect of UVIT about once per second. For selection of a band within each of the three channels a set of filters is mounted on a wheel; this wheel also carries a blind to block radiation. The wheels for NUV and FUV channels also carry gratings to provide low resolution (~ 100) slit-less spectroscopy. Photon counting imaging detectors are used in all the three channels to get a resolution of ~ 1.8" FWHM. The detectors can also be used with a low gain (called integration mode), but in this case individual photons are not detected and the spatial resolution is ~ 3". As the satellite is not stabilized to better than 10" (arcsec), it is also required that short exposures are taken and are integrated through a shift and add algorithm on ground: the shift is found by comparing successive images from VIS channel taken every second or so. The success of this algorithm depends on the absence of any jitter > 0.3" rms in attitude of the satellite (either due to some internal motions of any payload etc. or otherwise), and a drift free relative aspect of the three channels over periods of ~ 1000 s: a duration which is large enough to collects enough photons from sources in the UV images.
The 3 channels use MCP (Microchannel Plate)-based intensified CMOS imaging detectors for the recording of imagery in either (high gain) photon counting mode or in (low gain) integrating mode in which individual photons cannot be distinguished. Typically, the photon counting mode is used for the two ultraviolet channels which have a small flux, while the integration mode is used for the VIS channel which has a high flux. Special attention has been paid to minimize the photocathode/MCP gap to get a spatial resolution of ~ 25 µm FWHM Full Width Half Maximum), i.e. ~ 1 arcsec on the plate scale of the telescopes, in the photon counting mode.
The UV images are typically taken at ~ 30 frames/s; for specific observations, depending on the size of the selected field, images of a partial field can be taken up to a rate of 200 frames/s. The time of each frame can be tracked to an absolute accuracy of 5 ms.
The effective area of the telescope depends on the chosen channel and the filter: it is ~ 15 cm2 for the FUV (Far Ultraviolet) channel which only use crystal filters, and it is in range 15-40 cm2 for the various filters in NUV (Near Ultraviolet) & VIS channels.
Figure 12: Configuration of the UVIT assembly of two telescopes (image credit: AstroSat collaboration)
Figure 13: Photo of the UVIT engineering model (image credit: AstroSat collaboration)
Table 2: Key parameters of UVIT
Optical design: Each UVIT telescope is based on a Ritchey-Chretien configuration with an aperture of ~375mm and a focal length of ~ 4750 mm. Figures 14 and 15 illustrate the optical layout of the FUV and NUV-VIS telescope, respectively.
Figure 14: Optical layout of the FUV channel, f/12 Cassegrain, ~ 380 mm aperture (image credit: AstroSat collaboration)
Figure 15: Optical layout of the NUV & VIS channels, f/12 Cassegrain, ~ 380 mm aperture (image credit: AstroSat collaboration)
Legend of Figure 15: The optics of the NUV-VIS telescope; the position marked as ‘filter' carry a filter wheel; in NUV, the channel wheel has a selection of 6 filters and a grating and a block, while in VIS, the channel wheel has a selection of 5 filters and a block.
Figure 16: UVIT detector module (image credit: AstroSat collaboration)
In both telescopes, the primary mirror is a solid mirror with a diameter of 375mm and a central hole of 155 mm. The mirror material used is Zerodur with the surface error of better that λ/50 rms and the micro roughness is better than 15Å rms. The surface of primary is concave on-axis hyperboloid with a radius of curvature =3541mm and conic -1.129. The primary mirror is mounted in the telescope by side mounts; i.e 3 bipods @120º apart, the mounts are glued to the mirror and the mounts to be fixed on a ring.
The secondary mirror for both telescopes is also a solid mirror made of Zerodur material. The surface of the secondary mirror is convex on-axis hyperboloid with a radius of curvature = 1867mm and conic -6.3565. Its diameter is ~140 mm; it is mounted using a 3 blade cell mount, and the mount is glued to the mirror cylinder rim. The surface error and micro roughness of the secondary mirror are the same as for the primary mirror.
The average reflectivity of both mirrors is maintained identical; i.e., better than 60% for the wavelength band of FUV (130-180 nm), better than 70% for the wavelength band of NUV (180-200 nm), and better than 80% for VIS (200-600 nm).
