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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.

The science goals call for: 1) 2) 3) 4) 5) 6) 7) 8)

• 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.

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Figure 1: Illustration of the AstroSat spacecraft and its instrument complement (image credit: ISRO)

Spacecraft:

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 ~ 1600 kg, including 868 kg of payload mass. The expected operating life time of the satellite is five years.

AstroSat_Auto11

Figure 2: Artist's rendition of the deployed AstroSat spacecraft configuration (image credit: ISRO)

 

Launch: A launch of AstroSat is scheduled for 2013 on a PSLV vehicle (PSLV-22) of ISRO from SDSC (Satish Dhawan Space Center) SHAR, ISRO's launch site on the south-east coast of India, Sriharikota.

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.

The LAPAN-A2 microsatellite of LAPAN, Indonesia, is a secondary payload on this flight.

 


 

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. 9) 10) 11) 12) 13)

Parameter/Instrument

UVIT/OPT

SXT

LAXPC

CZTI

SSM

Detector

UV: photon counting CCD; Opt: CCD photometer

X-ray CCD (at the local plane)

Proportional counter

CdZnTe detector array

Position sensitive proportional counter

Imaging property

imaging

imaging

non-imaging

Imaging (<100 keV)

imaging

Optics

Twin Ritchey-Chretien 2 mirror system

Conical foil (Wolter-I mirrors)

Collimator

2D coded mask

1D coded mask

Bandwidth

130-320 nm

0.3-8 keV

3-100 keV

10-150 keV

2-10 keV

Geometric area

1250 cm2

250 cm2

10,800 cm2

1000 cm2

180 cm2

Effective area (cm2)

60 (depends on filter)

125 @ 0.5 keV
200 @ 1-2 keV
25 @ 6 keV

6000 @ 5-30 keV

500(<100 keV)
1000 (>100 keV)

~40 @ 2 keV
90 @ 5 keV
(Xe gas)

FOV

0.50º diameter

0.35º (FWHM)

1º x 1º

6 x 6 (<100 keV)
17º x 17º (>100 keV)

 

Energy resolution

< 100 (depends on filter)

2% @ 6 keV

9% @ 22keV

5% @ 10 keV

19% @ 6 keV

Angular resolution

1.8 arcsec

3-4 arcmin (HPD)

1-5 arcmin in scam mode only

8 arcmin

~10 arcmin

Time resolution

10 ms

2.6 s, 0.3 s, 1 ms

10 µs

1 ms

1 ms

Typical obs. time/target

30 min

0.5-1 day

1-2 days

2 days

5 min

Sensitivity (obs. time)

21st magnitude (5σ) (1800 s)

10 µCrab (5σ) (10000 s)

0.1 mCrab (3σ)(1000 s)

0.5mCrab (3σ) (1000 s)

~30mCrab (3σ) (300 s)

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. 14) 15) 16)

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 3), a focal length of ~ 4750 mm, and a plate scale of ~ 24 µm/arcsec. 17) 18) 19)

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.

AstroSat_Auto10

Figure 3: Configuration of the UVIT assembly of two telescopes (image credit: AstroSat collaboration)

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Figure 4: Photo of the UVIT engineering model (image credit: AstroSat collaboration)

Parameter → Channel

FUV

NUV

VIS

Detector

Intensified CMOS Photon Counting/ Integration

Intensified CMOS Photon Counting/ Integration

Intensified CMOS Photon Counting/ Integration

CMOS chip

Fillfactory/ Cypress STAR250, 512x512, 25Qm pixels

Telemetry optics

Ritchey-Chretien 2 mirror system

Ritchey-Chretien 2 mirror system

Ritchey-Chretien 2 mirror system

Bandwidth

130-180 nm

200-300 nm

320-550nm

Geometric area

~880 cm2

~880 cm2

~880 cm2

Effective area

>~15 cm2 at peak

>~50 cm2 at peak

>~50 cm2 at peak

FOV (Field of View)

~28 arcmin

~28 arcmin

~28 arcmin

Spectral resolution

<1000 Å (depends on choice of filters)

<1000 Å (depends on choice of filters)

<1000 Å (depends on choice of filters)

Spatial resolution

< 1.8 arcsec

< 1.8 arcsec

< 1.8 arcsec

Time resolution

< 10 ms (for partial field)

< 10 ms (for partial field)

< 10 ms (for partial field)

Typical observation time/target

30 minutes

30 minutes

30 minutes

Sensitivity (observation time)

> 20th magnitude (5σ) in 200 s

-

-

Photometry accuracy

10%

Instrument mass, power

230 kg, 85 W (peak 117 W)

Sun avoidance angle

45º

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 5 and 6 illustrate the optical layout of the FUV and NUV-VIS telescope, respectively.

