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CLASS on Chandrayaan-2 aims to map the abundance of elements on the lunar surface using XRF during solar flare. The instrument uses large area SCDs and is designed to provide better spatial resolution and sensitivity than the C1XS experiment on Chandrayaan-1.
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The Chandrayaan-2 Large Area Soft X-ray Spectrom-
eter (CLASS)
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How to cite:
Radhakrishna , V.; Narendranath, S. ; Tyagi, A. ; Bug, M. ; Unnikrishnan, U. ; Kulkarni, R. ; Sreekantha, C.
V. ; Kumar, ; Balaji, G. ; Athiray, P.S. ; Sudhakar, M. ; Manoj, R. ; Chetty, S. V. ; Thyagaraj, M. R. ; Howe,
C. ; Gow, J. and Sreekumar, P. (2011). The Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS).
In: 42nd Lunar and Planetary Science Conference, 7-11 March 2011, Houston, TX, USA.
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, S.
, A. Tyagi
, M. Bug
, U. Unnikrishnan
, R. Kulkarni
C.V. Sreekantha
, Kumar
, G. Balaji
, P.S.
M. Sudhakar
, R. Manoj
, S. V. Chetty
, M. R. Thyagaraj
, C. Howe
, J. Gow
, P. Sreekumar
, (email:
ISRO Satellite Centre,
University of Calicut,
Rutherford Appleton Laboratory, UK,
University, UK.
Introduction: The CLASS experiment on Chan-
drayaan-2, the second Indian lunar mission, aims to
map the abundance of the major rock forming elements
on the lunar surface using the technique of X-ray fluo-
rescence during solar flare events. CLASS is a continu-
ation of the successful C1XS [1] XRF experiment on
Chandrayaan-1. CLASS is designed to provide lunar
mapping of elemental abundances with a nominal spa-
tial resolution of 25 km (FWHM) from a 200 km polar,
circular orbit of Chandrayaan-2.
C1XS was developed at Rutherford Appleton La-
boratory, UK, in collaboration with ISRO and operated
for ~ 9 months in orbit. Although the instrument per-
formed very well [2], the lack of adequate solar activity
and a reduced mission life, prevented global coverage.
However, there are several observations that have
yielded good XRF data from C1XS.
Science objectives: The science objectives of
CLASS are to make global studies on the diversity and
distribution of lunar lithologies, quantitative estimate
of Mg abundance, essential for determining the distri-
bution of Mg suite rocks, bulk composition of the
crust, abundance patterns in the major crustal provinces
and mare basalt diversity. In addition to this regional
studies on the composition of central peaks of craters,
composition of large scale ejecta around basins, and to
study dark halo craters and possible associated crypto-
Results from C1XS indicate that the lunar highland
composition may contain more sodic plagioclase than
previously thought [3]. CLASS is hence designed to
operate down to 0.8 keV in order to also measure the
possible XRF signals from Na in addition to the majpr
elements Mg, Al and Si. Measurement of Ca, Ti and Fe
at the proposed spatial resolution can be achieved only
during strong flares; CLASS is expected to provide the
first distribution maps from direct chemical abundance
CLASS has a geometric area three times that of
C1XS which would enable it to measure lunar XRF
signals down to B flares promising better coverage
even if solar activity remains low.
CLASS Instrument: Our experience with C1XS
has proven swept charge devices as good choice for
experiments which demand large area (without imag-
ing) and require good spectral resolution even at tem-
peratures of -20 C (hence passive cooling is adequate).
SCDs of 1cm
area, CCD-54, manufactured by e2V
technologies, were used in C1XS. In CLASS, we pro-
pose to use large area SCDs (~20 x 20 mm
), sixteen of
which providing a total geometric area of 64 mm
From an orbital height of 200 kms, the collimators
placed over the detectors define a field of view of 14 x
14ÛNP):+0DWthe lunar surface. The collima-
tors over approximately one-fourth of the array are
designed to have a smaller field of view which would
yield a spatial resolution of 12 km on the lunar surface.
This would enable high resolution mapping during
strong solar flares. The CLASS payload is designed to
be operated in the range of -15 C to -25 C in the lunar
orbit using only passive cooling. A door is provided in
front of the detectors for protection of the SCDs from
high energy particles during transit through the radia-
tion belts. Thin aluminium foils were used as visible
light shields on C1XS. To prevent any contribution
from Particle Induced X-ray Emission (PIXE) from the
Al foil, we plan to use Be foils in CLASS. The sche-
matic of the CLASS payload is shown in Fig.1.
Figure 1. CLASS instrument showing the four qua-
drants with four SCDs each. The electronics is housed
in the box behind the detector units. An aluminum door
protects the detectors from radiation damage en-route
to the Moon. Passive radiators connected to heat pipes
provide the required low-temperature environment for
the detectors.