Appropriate baffling is provided at necessary places in the optical design. To avoid any cosmic and bright light hitting the detector, in each telescope there are 2 main baffles above the telescope tubes, one primary baffle near the primary mirror and one secondary baffle near the secondary mirror as shown in Figure 12.
Table 3: List of filers & gratings used in UVIT
Mechanical and thermal design: The mechanical configuration of the payload is shown in the Figure 12 and its subsystems interfaces and its specifications are presented. The primary and secondary mirror system separation is maintained by an Invar tubular structure (~1500 mm), made in 3 segments. The Telescope ring (TR ring) at the bottom supports the primary mirror, the secondary is supported at the top end by spider ring (SPDR). The TR ring also supports the focal volume elements by a system of 3 Invar rods (FR's), the bottom segment of the telescope tube (TT3) and the thermal cover. The SPDR is a 4 blades tangential system holding the secondary mirror at the center, the provision for required tilt and de-center are given at the secondary mirror interface.
The titanium satellite adapter on the UVIT structure provides the interface between UVIT and the spacecraft. The attachment is through 18 nos tabs on the titanium adapter, which are held against the satellite cylinder by M6 bolts, which get engaged in to the plate nuts riveted in the tabs. The satellite cylinder is provided with two cutouts to take the cable harness into the spacecraft. The titanium satellite adapter also has a master ref. cube fitted on it, to serve as a ref while integrating with spacecraft.
The mass of UVIT has two parts, the UVIT mass attached to the central cylinder of the satellite is 202 kg, and the UVIT electronics package mass on the satellite equipment panels is about 28 kg, amounting to the total UVIT mass of 230 kg.
Thermal design and analysis is concerned with predicting the temperatures of the payload in a specific orbital thermal environment. A numerical thermal model is used as the working tool in the development of the satellite thermal control system. It is used to predict temperature on a large scale, with most structures and other components interacting with one another and with the surrounding environment. The mode of heat transfer in the payload system is generally through conduction and radiation heat transfer, except in the case at the launch pad. The ambient temperature and heat loads influence overall temperature distribution in the payload.
The thermal control system of the payload employs a passive method of isolation by MLI (Multi Layer Insulation), and heaters under closed loop control. The UVIT is wrapped all over with MLI. The heaters and the OSR (Optical Solar Reflector) are used to maintain the temperature. The thermal loads are the internal and external loads. Internal loads due to internal power dissipation of the filter motor, detector and high voltage box, which are located in the focal volume unit. The external loads are due to the sun, albedo of the Earth and earthshine and are evaluated based on the orbit parameters.
Table 4: Thermal requirements and achieved thermal model results
Figure 17: Photo of the assembled UVIT (FUV and NUV-VIS) flight model telescopes (image credit: AstroSat collaboration, Ref. 31)
The UVIT project is collaboration between the following institutes from India:IIA (Indian Institute of Astrophysics), Bengaluru, IUCAA (Inter University Centre for Astronomy and Astrophysics), Pune, and NCRA (National Centre for Radioastrophysics), Pune, TIFR ((Tata Institute of Fundamental Research), Mumbai), and CSA (Canadian Space Agency). The detector systems are provided by CSA. The mirrors are provided by LEOS, ISRO, Bengaluru and the filter-wheels drives are provided by IISU, ISRO, Trivandrum. Many departments from ISAC, ISRO, Bengaluru have provided direct support in the design and implementation of the various subsystems.
SXT (Soft X-ray imaging Telescope):
The SXT assembly employs focussing optics and a deep depletion CCD camera at the focal plane to perform X-ray imaging in 0.3-8.0 keV band. The optics consist of 41 concentric shells of gold-coated conical foil mirrors in an approximate Wolter-I configuration. The focal plane CCD camera is very similar to that flown on SWIFT XRT (X-Ray Telescope) of NASA. The CCD will be operated at a temperature of about -80ºC by thermoelectric cooling.
Telescope optics at the soft X-ray bands employ grazing incidence reflection from metal surfaces. The refractive index of metals in X-rays is slightly less than one so it is possible to get a total external reflection at a vacuum-metal interface if the X-rays are incident nearly parallel to the metal surface. The limiting angle of grazing incidence lies between a few degrees at ~0.1 keV to a few arcminutes at ~10 keV. 35) 36) 37)
Table 5: Summary of the main SXT characteristics
Figure 18: Illustration of the SXT structure (image credit: AstroSat collaboration)
Figure 19: Effective area of the SXT as a function of photon energy (image credit: AstroSat collaboration)
At its focal plane, the SXT carries a thermoelectrically cooled X-ray CCD camera, based on the e2V Technologies CCD-22 chip. The CCD has 600 x 600 pixels each of 40 micron square. It is a frame transfer device - an image transferred from image to store section can be read out while a new image is being acquired.