AstroSat_AutoE

Figure 5: Optical layout of the FUV channel, f/12 Cassegrain, ~ 380 mm aperture (image credit: AstroSat collaboration)

AstroSat_AutoD

Figure 6: Optical layout of the NUV & VIS channels, f/12 Cassegrain, ~ 380 mm aperture (image credit: AstroSat collaboration)

Legend of Figure 6: 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.

AstroSat_AutoC

Figure 7: 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 3.


FUV channel

Slit No

Filter type

Filter thickness

Passband

Material

1

Block with Aluminum

 

 

 

2

Calcium Fluoride - 1

2.50 mm

>125 nm

 

3

Barium Fluoride

2.40 mm

>135 nm

 

4

Sapphire Window

2.00 mm

> 142 nm

 

5

Grating - 1

4.48 mm

 

 

6

Silica

2.70 mm

>159 nm

 

7

Grating – 2

4.48 mm

 

 

8

Calcium Fluoride – 2

2.50 mm

>125 nm

 

NUV channel

1

Block with Aluminum

 

 

 

2

Fused Silica Window

3.00 mm

>159 nm

 

3

NUVB15

2.97 mm

200-230 nm

Silica (UV)

4

NUVB13

3.15 mm

230-260 nm

Silica (UV)

5

Grating

4.48 mm

 

 

6

NUVB4

3.33 mm

250-280 nm

Silica (UV)

7

NUVB2

3.38 mm

275-285 nm

Silica (UV)

8

Fused Silica Window

3.30 mm

>159 nm

 

VIS channel

1

Block with Aluminum

 

 

 

2

VIS 3

3.00 mm

400-500 nm

UBK7

3

VIS 2

3.00 mm

370-410 nm

UBK7

4

VIS 1

3.00 mm

320-360 nm

UBK7

5

Neutral Density Filter

3.00 mm

 

 

6

BK7 Window

3.00 mm

 

 

Table 3: List of filers & gratings used in UVIT

Mechanical and thermal design: The mechanical configuration of the payload is shown in the Figure 3 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.

Requirements

Achieved values

Temperature of telescope tubes to be between 18-22ºC

17.5ºC(min) and 22.8ºC(max) in cold Invar case

Axial variation of temperature on telescope tubes to be within ±2ºC

2.3ºC on NUV side in cold focal case and cold Invar cases

Circumferential variation of temperature on telescope tubes to be within 5ºC

2.8ºC in cold focal case

Temporal variation of temperature at a given point within 1000 s (~15 minutes) (in quasi steady state) to be within 0.3ºC

0.77ºC (TT2 bottom portion in FUV side) in hot focal case (max) and 0.02ºC (TT2 top portion in NUV side) in hot focal case (min)

Temperature of elements in the focal plane volume to be between 15-20ºC

12.7ºC (min) 20.6ºC (max)

Temperature (during operation) of detectors (CPU’s) between 0-20ºC

16.4ºC (min) 17.9ºC (max)

Temperature (during operation) of High Voltage Units (HVU’s) between 0-30ºC

12.7ºC (min) 18.4ºC (max)

Duty cycle of heaters not to exceed 65%

64% in MB1 in cold Invar case

Table 4: Thermal requirements and achieved thermal model results

AstroSat_AutoB

Figure 8: Photo of the assembled UVIT (FUV and NUV-VIS) flight model telescopes (image credit: AstroSat collaboration, Ref. 16)

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. 20) 21) 22)

Telescope length

2465 mm (including baffle, door and camera)

Telescope mirrors

Conical shells

Focal length

2000 mm

Telescope PSF (Point Spread Function)

1.5 - 2.5 arcmin (rms)

FOV (Field of View)

41.3 x 41.3 arcmin

Energy range

0.3-8.0 keV

Detector

E2V CCD-22 (Frame Store)

Detector format

600 x 600 pixels

Pixel scale

4.13 arcsec/pixel

CCD readout modes

Photon Counting, Imaging, Timing

Effective area

200 cm2 @ 1.5 keV

Position accuracy

30 arcsec

Sensitivity expected

10 µCrab or better

Table 5: Summary of the main SXT characteristics

AstroSat_AutoA

Figure 9: Illustration of the SXT structure (image credit: AstroSat collaboration)

AstroSat_Auto9

Figure 10: 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.

