Complete information on ASTROSAT – Spacecraft fully assembled and tests initiated

Introduction

ASTROSAT is the first dedicated Indian astronomy mission aimed at studying distant celestial objects. The mission is capable of performing observations in Ultraviolet (UV), optical, low and high energy X-ray wavebands at the same time.

The project was conceived following the success of the Indian X-Ray Astronomy Experiment, carried on a remote sensing satellite launched in 1996.
The government cleared the Astrosat project In 2004 at a cost of Rs. 178 crore. The satellite was planned to be launched in about 4 years.

ASTROSAT would be India's first multiwavelength astronomy satellite. It will facilitate simultaneous observations of celestial bodies and cosmic sources in X-ray and UV spectral bands. The uniqueness of ASTROSAT lies in its wide spectral coverage extending over visible (3500-6000 Å), UV (1300-3000 Å), soft X and hard X ray regions (0.5-8 keV; 3-80 keV).

The following extract from a page on NASA's High Energy Astrophysics Science Research Archive Center (HEASRAC) website gives a perspective on the Astrophysics website.

"Most astronomical objects in the known Universe emit radiation spanning the complete electromagnetic spectrum stretching from long wavelength radio emission to extremely short wavelength gamma rays. Hence for a detailed understanding of the physical processes that give rise to frequency-dependent, time-variable phenomena, it is essential to carrry out nearly simultaneous multi-frequency observations. Important areas requiring broad band coverage include studies of astrophysical objects ranging from the nearby solar system objects to distant stars, to objects at cosmological distances; timing studies of variables ranging from pulsations of the hot white dwarfs to active galactic nuclei (AGN) with time scales ranging from milliseconds to few hours to days."


Research Institutions Involved with the Project
Other research institutions contributing to the collaborative effort of the project include:

1. ISRO
2. Indian Institute of Astrophysics, Bangalore
3. Raman Research Institute, Bangalore
4. Inter-University Center for Astronomy and Astrophysics, Pune
5. Nuclear Research Laboratory, Bhabha Atomic Research Centre, Mumbai
6. S.N. Bose National Center for Basic Sciences, Kolkata
7. Canadian Space Agency.

Launch and Orbital Parameters
The 1,650 kg satellite with a science payload of 750 kg is scheduled to be launched in the second half of 2015 atop PSLV C-34.
It will be placed in a 650-km (400 miles) orbit with an 8° inclination for spectroscopic studies of X-ray binaries, supernova remnants, quasars, pulsars, galaxy clusters and active galactic nuclei at a number of different wavelengths simultaneously, from the ultraviolet band to energetic x-rays.

Project Progress
The satellite is planned to be launched during the second half of 2015 by PSLV C-34 to a 650 km near equatorial orbit around the Earth. It is significant to note that ASTROSAT is the first mission to be operated as a space observatory by ISRO.

ISRO in a press release on May 19, 2005 stated, "All the payloads and sub-systems are integrated to the satellite. Mechanical fit checks of the satellite with PSLV payload adaptor were performed successfully. Last week, the spacecraft was fully assembled and switched ON. Spacecraft parameters are normal, which indicates everything is functioning well. In the coming days, Spacecraft will undergo several environmental tests like Electromagnetic Interference (EMI) – Electromagnetic Compatibility (EMC), Thermal Vacuum, Vibration, Acoustic tests before shipment to Satish Dhawan Space Centre, Sriharikota.

The Hindu in July 2012 that the telescope took 11 years to develop. It has 320 mirrors of aluminium that had to be made with great precision and given a fine gold coating. These mirrors were arranged in the form of concentric shells, with struts to hold them in place. They had to be positioned with an accuracy of 20 microns, which was less than the width of a human hair.

Attitude Control System
TIFR is also developing the satellite’s attitude control system, consisting of two star trackers and gyros, to facilitate accurate pointing of the instruments towards a specific direction in the sky.
The challenges associated with developing the Attitude Control System have been overcome and delivery of the payload to the ISRO satellite center will begin from the middle of this year.
In particular, it will train its instruments at active galactic nuclei at the core of the Milky Way that is believed to have a super massive black hole.

ASTROSAT will carry five astronomy payloads for simultaneous multi-band observations:

•  Twin 40-cm Ultraviolet Imaging Telescopes (UVIT) covering Far-UV to optical bands.

