Table Of ContentJ. Astrophys. Astr. (2021) © Indian Academy of Sciences
https://doi.org/10.1007/s12036-021-09745-z
J.Astrophys.Astr. (2021) 42:15 (cid:2)Indian Academyof Sciences
https://doi.org/10.1007/s12036-021-09753-z
Sadhana(0123456789().,-volV)F]T(30123456789().,-volV)
Editorial
It gives us great pleasure to present this Special Issue It was wonderful to receive a very enthusiastic
on ‘‘AstroSat: Five Years in Orbit’’. The first Indian responsetothiscall,whichhasenabledustoassemble
multi-wavelength space observatory, AstroSat, has this bouquet of over sixty peer-reviewed articles
been the realisation of the dream of many scientists addressing various aspects of the mission and its sci-
and engineers. This issue is a compendium of articles ence outcome.
ontherecentscienceresultsbasedontheobservations This issue begins with an article tracing the history
made with this observatory, while also providing a of initiation of the idea of AstroSat, and a brief
glimpse of the methods adopted to operate the overview of some of the results from the first five
satellite. years of operation. This is followed by four review
AstroSat, the first dedicated astronomy mission of articles providing an overview of the unique aspects
the Indian Space Research Organisation (ISRO), was and the major science achievements of the LAXPC,
launched from the Satish Dhawan Space Centre, Sri- UVIT and SXT instruments.
harikota, India on 28 September, 2015, by the PSLV- A series of nearly twenty articles that follow pro-
C30 launch vehicle into a 650-km circular orbit with vide a broad perspective of various aspects of the
an inclination of 6 degree. It is the first satellite to project implementation and new developments that
combine both NUV and FUV capabilities along with were undertaken to operate this spacecraft as an
broad spectral coverage in X-rays. It has many more observatory, present several new calibration results
firsts in the Indian context—the first time several includingthatoftheUVgratinganddescribeavariety
major Indian scientific institutes and international of data analysis pipelines.
partners have contributed to the design, development, Wide-rangingoriginalscienceresultsarepresentedin
testing and qualification of the payloads in addition to the subsequent forty articles or so. The largest group of
conducting scientific research with the observations, papers is devoted to UV studies of galaxies in different
the first Indian satellite to have a payload mass frac- environments, their structure and star formation—in-
tion greater than 50%, the first time the PSLV was cluding the Milky Way, its satellites and the neigh-
used to launch a 1500 kg class satellite in a near- bouring Andromeda galaxy. Several papers also report
equatorial orbit, and the first Indian spacecraft oper- multi-wavelength studies of Active Galaxies, providing
ated as a proposal-driven space observatory. new constraints on their central engines. Stellar popu-
AstroSat was in performance verification mode for lationsindifferentopenandglobularclustersarestudied
the first six months in orbit, followed by one year of in a number of contributions, revealing their evolution-
observations for Guaranteed Time observers, primar- ary history. Fascinating new details of Planetary Nebu-
ily from the instrument developing teams. After this lae,andinterestingresultsfromthestudiesofindividual
the observatory time was gradually opened up for stars are also presented.
researchers from India and abroad, and with the Fast, broadband X-ray timing capability is a key
completionoffiveyearsinorbit,therehavebeenmore strength of AstroSat and several articles in this issue
than 150 refereed publications using observations employ this to study diverse aspects of stellar mass
from AstroSat. compact objects—white dwarfs, neutron stars and
On the completion of five years of AstroSat, the black holes—in binary systems. Some of the articles
Journal of Astrophysics and Astronomy circulated an also discuss the detection and detailed study of fast
invitation to contribute articles to mark the occasion. high energy transient sources with AstroSat.
15 Page 2 of 2 J. Astrophys. Astr. (2021) 42:15
The success of AstroSat has contributed to a major heartfelt gratitude to all the contributors for enthusi-
growth in the Indian Space Astronomy community. astically agreeing to write for this issue despite their
The resultingmaturityand confidence hasgenerated a other commitments. We are also grateful to all the
slew of ideas for future Indian astronomy missions. team members of ISRO, and the various institutions,
This volume concludes with a discussion of the who have made AstroSat what it is today. Special
exciting prospects of future growth in this area. thanks are due to the contributors for the cover page,
Finally, words are insufficient to acknowledge the and to Ms. Shylaja, Ms. Cicilia and Ms Srimathi and
help we have received from a large number of con- the entire editorial team at the Indian Academy of
tributors, without whom this issue would not have Sciences for their invaluable help in bringing out this
beenpossible.WeareextremelythankfultoChairman issue.