AstroSat_Auto8

Figure 11: Schematic diagram of the CCD-22 detector (image credit: E2V)

AstroSat_Auto7

Figure 12: 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.

AstroSat_Auto6

Figure 13: Focal plane camera assembly of SXT (image credit: AstroSat collaboration)

AstroSat_Auto5

Figure 14: Photo of the SXT flight model optics entrance aperture (image credit: AstroSat collaboration)

 

LAXPC (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: 23) 24)

1) a wide energy band of 3-80 keV

2) high detection efficiency over the entire energy band

3) narrow field of view to minimize source confusion

4) moderate energy resolution

5) small internal background and

6) 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. 25) 26)

AstroSat_Auto4

Figure 15: Photo of the LAXPC wired Anode Assembly (image credit: AstroSat collaboration)

Legend to Figure 15: 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 16.

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. 27)

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.

AstroSat_Auto3

Figure 16: Effective area of the LAXPC instrument as a function of energy (image credit: AstroSat collaboration)

AstroSat_Auto2

Figure 17: 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. 28)

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.

Detector

CZT (Cadmium-Zinc-Telluride) detector array

Energy range

10 - 150 keV, up to 1 MeV (photometric)

Energy resolution

5% @ 100 keV

Pixel size, number of pixels

2.4 mm x 2.4 mm (5 mm thick)

Number of pixels

16384

Geometric area

1024 cm2

FOV (Field of View)

6º x 6º (10-100 keV) (defined by collimator)
17º x 17º (> 100 keV) (defined by coded mask housing)

Angular resolution

8 arcmin (< 100 keV)

Veto layer

2 cm thick CsI crystal+PMT (Photo Multiplier Tube)

Read-out

ASIC based (128 chips of 128 channels)

Imaging method

CAM (Coded Aperture Mask)

Overall size

50 cm x 50 cm x 70 cm (height), without radiator plate

Instrument mass, power

50 kg, 50 W

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.

AstroSat_Auto1

Figure 18: 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. 29)

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.

AstroSat_Auto0

Figure 19: Schematic view of the SSM instrument (image credit: AstroSat collaboration)

Detector

Proportional counters with resistive anodes; ratio of signals on either ends of anode gives position

Energy range

2 - 10 keV

Position resolution

1.5 mm

Position determination

~0.5 mm

FOV

10º x 90º (FWHM)

Sensitivity

30 mCrab (5 minute integration)

Best time resolution

1 ms

Angular resolution

~ 10 arcmin

Instrument mass, power

48 kg, 30 W

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 19).

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. 30)

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.

Scintillator

1cm x 1cm x 1cm CsI (Tl) crystal

Light collector

Photodiode with pre-amp (Hamamatsu s3590-08+eV5152)

Window

1 mil Kapton

Low energy threshold

1.2 MeV

Time resolution

5 s

Expected count rate

1 s-1 (in non-SAA region)

Maximum count rate

1000-1

Instrument size, mass, power

18 cm x 15 cm x 5 cm, 2 kg, 2.3 W

Table 8: Key parameters of the CPM instrument


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2) V. Kosteswara Rao, P. C. Agrawal, P. Sreekumar, K. Thyagarajan, “The scientific objectives of the AstroSat mission of ISRO,” Acta Astronautica, Vol. 65, Issues 1-2, July-August 2009, pp. 6-17

3) Paul O'Brien, “AstroSat,” Neutron Stars & Gamma Ray Bursts 2009, Cairo, Egypt, March 30-April 1, 2009, URL: http://www.ns-grb.com/PPT/obrien-astrosat.pdf