•  Three units of Large Area Xenon Proportional Counters (LAXPC) covering medium energy X-rays from 3 to 80 keV with an effective area of 6000 sq.cm. at 10 keV.

•  A Soft X-ray Telescope (SXT) with conical foil mirrors and X-ray CCD detector, covering the energy range 0.3-8 keV. The effective area will be about 120 sq.cm. at 1 keV.

•  A Cadmium-Zinc-Telluride coded-mask imager (CZTI), covering hard X-rays from 10 to 150 keV, with about 10 deg field of view and 500 sq.cm. effective area.

•  A Scanning Sky Monitor (SSM) consisting of three one-dimensional position-sensitive proportional counters with coded masks. The assembly will be placed on a rotating platform to scan the available sky once every six hours in order to locate transient X-ray sources.

ASTROSAT will be a proposal-driven general purpose observatory, with main scientific focus on:

•  Simultaneous multi-wavelength monitoring of intensity variations in a broad range of cosmic sources.
•  Monitoring the X-ray sky for new transients.
•  Sky surveys in the hard X-ray and UV bands.
•  Broadband spectroscopic studies of X-ray binaries, AGN, SNRs, clusters of galaxies and stellar coronae.
•  Studies of periodic and non-periodic variability of X-ray sources.

 Open observing time on ASTROSAT will start one year after launch, for which proposals will be invited from the astronomy community. The primary data archive for ASTROSAT will  be located at the Indian Space Science Data Centre (ISSDC) near Bangalore, India.

The Ground Command and Control Centre for ASTROSAT will be located at ISTRAC, Bangalore, India. Commanding and data download will be possible during every visible pass over Bangalore. Ten out of 14 orbits per day will be visible to the ground station.

Astrosat is India's first dedicated astronomy satellite and is scheduled to launch on board the PSLV in 2015. Based on the success of the satellite-borne Indian X-ray Astronomy Experiment (IXAE), which was launched in 1996, the Indian Space Research Organization (ISRO) approved (in 2004) further development for a full-fledged astronomy satellite ASTROSAT 

An X-ray astronomy experiment for the study of spectral and temporal characteristics of cosmic X-ray sources was developed jointly by TIFR and ISRO Satellite Center (ISAC). The payload consists of three identical pointed proportional counters (PPC) and one X-ray Sky Monitor (XSM). Each of the detectors are controlled by independent microprocessor based processing electronics. A common electronics sub-system acts as an interface with the satellite bus. An oven controlled oscillator (accuracy one part in 109) provides high timing accuracy. The Indian satellite IRS-P3, carrying the X-ray astronomy instruments was launched on 1996, March 21 with Indian Polar Satellite Launch Vehicle PSLV-D3 from Shriharikota Range, India.​

A large number of leading astronomy research institutions in India and abroad are jointly building various instruments for the satellite. Important areas requiring broad band coverage include studies of astrophysical objects ranging from the nearby solar systemobjects to distant stars, to objects at cosmological distances; timing studies of variables ranging from pulsations of the hot white dwarfs to active galactic nuclei with time scales ranging from milliseconds to few hours to days.

Astrosat is currently proposed as a multi-wavelength astronomy mission on an IRS-class satellite into a near-Earth, equatorial orbitby the PSLV. The 5 instruments on-board cover the visible (320-530 nm), near UV (180-300 nm), far UV (130-180 nm), soft X-ray(0.3-8 keV and 2-10 keV) and hard X-ray (3-80 keV and 10-150 keV) regions of the electromagnetic spectrum.

Participants

The Astrosat project is a collaborative effort of a growing list of research institutions. The current participants are:

• Indian Space Research Organization
• Tata Institute of Fundamental Research, Mumbai
• Indian Institute of Astrophysics, Bangalore
• Raman Research Institute, Bangalore
• Inter-University Centre for Astronomy and Astrophysics, Pune
• Bhabha Atomic Research Centre, Mumbai
• S.N. Bose National Centre for Basic Sciences, Kolkata
• Canadian Space Agency
• University of Leicester (UoL), UK

Mission

ASTROSAT will be a proposal-driven general purpose observatory, with main scientific focus on:

• Simultaneous multi-wavelength monitoring of intensity variations in a broad range of cosmic sources
• Monitoring the X-ray sky for new transients
• Sky surveys in the hard X-ray and UV bands
• Broadband spectroscopic studies of X-ray binaries, AGN, SNRs, clusters of galaxies and stellar coronae
• Studies of periodic and non-periodic variability of X-ray sources

​

Astrosat will carry out multi-wavelength observations covering spectral bands from radio, optical, IR, UV, X-ray and Gamma ray regions both for study of specific sources of interest and in survey mode. While radio, optical, IR observations would be coordinated through ground-based telescopes, the high energy regions, i.e., UV, X-rays and Gamma rays would be covered by the dedicated satellite borne instrumentation of Astrosat.