The CCD detector operated in single photon counting mode. Each X-ray photon, depending on its energy, will liberate about 100 to 1000 electron-hole pairs. Preserving this total charge information for each photon will lead to the measurement of its energy, thus enabling spectroscopic studies. The energy resolution is strongly degraded by system noise. To reduce thermal noise in the CCD it will be thermoelectrically cooled to an operating temperature of -80oC, which is expected to yield an energy resolution of about 2% at 6keV.
Figure 20: Schematic diagram of the CCD-22 detector (image credit: E2V)
Figure 21: Photo of the theromoelectric cooler and CCD assembly (image credit: AstroSat collaboration)
The focal plane camera assembly consists of the CCD and its cooling arrangement housed in a cryostat, which will also contain four Fe55 calibration sources, an optical blocking filter for the CCD and an aluminum proton shield to protect the CCD from proton damage while passing through the South Atlantic Anomaly region. The optical blocking filter is made of a single fixed polyamide film of thickness 184 nm, with a 48.8 nm thick aluminum coating on one side. This yields an optical transmission of about 0.25%, limiting the background light reaching the detector. The entire cryostat body is made of aluminum alloy, gold plated for thermal insulation.
Figure 22: Focal plane camera assembly of SXT (image credit: AstroSat collaboration)
Figure 23: Photo of the SXT flight model optics entrance aperture (image credit: AstroSat collaboration)
LAXPCs (Large Area Xenon Proportional Counters):
The instrument is used for X-ray timing and low-resolution studies. The assembly consists of a cluster of three coaligned identical Large Area X-ray Proportional Counters (LAXPCs), each with a multi-wire-multi-layer configuration and a FOV of 1º x 1º. These detectors are designed to achieve: 38) 39)
6) a wide energy band of 3-80 keV
7) high detection efficiency over the entire energy band
8) narrow field of view to minimize source confusion
9) moderate energy resolution
10) small internal background and
11) long life time in space.
A Xenon-based gas mixture at a pressure of two atmospheres will be filled in multilayer 15 cm deep detectors to achieve an average detection efficiency of close to 100% below 15 keV and about 50 % up to 80 keV. A thin (thickness of 25/50 µm) aluminized Mylar window for X-ray entrance ensures a low energy threshold of about 2-3 keV. The Mylar film is supported by a honeycomb shaped window support collimator with a 5º x 5º FOV. A FOV of 1º x 1º is provided by using mechanical collimators made of a sandwich of tin,copper and aluminum coaligned with the window support collimator and sitting above it. 40) 41)
Figure 24: Photo of the LAXPC wired Anode Assembly (image credit: AstroSat collaboration)
Legend to Figure 24: LAXPC X-ray detector anode assembly with veto layer on 3 sides mounted on the back plate. 60 anode cells are arranged in 5 layers to make the X-ray detection volume, 37 µm diameter Au-plated SS wires under tension used for anodes.
The total effective area of the 3 LAXPCs is ~ 6000 cm2 at 5 keV. Due to its large depth and high gas pressure the LAXPC will have high detection efficiency right up to about 80 keV, as shown in Figure 25.
To achieve good energy resolution of the detectors, it is necessary to have a uniform gain over the entire area and the gas needs to be free from impurities like oxygen and water vapor. The former is achieved by precision placement of the anode wires at the center of the cells and by the use of anode wire of uniform diameter. An onboard purifier is being used to purify the gas from time to time; it will prevent degradation of energy resolution due to slow outgassing from detector walls.
The high sensitivity of the LAXPC instrument will allow the detection of a 0.1 mCrab source at the 5σ level in an exposure of about 104 seconds. This will enable the LAXPC to address a wide variety of science topics. 42)
Four modes of operation:
• Broad band counting with variable integration time in many energy channels
• Onboard pulse height histograms with variable integration time
• Time tagging of each photon to 10 µs accuracy
• Fast counting mode to handle high counting rates from bursts.