ISRO, Dr. K. Sivan, for providing the Foreword. We We believe this is only the beginning. We look
also express our sincere thanks to the Chief Editor of forward to many more interesting results from
Journal of Astrophysics and Astronomy, Prof. Anna- AstroSat in the years to come.
purniSubramaniam,whohasbeeninstrumentalin not
S. Seetha
only guiding us through, but also contributing signif-
D. Bhattacharya
icantly to the editorial activities of this issue. We
Guest Editors
would also like to take this opportunity to express our
J.Astrophys.Astr. (2021) 42:19 (cid:2)Indian Academyof Sciences
https://doi.org/10.1007/s12036-021-09692-9
Sadhana(0123456789().,-volV)FT3](0123647589().,-volV)
REVIEW
AstroSat: Concept to achievements
S. SEETHA1,* and K. KASTURIRANGAN2
1Raman Research Institute, C.V. Raman Avenue, Bengaluru 560 080, India.
2ISRO Headquarters, Antariksh Bhavan, New BEL Road, Bengaluru 560 231, India.
*Corresponding Author. E-mail: [email protected]
MS received 16 November 2020; accepted 1 January 2021
Abstract. AstroSat has completed 5 years of successful in-orbit operation on 28 September 2020. AstroSat
is ISRO’s first Indian multi-wavelength satellite operating as a space Observatory. It is the only satellite
which can simultaneously observe in the Far UV and a wide X-ray band from 0.3 to 80 keV using different
instruments. This astronomy mission was conceived, following the success of several piggy back astronomy
experiments flown earlier on Indian satellites. AstroSat is the result of collaboration between ISRO and
several astronomy institutions within India and abroad. There are over 150 refereed publications resulting
from data from AstroSat, in addition to Astronomy Telegrams, Circulars and Conference proceedings. This
paper provides a brief summary of the evolution of the concept of AstroSat, how it was realized and scientific
outcome from this mission.
Keywords. AstroSat—space mission—accretion—X-ray binaries—stars—stellar clusters—galaxies.
1. Introduction been intertwined with this program, starting with
sounding rockets for upper atmospheric studies and
AstroSat is ISRO’s first Indian multi-wavelength X-ray astronomy, moving on to Indian satellites for
astronomysatellitebeingoperatedasaspaceobserva- X-ray and Gamma ray piggyback experiments,
tory. It has completed the design life of five years in finally leading to dedicated missions. In parallel,
orbit on 28 September 2020 and holds promise for Tata Institute of Fundamental Research (TIFR)
further successful operations leading to significant Mumbai has developed a balloon facility at Hyder-
science for at least 5 more years, if not more. It is a abad dedicated for conducting science experiments
satellitehavingmultipleinstrumentscoveringboththe with balloons, which can now be launched to heights
UV and the X-ray wavebands. This paper provides a of 42 km.
brief summary of the environs under which AstroSat The developments of satellites and launch vehicles
was conceptualized, the mode in which it was devel- have largely been driven by the national priority of
oped, tested, and is being operated. This paper also space applications for development. However the
givesaglimpseofsomeofthesignificantscienceresults recognition that these could in turn provide opportu-
fromAstroSat. nities fornew spacescience experimentswas inherent
in the vision of the leaders of the space program. This
unique approach of developing scientific capabilities
1.1 Historical perspective without making science as the prime reason for
resourceinvestment,gavetheIndianSpaceprogram a
India’s space program has completed over 50 years distinct character.
since its inception. Space Science experiments have The scientific heritage and the instrumentation
capabilities in the country ensured that the space sci-
This article is part of the Special Issue on ‘‘AstroSat: Five ence program matured into an indispensable compo-
Years in Orbit’’. nent of the Indian Space Program.
19 Page 2 of 19 J. Astrophys. Astr. (2021) 42:19
1.2 Heritage signal to noise ratio (Sinha et al. 2001). Gamma ray
burstsbeingtransientsourceswithlittlepriorknowledge
Starting from the 1940s, the foundation for ‘space ofthe direction, requiredlarge field of view instruments
science’ began with the experiments to study cosmic to detect the bursts wherever they occurred in the sky.
rays. This led to development of Balloon fabrication Some of the bursts detected by this experiment were
and launch facility by TIFR at Hyderabad that has the usefulfortriangulationwithotherinternationalsatellites
capability of carrying a payload of up to several for localization of the sources.