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6) Dipankar Bhattacharya, “AstroSat Science and synergy with NuSTAR,” URL: http://www.astro.caltech.edu/Robo-AO/docs/DB_Astrosat_NuSTAR.pdf

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8) Hardik Panchal, “Astrosat: a telescope on a satellite,” Current Science, Vol. 104, No 4, February 25, 2013, p. 412, URL: http://www.currentscience.ac.in/Volumes/104/04/0412.pdf

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10) http://www.iucaa.ernet.in/~astrosat/Xrayconf2011/Talks.html

11) S. N. Tandon, “Introduction to AstroSat,” Metteing: Adaptive Optics with Moderate¿sizes Telescopes, August 22¿25, 2011, URL: http://www.astro.caltech.edu/Robo-AO/docs/introduction_astrosat_august2011.pdf

12) Dipankar Bhattacharya, “AstroSat - A forthcoming Indian Astronomy Mission,” URL: http://www.astro.caltech.edu/~srk/MiniSat/Presentations/astrosat_overview.pdf

13) K. P. Singh, “AstroSat: A Multi-Wavelength Satellite,” July 9-10, 2010, URL: http://www.star.le.ac.uk/conf50/talks/S08_0930_KP_Singh.pdf

14) “UVIT - Ultra-Violet Imaging Telescope, NRC (National Research Council) Canada,” 2009, URL: http://archive.nrc-cnrc.gc.ca/eng/projects/hia/uv-imaging.html

15) Leonardo Ubeda, Carmelle Robert, Laurent Drissen, and the UVIT Canadian Science Team, “Introducing UVIT,” URL: http://www.astro.phy.ulaval.ca/staff/laurent/ARTICLES/lubeda.pdf

16) Amit Kumar, S.K.Ghosh, J. Hutchings,P. U. Kamath, S. Kathiravan,P. K. Mahesh,J. Murthy, Nagbhushana S. A. K. Pati,M. N. Rao, N. K. Rao,S. Sriram, S. N. Tandon, “Ultra Violet Imaging Telescope (UVIT) on ASTROSAT,” 2012, URL: http://arxiv.org/ftp/arxiv/papers/1208/1208.4670.pdf

17) “UltraViolet Imaging Telescope,” IIAP, URL: http://www.iiap.res.in/Uvit

18) “Critical Design Review (CDR) Of Ultra Violet Imaging Telescope (UVIT),” IIA, June 17-18, 2011

19) “Filter Wheel Drive Mechanism and Electronics for Ultra Violet Imaging Telescope of AstroSat,“ ISRO, June 2011

20) “Soft X-ray Telescope for AstroSat,” Poster, URL: http://meghnad.iucaa.ernet.in/~astrosat/SXT_files/SXT_poster1.pdf

21) “Soft X-ray Telescope CCD camera,” URL: http://www.iucaa.ernet.in/~astrosat/SXT_files/SXT_CCD.html

22) “Leicester set to fly high in India's first-ever national astronomy mission,” Dec. 2011, URL: http://www2.le.ac.uk/offices/press/press-releases/2011/december/leicester-set-to-fly-high-in-indias-first-ever-national-astronomy-mission

23) “The LAXPC Instrument,” ISRO, URL: http://www.iucaa.ernet.in/~astrosat/laxpc_specs.html

24) “Science with AstroSat LAXPC,” URL: http://www.iucaa.ernet.in/~astrosat/laxpc_files/laxpc_science.html

25) R. K. Manchanda, “Large Area X-ray Proportional Counter payload (LAXPC),” Sept. 27, 2006, URL: http://www.iiap.res.in/uvit/uvitday1/IIA_conf.pdf

26) http://www.iucaa.ernet.in/~astrosat/laxpc_specs.html

27) “Science with AstroSat LAXPC,” URL: http://www.iucaa.ernet.in/~astrosat/laxpc_files/laxpc_science.html

28) “AstroSat Cadmium Zinc Telluride Imager (CZTI),” URL: http://www.iucaa.ernet.in/~astrosat/czti_specs.html

29) “AstroSat Scanning Sky Monitor (SSM),” URL: http://www.iucaa.ernet.in/~astrosat/ssm_specs.html

30) “AstroSat Charged Particle Monitor (CPM),” URL: http://www.iucaa.ernet.in/~astrosat/cpm_specs.html


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.