Figure 25: Effective area of the LAXPC instrument as a function of energy (image credit: AstroSat collaboration)
Figure 26: Photo of the LAXPC collimator (image credit: AstroSat collaboration)
The LAXPC instrument has a mass of ~ 390 kg.
CZTI (Cadmium-Zinc-Telluride coded-mask Imager):
The CZTI instrument consists of a pixelized CdZnTe (Cadmium-Zinc-Telluride) detector array of ~1000 cm2 in geometric area. These detectors have very good detection efficiency, close to 100% up to 100 keV, and have a superior energy resolution (~2% at 60 keV) when compared to scintillation and proportional counters. Their small pixel size also facilitates medium resolution imaging in hard X-rays. The CZTI will be fitted with a two dimensional coded mask, for imaging purposes. The sky brightness distribution will be obtained by applying a deconvolution procedure to the shadow pattern of the coded mask recorded by the detector. 43)
The coded mask imaging technique is one possible way of performing wide field imaging with photons of energy greater than a few keV. It comprises of utilizing the shadows of a multiple pinhole mask plate cast on the detector, with the shift in the shadows encoding the location of the source in the sky. The CZTI comprises of a two dimensional mask plate mounted on top of a pixelized CZT detector array.
Table 6: Key parameters of the CZTI instrument
The CTZI instrument is fabricated in four identical, independent quadrants which are joined together in the final configuration. Each quadrant has a 64 x 64 element coded mask and a detector array of the same number of pixels. The mask pattern of adjacent quadrants are rotated by 90º with respect to each other.
Figure 27: Schematic view of the CTZI instrument (image credit: AstroSat collaboration)
SSM (Scanning Sky Monitor):
The SSM instrument consists of three position sensitive proportional counters, each with a one dimensional coded mask, very similar in design to the ASM (All Sky Monitor) on NASA's RXTE (Rossi X-ray Timing Explorer) satellite (launch Dec. 30, 1995 ). The gas-filled proportional counter features resistive wires as anodes. The ratio of the output charge on either ends of the wire provide the position of the X-ray interaction, providing an imaging plane at the detector. The coded mask, consisting of a series of slits, casts a shadow on the detector, from which the sky brightness distribution can be derived. 44)
The objectives of SSM are:
• To detect, locate and monitor x‐ray transients (nearly half of known x‐ray binaries are transients)
• Monitor known bright sources (several samples/day; monitor for many months)
• Alert other instruments for detailed studies.
Figure 28: Schematic view of the SSM instrument (image credit: AstroSat collaboration)
Table 7: Key parameters of the SSM instrument
The operation of the SSM is to scan the sky continuously irrespective of the functions of the other instruments on the spacecraft. A mounting arrangement is therefore necessary to enable these detectors to scan as much of the sky as possible, independent of the satellite pointing.
The three counters are mounted on a rotating platform providing a stepped rotation at discrete steps about one axis. One of the monitors (the boom camera: SSM3) is aligned with the rotation axis while the other two are mounted with their field of view forming an 'X' in the sky (Figure 28).
Typical scan pointings will be ~10 º apart with ~10 minute integration at each location. This enables nearly half of the sky coverage, about 4 times per day (including nominal SAA exclusion orbits).
CPM (Charged Particle Monitor):
A CPM, an auxiliary instrument, is included in the sensor complement of AstroSat to control the operation of the LAXPC, SXT and SSM instruments. Even though the orbital inclination of the satellite is 8º, in about 2/3rd of the orbits, the satellite will spend a considerable time (15 - 20 minutes) in the SAA (South Atlantic Anomaly) region which has high fluxes of low energy protons and electrons. The high voltage will be lowered or put off using data from CPM when the satellite enters the SAA region to prevent damage to the detectors as well as to minimize the ageing effect in the Proportional Counters. 45)
A Scintillator Photodiode Detector (SPD) with a Charge Sensitive Preamplifier will be used to detect the charged particles.
In the CPM, a cube of 10 mm side length of CsI (Tl) crystal (wavelength = 550 nm) with Teflon reflective material is coupled to the same area window of a Si-PIN diode. The incident charged particle energy is converted into light in CsI, and the light, seen by the photodiode, is converted into an electrical pulse with the help of a CSPA (Charge Sensitive Pre-Amplifier). The electrical signal is then passed through a LLD (Lower Level Discriminator) with a threshold level commandable from ground. The output is made available to all other instruments on board, and is also recorded as a part of the satellite housekeeping data.