hundred kg to an altitude of 30 to 40 km. This enabled Although it turned out that this effort was super-
Indian scientists of TIFR and Physical Research sededbythelaunchofthelargeComptonGammaRay
Laboratory (PRL) to develop instruments for studies Observatory (CGRO) by NASA, it must be said our
of hard X-rays ([20 keV) from bright X-ray experiment was an unqualified success.
sources. A large number of Balloon experiments Encouraged by the success of the GRB instrument,
for X-ray astronomy studies were conducted from the TIFR and ISAC groups submitted a joint proposal
Hyderabad during 1967-1980 resulting in several new toISROforanX-rayAstronomyexperimentweighing
and interesting results. In parallel the PRL group about 50 kg for launch on an Indian satellite. It was
initially and later the TIFR group developed aimed at studies of timing and spectral characteristics
instruments for the rocket borne X-ray experiments of X-ray binaries. This also required inertial pointing
launched from ISRO’s launch facilities at Thumba and of the satellite to specific sources in the sky.
Shriharikota in late sixties and up to early eighties for Thisdemandedprovisionforhighermassandpower
studies of cosmic sources in *1–10 keV energy on the satellite. The development of the Polar Satellite
range. These provided invaluable training and expe- Launch Vehicle (PSLV) by ISRO primarily for remote
rience to the Indian scientists for the design and fab- sensing satellites, enabled the launch of satellites with
rication of the satellite-borne instruments. mass of about 1000 kg into a polar sun synchronous
Meanwhile, scientists at the PRL, Ahmedabad also orbit. When the PSLV became operational, ISRO
conducted research on similar areas of study with agreed to accommodate the proposed Indian X-ray
emphasis on the solar activity influence on cosmic Astronomy Experiment (IXAE) to be flown on the
rays. Further X-ray astronomy experiments from PRL Remote sensing satellite IRS-P3. Four argon-filled
were flown on sounding rockets. Both the teams at proportionalcounters(area*1600sqcm)andthefront
TIFR and PRL, also placed a lot of stress on develop- endelectronicswasdesignedandprovidedbytheTIFR
ment of instrumentation for these experiments. It is group and the ISAC group designed and supplied the
worth pointing out that both the rocket facility at signal processing electronics (Agrawal et al. 1997).
Thumba and the balloon facility at Hyderabad had Developed in a record time of 18 months, this instru-
specific locational advantage for conducting X-ray ast- ment named as Indian X-ray Astronomy Experiment
ronomy studies owing to the low background resulting (IXAE), was launched on March 21, 1996 in a polar
from the low latitude cut off of the cosmic ray intensity orbit by the PSLV. Using a star sensor the IXAE
and therefore the reduction of secondary background. detectors were pointed towards the sky for about 2
The ground was therefore well prepared when the monthseveryyeartostudyspecificsources.TheIXAE
opportunity arose to fly instruments on board the first performed well for about 5 years and studied about 20
Indian satellite ‘Aryabhata’, and both these groups at X-ray sources. This experiment produced many new
PRL and TIFR contributed to two of the astronomy results especially on a new transient black hole binary
instruments on this satellite. GRS 1915?105 (Paul et al. 1998). The success of the
As the newly created Indian Space Research IXAE was a major milestone in the evolution of the
Organisation (ISRO) gathered momentum, indigenous idea of AstroSat as it gave confidence that Indian sci-
experimental rockets and satellites provided opportu- entists can make the gas filled detectors that can work
nities for several ‘‘piggy back experiments’’. in space (Agrawal 2017). One of the scientists associ-
ated with the IXAE proposed to ISRO a dedicated
The first of these was the Gamma Ray Burst (GRB)
Indian X-ray astronomy satellite with a more complex
experiment developed at ISRO Satellite Centre (ISAC,
and heavier X-ray experiment which will lead to
now U.R. Rao Satellite Centre (URSC)) flown on the
internationally competitive science. ISRO suggested
SROSS series of satellites starting in 1987, with a
expanding the scope by convening a meeting of Indian
capability of carrying a payload of 5–10 kg. The GRB
astronomers where this and other proposals were fur-
experiment onboard the SROSS-C2 satellite (Kasturi-
ther deliberated, for a dedicated astronomy mission.