Large area SCDs, with larger pixel size and two
phase clocking are expected to give good spectral reso-
lution (< 200 eV @ 6 keV) and low energy threshold
(~ 0.8 keV). This has already been demostrated in the
laboratory with a CCD-236 device manufactured by
e2V Technologies. A comparison of spectra recorded
1708.pdf42nd Lunar and Planetary Science Conference (2011)
for CCD-54 used in C1XS and that from a large area
SCD, is shown in Fig.3. It is seen that the large number
of partially absorbed (split) events recorded for CCD-
54, is significantly reduced in the large area SCD. The
reduced fraction of split events in the large area SCD is
due to its larger pixel size. This is significant and one
can expect improved resolution and higher sensitivity
Figure 2. CLASS electronics block diagram
Electronics: The electronics is packed behind the
detector unit into four PCB cards. The large area SCD,
is a single output, diagonally-read x-ray CCD with 2
phase clocks and is non-imaging. Charge generated by
an event is continuously clocked (~ 100 kHz) and
transported along the buried channels towards a node
to generate a voltage pulse. The output from an SCD is
similar to a conventional CCD and a correlated double
sampling (CDS) circuit is used for noise reduction be-
fore feeding the signal into ADC. The signal generation
and data processing is done using Actel’s RTAX-
1000S FPGA based system. The temperature interface
provides the continuous monitor of the SCD tempera-
ture for operation in favorable temperature range using
onboard logic. While spacecraft telecommand interface
provides different commands for ON / OFF and mode
of operation Telemetry interface is used to monitor
critical parameters of the payload. Base band data han-
dling interface is used to transfer the science data from
the spacecraft Solid State Recorder.
Performance: The large area SCDs on CLASS are
an improved version of that on C1XS. They operate
with 2 phase clocking and have larger pixels thus re-
ducing split events. Figure 3 shows spectra obtained
with C1XS SCDs compared to that with CLASS SCDs.
The fraction of events outside the photopeak is consi-
derably reduced and detection efficiency at the photo-
peak increased. Photopeak fraction for the recored
spectra increased by a factor of two for the large area
SCD compared to CCD-54.
Figure 3. Large area SCD(CCD-236) spectrum along
with C1XS SCD (CCD-54) spectrum showing lesser
contribution from split events to the photopeak.
Conclusions and future work: CLASS is cur-
rently under development at the Space Astronomy
Group, ISRO Satellite Centre, India. It is expected to
provide the first moderate resolution (25 km or better)
global map of lunar chemistry.
[1] Grande, M. et al. (2007) PSS, 55, 494-502. [2]
Howe et al (2009) PSS, 753. [3] Narendranath et al
(2010) submitted to Icarus. [4] Tyagi, A. et al (2010)
Nat. Symp. Nucl.Instrum Proceedings, 593-595.
Split events
circuit for
clock and
Power Interface Module
DC-DC power supply
clock driver
Analog front end
and CDS voltages
CDS based ADC
Telemetry Telecommand Interface
Baseband Data
Handling Interface
FPGA system
Clock and
generation for
SCD 16
Temperature sensor
interface with ADC
Analog read out
1708.pdf42nd Lunar and Planetary Science Conference (2011)

Supplementary resource (1)

... The CLASS instrument aboard Chandrayaan-2 [1] uses 16 CCD236 swept charge devices (SCDs) to monitor X-ray fluorescence of the surface of the Moon to map its elemental composition [2]. The Centre for Electronic Imaging (CEI) completed a set of ground irradiations of the CCD236 in order to demonstrate its suitability for the instrument [3][4][5][6], and identified a linear trend of fluorescence peak FWHM for proton irradiation fluence [7]. ...
Full-text available
India’s Chandrayaan-2 Large Area Soft X-ray Spectrometer (CLASS) employs 16 CCD236 Swept Charge Devices (SCDs) similar in structure to Charge Coupled Device (CCD) image sensors. The CCD236 permits X-ray detection over a large surface area, intended to improve low flux performance, with simplified control interfaces and improved warm temperature performance. These devices were the subject of ground testing and performance evaluation before flight. Data that was recently made available by the Indian Space Research Organisation (ISRO) has permitted the analysis of the performance of the CLASS SCDs after over a year of operations around the Moon. Of particular interest is the change in device performance and behaviour during transit and in lunar orbit. Preliminary analysis has indicated that device FWHM, representing the aggregate of different noise sources, has increased in line with predictions based on ground irradiation and testing.
... Due to limited mission life and low solar activity, CIXS could not achieve the goal of global mapping of elements abundance of the Moon. CLASS by detecting X-ray emission in the 0.8--15 keV energy range and spatial resolution of 12.5 km, examines the presence of major elements (Mg, Al, Si, Ca, Ti, Fe Na) on the Moon (Radhakrishna et al. 2020). Solar X-ray Monitor (XSM) is a companion payload to support CLASS. ...
Chandrayaan-2 Mission, various sensors and their objectives
... Therefore, X-ray CCDs have a good reputation in modern astronomical observation and high-energy particle detection due to their excellent spatial resolution and sufficient energy resolution [1]. They have been successfully utilized in recent astronomical satellites such as the missions of ASTRO-H from Japan [2], CLASS from India [3], and HXMT from China [4]. ...