The mission would also study near simultaneous muti-wavelength data from different variable sources. In a binary system, for example, regions near the compact object emit predominantly in X-rays, the accretion disc emitting most of its light in the UV/optical waveband, whereas the mass of the donating star is brightest in the optical band.

The observatory will also carry out:

1. Low to moderate resolution spectroscopy over wide energy band with the primary emphasis on studies of X-ray emitting objects
2. Timing studies of periodic and aperiodic phenomenon in X-ray binaries
3. Studies of pulsations in X-ray pulsars
4. QPOs, flickering, flaring, and other variations in X-ray binaries
5. Short and long term intensity variations in AGNs
6. Time lag studies in low/hard X-rays and UV/optical radiation
7. Detection and study of x-ray transients.

In particular, the mission will train its instruments at active galactic nuclei at the core of the Milky Way that is believed to have a super massive black hole.

Payloads

The scientific payload has a total mass of 750 kg and contains six instruments.

• The UltraViolet Imaging Telescope (UVIT) - The UltraViolet Imaging Telescope will perform imaging simultaneously in three channels: 130-180 nm, 180-300 nm, and 320-530 nm. The field of view is a circle of ~ 28 arcmin diameter and the angular resolution is 1.8" for the ultraviolet channels and 2.5" for the visible 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 slitless spectroscopy with a resolution of ~100.

• Soft X-ray imaging Telescope (SXT)- The soft X-ray telescope on Astrosat will employ 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 will consist of 41 concentric shells of gold-coated conical foil mirrors in an approximate Wolter-I configuration. The focal plane CCD camera will be very similar to that flown on SWIFT XRT. The CCD will be operated at a temperature of about −80 °C by thermoelectric cooling.

• The LAXPC Instrument - For X-ray timing and low-resolution spectral studies over a broad energy band (3-80 keV) Astrosat will use a cluster of 3 co-aligned identical Large Area X-ray Proportional Counters (LAXPCs), each with a multi-wire-multi-layer configuration and a Field of View of 1° × 1°. These detectors are designed to achieve (I) wide energy band of 3-80 keV, (II) high detection efficiency over the entire energy band, (III) narrow field of view to minimize source confusion, (IV) moderate energy resolution, (V) small internal background and (VI) long lifetime in space.

• Cadmium Zinc Telluride Imager (CZTI) - Astrosat will carry a hard X-ray imager in the form of CZTI. It will consist of a Pixellated Cadmium-Zinc-Telluride detector array of ~1000 cm2 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) 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. One of the major scientific achievements of CZTI would be the polarization measurements for bright galactic X-ray sources.

• Scanning Sky Monitor (SSM) - The Scanning Sky Monitor will consist of three position sensitive proportional counters, each with a one-dimensional coded mask, very similar in design to the All Sky Monitor on NASA's RXTE satellite. The gas-filled proportional counter will have resistive wires as anodes. The ratio of the output charge on either ends of the wire will provide the position of the x-ray interaction, providing an imaging plane at the detector. The coded mask, consisting of a series of slits, will cast a shadow on the detector, from which the sky brightness distribution will be derived.

• Charged Particle Monitor (CPM) - A charged particle monitor (CPM) will be included as a part of Astrosat payloads to control the operation of the LAXPC, SXT and SSM. Even though the orbital inclination of the satellite will be 8 deg or less, in about 2/3 of the orbits, the satellite will spend a considerable time (15 – 20 minutes) in the South Atlantic Anomaly (SAA) 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 ageing effect in the Proportional Counters.

Ground support

The Ground Command and Control Centre for Astrosat will be located at ISAC, Bangalore, India. Commanding and data download will be possible during every visible pass over Bangalore. Ten out of 14 orbits per day will be visible to the ground station. The satellite is capable of gathering 420 gigabits of data every day that can be down loaded in 10 to 11 orbits visible at Tracking and Data receiving center of ISRO in Bangalore. A third 11-meter antenna at the Indian Deep Space Network (IDSN) was operational in July 2009 to track Astrosat.