rangan et al. 1997) recorded over 50 GRBs, of high
J. Astrophys. Astr. (2021) 42:19 Page 3 of 19 19
2. AstroSat Often there are logistic problems in making simul-
taneous and coordinated studies of a specific object
This is the golden age for multi-wavelength astron- from different satellites and ground based tele-
omy. Ground-based optical telescopes have extended scopes. The most efficient and effective way to
to tens of meters in size, radio astronomy telescopes pursue multiwavelength studies is to have a dedi-
extending their capabilities to long wavelengths, huge cated satellite mission which will carry several
arrays and improving the spatial resolution with instruments covering the desired spectral bands so
interferometry, new facilities like mm wave astron- that simultaneous observations in all the desired
omy making a mark and a large number of space wavebands can be made from the same satellite.
astronomy satellites covering almost the entire There are however, different observational con-
wavelength range from optical to high energy gamma straints for instruments operating in different wave-
rays. These have been made possible by the aspira- bands and therefore it was also decided that the
tions of the scientists world over to continuously scientific objectives could include some specific aims
improve on the capabilities and broaden and sharpen which could also be realized using individual instru-
the knowledge base already achieved. ments/wavebands.
The success of the GRB and the IXAE propelled a
series of meetings of the Indian astronomical com-
2.1 Objectives
munity to explore the possibility of a major Indian
astronomical observatory in space. By mid 1990s,
The AstroSat was therefore proposed as a multi-
NASA’s Hubble Space Telescope and CGRO were
wavelength astronomy mission with wide spectral
already operational in orbit and the Chandra X-ray
coverage extending over Near and Far Ultraviolet
missionandESA’sXMM-Newtonwerewellintofinal
(NUV, FUV), soft and hard X-ray bands (Agrawal
stagesofdevelopment.Evenmoreambitiousmissions
2006; Rao et al. 2009).
were being funded or under planning world over.
It has provided an opportunity to astronomers to
Given this international scenario, and the proven
carry out cutting edge research in the frontier areas of
capabilities and resources within India, the critical
X-rayastronomyandUltravioletastronomyandallow
question was to ask ‘‘What should be the nature and
them to address some of the outstanding problems in
scope of the Indian mission?’’
high energy astrophysics.
Traditionally, the quest for higher sensitivity and
The scientific objectives of AstroSat are the
higher angular resolution has been the motivating
following:
factor for newer missions. It became clear from the
discussionsthat this is not the only goal we could aim (1) Understand high-energy emission processes in
for. Indian astronomers worked towards defining a various astrophysical systems: Multiwavelength
mission that could, while supplementing other mis- studies of various cosmic sources over a wide
sions, have its own niche objectives which would spectral band extending over visible, UV and
encompass the research interests of the scientific X-ray bands.
community and technical capabilities of the engi- (2) Correlated time variations of intensity in UV, soft
neering teams within the country. and hard X-ray bands to investigate the origin and
The final consensus was a multiwavelength mechanism of the emission of radiation in different
astronomical observatory, based on the following wave bands.
reasoning. To understand the nature of cosmic (3) Understand variability timescale in various astro-
sources, their radiation processes and environment, physical systems through broad-band and long-
it is necessary to measure their emission over a duration observations: studies of periodic (pulsa-
wide range of the electromagnetic spectrum. Since tions, binary light curves, QPOs etc.) and aperiodic
intensity of several classes of cosmic sources varies (flaring activity, bursts, flickering and other chaotic
with time, it is necessary to make simultaneous variations) variability.
observations in different wavebands. Most of the (4) Search for black hole sources by limited surveys in
space observatories are dedicated to a particular galactic plane; detection and detailed studies of
waveband, e.g. X-ray, UV etc. Consequently, mul- stellar-mass black holes.
tiwavelength studies usually have to be made from (5) Study of non-thermal emission in supernova
coordinated observations with different satellites. remnants and galaxy clusters.
19 Page 4 of 19 J. Astrophys. Astr. (2021) 42:19
(6) Study magnetic fields in strongly magnetized models of the main five payloads and in the center is
systems: Measuring magnetic fields of neutron the picture of assembled satellite on ground.
stars by detection and studies of cyclotron lines in The realization of the above instruments was made
the X-ray spectra of X-ray pulsars. possible with the involvement of many institutions in
(7) Detection of X-ray transients and long duration addition to various Centres of Indian Space Research
temporal studies. Organisation (ISRO). They are Tata Institute of Fun-
(8) High-resolution UV studies of stars, emission damental Research (TIFR), Mumbai, Indian Institute
nebulae and galaxies: study of UV emission from of Astrophysics, (IIA) Bengaluru, Inter-University
hot stars (WD, CV, WR, LBV, b-Cephei, etc.) in Centre for Astronomy and Astrophysics, (IUCAA)
galaxies, morphological studies of nebulae and Pune, Raman Research Institute, (RRI) Bengaluru,
supernova remnants and nearby galaxies. Physical Research Laboratory, (PRL) Ahmedabad
(9) Limited sky survey in UV: multi-band limited sky along with a collaboration with Canadian Space
survey in ultra-violet in 130–300 nm band. Agency for the detectors and electronics of UVIT and
with University of Leicester, UK for the detectors of
To accomplish the above scientific objectives, the
SXT.
instrument configuration was flown as a combination
There were several challenges in realizing the
of four types of X-ray detectors and twin telescopes
payloads and the satellite, a few of them being:
covering visible, NUV and FUV bands (Singh et al.