Table 8: Key parameters of the CPM instrument
AstroSat Ground Segment:
ISTRAC (ISRO Telemetry Tracking and Command Network) is bestowed with the responsibility of providing the ground support for all the phases of this mission. AstroSat Ground Segment comprises TT&C (Tracking Telemetry and Telecommand) and Payload Data reception stations, SCC (Satellite Control Center), ISSDC (Indian Space Science Data Center) and the POCs (Payload Operations Centers). 46) 47)
The AstroSat TT&C and Payload Ground Station along with ISSDC is collocated in the Indian Deep Space Network Complex (IDSN), Bylalu. All these four operational areas are intimately connected through communication links. In addition to the above, there is an arrangement for the pre-processing and approval of the received request for the satellite time that streamlines the sequence of observations to be carried out considering the merit, exigency of the proposals and the constraints posed by the satellite and the geometry. These ground systems function in unison towards fulfilling the mission objectives.
TT&C and Payload Data Reception Station: The AstroSat antenna (11 m diameter) is dedicated for AstroSat TT&C data &payload data reception operations. The station is tailored to meet the mission requirements in conjunction with the onboard RF systems. A standby TT&C and payload data reception station is also made available at SCC, Bangalore. For the purpose of improved visibility and extended TT&C support, the rest of the ISTRAC S-band network will be utilized. The ground station transfers the TT&C data to the control center and the exclusive payload data reception stations transmit instrument-wise de-mulitplexed data to the Science Data Center, after due conditioning.
Figure 29: Photo of the AstroSat antenna (image credit: ISRO)
SCC (Satellite Control Center): The SCC comprises computers, technical facilities, Mission Control & Mission Analysis area and a dedicated mission control area. At the SCC, the satellite health is monitored, telecommands uplinked towards payload operations and configuration changes onboard, attitude determination, visibility generation, scheduling and archival of all TT&C data sets. The timetabling for various source observations and operating various instruments is approved by a separate committee identified for this purpose. This center houses all necessary and approved mission operation software and analysis tools. The Pre-launch simulations are also organized from this center. The ISTRAC communication network is comprising both terrestrial and satellite links, it provides real time voice / data/ fax connectivity for all ground elements inclusive of the launcher agency.
Payload Operations Planning: AstroSat observations are basically proposal –driven and Announcement of Opportunity comes in to effect, a year after the launch.
The APPS (AstroSat Proposal Processing System) accepts all the proposals in to its proposal database and takes up further processing based on the predetermined criteria. It is also interfaced with other planning tools such as Astroviewer and Exposure Time Calculator, along with details of the instruments mode, constraints etc. The source selection and finalization shall be carried out by the ATAC (AstroSat Time Allocation Committee) assisted by the peer reviewing mechanism that assess all the proposals on their scientific merit and feasibility, and taking into account the comments of the ATC (AstroSat Technical Committee) on the proposals. The proposals cleared by ATAC will flow to ATC, which studies the feasibility of observing the sources, meeting all the spacecraft constraints. The approved proposals shall be passed on to Payload Programming System for the generation of time-lined command sequence output to be uplinked to the satellite.
ISSDC & POCs: The ISSDC (Indian Space Science Data Center) is bequeathed with the responsibility of governing the ingest, Quicklook Display (QLD), processing (for level-0/1), archival (all levels, along with the auxiliary data) and dissemination of the payload data. It also receives and archives the higher level products generated by POCs/Guest Observers. ISSDC also supports Scientists with Payload and instrument health monitoring facilities, in addition to serving as an alternate control center.
POCs (Payload Operation Centers) are the instrument-wise focal points. They monitor the health of the instruments, coordinate the instruments related operations, plan periodic calibrations and higher level data processing. They receive the processed data from ISSDC and return higher level products generated at the POCs for archival and dissemination to the general public after a lock-in period.
Thus, end-to-end mission activities, involving, payload operation proposal reception & processing, command generation and uplink, payload operations, satellite orbit and attitude maintenance, payload data reception and processing, dissemination and archival activities - are carried out by the unified ground segment. Above all, the ground segment is responsible for the up-keeping of this space asset that provides a conductive environment for the instrument operations, inclusive of all desired orbit and attitude.
Figure 30: Overview of the AstroSat Ground Segment (image credit: ISRO, Ref. 10)
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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)