2014; Agrawal 2017). The instrumentation flown had • Achieving an overall angular angular resolution
the following features: of\1.8 arc-sec for the UVIT telescope with a
field-of-view of 28 arcmin, with indigenous
• Large Area X-ray Proportional Counters
development of UV mirrors for the first time.
(LAXPC) (3 nos) to cover the energy range
• Development and testing of indigenous gold-
3–80 keV, and to have an effective area of
coated foil optics for soft X-ray telescope, and
*6000 cm2 at 15 keV and a time resolution of
ensure an aligned assembly of these mirrors.
10 microsecond (Agrawal et al. 2017; Yadav
• Development of large high pressure gas filled
et al. 2017).
counters with effective area *6000 cm2 with
• Twin Ritchey Chretian UV Imaging Telescope
energy measurement up to 80 keV, and a
(UVIT) covering the wavelength band of
temporalresolutionof10microsecond.Amajor
130–180 nm and 200–300 nm (using several
challenge in the LAXPC instrument was also to
filters) and having an angular resolution better
developalightweightmulti-layercollimatoryet
than 1.8 arcsec (Tandon et al. 2017).
opaque to X-rays up to 80 keV.
• Soft X-ray Telescope (SXT) covering the
• Qualification of position sensitive gas-filled
energy range of 0.3–8 keV with an angular
counters and rotation mechanism for SSM.
resolution of 3–4 arcmin (Singh et al. 2017).
• Qualifying of commercial Cadmium Zinc Tel-
• Pixellated Cadmium Zinc Telluride Imager
luride detectors for astronomy purposes, and
(CZTI) operating in the energy range 20–100
demonstrating the polarisation capabilities of
keV with an area of 1000 sq cm, with a
CZTI.
collimator and coded mask. Above 100 keV, it
• Contamination control of UVIT optics.
operates as a detector without collimation
• Development of doors for the UVIT and SXT,
(Bhalerao et al. 2017a, b).
and deployment mechanism.
• Scanning Sky Monitor (SSM) operating in the
• Capability for large onboard data storage with
X-ray range of 2.5–10 keV, with a one-dimen-
read/write capability.
sional coded mask (Ramadevi et al. 2017).
• Inter-payload alignment and measurement on
• A Charge Particle Monitor (CPM) capable of
ground and measurement after launch using
detecting protons[1 MeV (Rao et al. 2017).
celestial sources.
Design and fabrication of three of the X-ray • S/C maneuvering with avoidance of the Sun
astronomy instruments namely LAXPC, SXT and along two axes, and S/C pointing and stability.
CZTI and the CPM were the responsibility of TIFR, • Enabling data storage in photon counting mode
and the responsibility for UVIT was with the Indian for the payloads.
Institute of Astrophysics, (IIA) and the SSM instru- • Backgroundsimulations,estimateandmodeling
ment by ISAC. Figure 1 is a collage of the flight for the various detectors.
J. Astrophys. Astr. (2021) 42:19 Page 5 of 19 19
Figure 1. The fivemain payloads ofAstroSat, andthe assembled satelliteinthe centre, beforelaunch. Image Courtesy:
Payload and project teams and S. Megala.
It may be noted here that the Neil Gehrels Swift the first time the payload mass was [50% of the
Observatory and XMM-Newton do have capability satellite mass on an Indian satellite launched by
to observe in NUV bands in addition to X- PSLV. The inclination of the orbit was chosen to
rayband. HowevertheydonothaveFUVcapab- avoid the satellite traversing through the inner por-
ilities. On the other hand, the Hubble Space tionsoftheSouthAtlanticAnomalyregionwithhigh
Telescope has extremely good resolution in Far UV, count rates of charged particles. The altitude was
but over a narrow field-of-view. The advantage of chosen to minimise the effect of atomic oxygen on
UVIT is it can observe faint objects with ~1.5 the UVIT optics. This was also the first time PSLV
arcsec resolution with an FoV of nearly 0.5 degree, launched a satellite in a low Earth orbit at a near
ideal for imaging galaxies. UVIT also has about 3 equatorial inclination.
times better angular resolution than the earlier Four of the payloads namely the LAXPC, SXT,
GALEX mission. UVIT and CZTI have their view direction aligned to
LAXPC with its large area of 3 counters enabled an the ?ve roll axis of the satellite (see Fig. 2). These
effective area of LAXPC is ~6000 sq. cm at 15 keV payloads therefore are pointed towards the same
(Antia et al. 2017). Above 30 keV, the effective area sourceforobservation.Theskymonitorispointedina
of the 3 LAXPCs together was four to five times perpendicular direction and is rotated in a step and
greater than that of the Proportional Counter Array stare mode to scan as much of the sky in a rotation.
(PCA) on Rossi X-ray timing Explorer (RXTE). For further details of pointing and maintenance of
orbit, see Seetha and Megala (2017) and other papers
in this volume.
2.2 AstroSat mission management The satellite was operated for the first six months
for performance verification of the payloads, and the
AstroSat with a mass of 1513 kg and a payload mass next six months was guaranteed time (GT) for the
of 855 kg (Navalgund et al. 2017) was launched on instrument teams (Pandiyan et al. 2017). In ensuing
28 September 2015 by the Polar Satellite Launch years, the GT gradually reduced and the observing
Vehicle, PSLV-XL C30, into an orbit with inclina- timewasgraduallymadeopentobothIndianandlater
tion of 6 degree and an altitude of 650 km. This was international astronomers too.
19 Page 6 of 19 J. Astrophys. Astr. (2021) 42:19
Figure 2. A drawing showing the post launch configuration of the payloads, with satellite axes reference. Image
Courtesy: Project team.
2.3 AstroSat as an Observatory bright object, and sensitivity in the various instru-
ments etc. Based on the approvals, the mission and
AstroSat is operated as the first Indian Space Obser- operations team at U.R. Rao Satellite Centre, and ISRO
vatory. Observations are proposal driven. The Astro- Telemetry, Tracking and Command Network
Sat Proposal Processing System (APPS) is a web (ISTRAC) create the necessary list of targets to be
based software facility, developed through IUCAA, observed, and generate command files, taking into
Pune, and hosted and administered by ISRO Space consideration Sun avoidance, RAM angle constraints,
Science Data Centre (ISSDC), Bangalore. All pro- data readout capability and thermal constraints. The
posal related activities like proposal submission, operations of the payload and spacecraft are com-
review, approval etc. are done through APPS. The call manded at ISTRAC. Data is received and data prod-
for proposals for observations are made through an ucts are again generated at ISSDC, and verified
Announcement of Opportunity (AO) typically issued through Payload Operation Centres (POCs) and fur-
in March of each year for an year’s observation cycle ther made available for dissemination to the respective
starting from October. Announcement for various proposal Principal Investigators (PI). The data of
observation cycles are made by ISRO HQ in co- observation under AO proposals has a lock-in period
ordination with ISSDC and the Science Working of twelve months with proposal PIs, after which it is
Group (SWG). Proposals can be submitted by any made available to the public as archival data. The
scientist capable of utilising the data, with 55% of the TOO data do not have any lock-in period and is
available time being allotted for proposals from India. released to public, along with proposer. The obser-
In addition to proposals for AO, proposals can be vations of TOOs have to be scheduled by sliding/
submitted anytime for Targets of Opportunity (TOO), shifting the pre-planned observations. Both propri-
(in case of any sudden outburst or state change to be etary data (during lock-in) and archival data are hos-
observed in specific sources) and Calibration propos- ted on https://astrobrowse.issdc.gov.in maintained by
als (Cal). Submitted proposals are reviewed and ISSDC at ISTRAC. Anybody can download data
approved through the APPS utility, by the AstroSat through this website after registering in APPS.
Time Allocation Committee (ATAC) for AO propos- The AstroSat calibration database (CALDB) and
als, TOO committee for TOO proposals and Calibra- other tools/software required for analysing the Astro-
tion Scientist for the Cal. Proposals. These committees Sat data are hosted/linked at the AstroSat support cell
are supported by recommendations by the AstroSat (ASSC, http://astrosat-ssc.iucaa.in/) hosted by IUCAA
Technical Committee (ATC), which considers pay- Pune. ASSC team also provides assistance to pro-
load operation constraints like sun angle, avoidance of posers and conducts workshops for training users for