A fully integrated 100 kHz X-ray charge coupled device (CCD) readout application specific integrated circuit (ASIC) employing delta sigma ($Delta Sigma $) digitization is presented. To achieve high linearity with small chip size and low power consumption, the correlated double sampling (CDS) is realized by the $Sigma Delta{rm ADC}$ instead of the analog front end (AFE) as in conventional CCD readout circuits. Besides, the proposed decimation filter features simple structure and eases the integration. The chip is fabricated in $0.35 upmuhbox{m}$ CMOS technology and the measured integral nonlinearity (INL) throughout the input dynamic range of ASIC is 0.055% with $35.1 pm 0.3 upmuhbox{V}$ input referred noise. A CCD detection system is built and tested with the sensitivity of CCD being $4 upmuhbox{V}/hbox{e}^-$. The integration test results show that the readout noise is $11.8 hbox{e}^ {-} $ at 100 kHz readout pixel rate and the achieved energy spectrum resolution is $168 hbox{eV} pm 4.7 hbox{eV}$ (Full Width at Half Maximum: FWHM) at 5.9 keV.
... The first generation of the SCD, the CCD54 was used in the European Space Agency's Demonstration of a Compact X-ray Imaging Spectrometer (DCIX) instrument [4] in 2003 and India's Chandrayaan-1 X-ray Spectrometer (C1XS) instrument in 2008 [5]. The second generation of device, the CCD236 will be used in India's Chandrayaan-2 Large Soft X-ray Spectrometer (CLASS) instrument [6] and China's Hard X-ray Modulation Telescope (HXMT) [7]. ...
Full-text available
The e2v CCD236 is a swept charge device (SCD) designed as a soft X-ray detector for spectroscopy in the range 0.8 keV to 10 keV [1]. It benefits from improvements in design over the previous generation of SCD (the e2v CCD54) [2] to allow for increased detector area, a reduction in split X-ray events and improvements to radiation hardness [3]. To enable the suppression of surface dark current the device is clocked continuously, therefore there is no positional information making responsivity variations difficult to measure. This paper describes investigated techniques to achieve a responsivity map across the device using masking and XRF, and spot illumination from an organic light-emitting diode (OLED). The results of this technique should allow a deeper understanding of the device sensitivity and allow better data interpretation in SCD applications.
Solar X-ray Monitor (XSM) instrument of India’s Chandrayaan-2 lunar mission carries out broadband spectroscopy of the Sun in soft X-rays. XSM, with its unique features such as low background, high time cadence, and high spectral resolution, provides the opportunity to characterize transient and quiescent X-ray emission from the Sun even during low activity periods. It records the X-ray spectrum at one-second cadence, and the data recorded on-board are downloaded at regular intervals along with that of other payloads. During ground pre-processing, the XSM data is segregated, and the level-0 data is made available for higher levels of processing at the Payload Operations Center (POC). XSM Data Analysis Software (XSMDAS) is developed to carry out the processing of the level-0 data to higher levels and to generate calibrated light curves and spectra for user-defined binning parameters such that it is suitable for further scientific analysis. A front-end for the XSMDAS named XSM Quick Look Display (XSMQLD) is also developed to facilitate a first look at the data without applying calibration. XSM Data Management-Monitoring System (XSMDMS) is designed to carry out automated data processing at the POC and to maintain an SQLite database with relevant information on the data sets and an internal web application for monitoring data quality and instrument health. All XSM raw and calibrated data products are in FITS format, organized into day-wise files, and the data archive follows Planetary Data System-4 (PDS4) standards. The XSM data is made available along with the XSM Data Analysis Software from ISRO Science Data Archive (ISDA) hosted at Indian Space Science Data Center (ISSDC). Here we discuss the design and implementation of all components of the software for the XSM data processing and the contents of the XSM data archive.
Full-text available
The Chandrayaan-2 Large Area Soft X-ray spectrometer (CLASS) is due to be launched by the Indian Space Research Organisation (ISRO) in 2014. It will map the elemental composition of the lunar surface, building on the Chandrayaan-1 X-ray spectrometer (C1XS) heritage. CLASS will use an array of e2v technologies CCD236 swept charge devices (SCD) providing an active detector area of approximately 64 cm2, almost three times the active area of C1XS which used the first generation of SCD, the CCD54. The CCD236 is designed as a soft X-ray detector, 0.8 keV to 10 keV, and benefits from improvements in design to allow for increased detector area, a reduction in split X-ray events and improvements to radiation hardness. This paper describes the investigation into the performance requirements of the CCD236, focussing on an optimisation of the energy resolution of a device irradiated to the estimated worse case end of life proton fluence.
The remote X-ray fluorescence spectroscopy is a powerful technique to investigate the elemental abundances in the atmosphere-less planetary bodies. The experiment involves measuring spectra of fluorescent X-rays from lunar surface using a low energy X-ray detector onboard an orbiting satellite. Since the flux of fluorescent X-ray lines critically depend on the flux and spectrum of the incident solar Xrays, it is essential to have simultaneous and accurate measurement of X-ray from both Moon and Sun. In the context of Moon, this technique has been employed since early days of space exploration to determine elemental composition of lunar surface. However, so far it has not been possible to exploit it to its full potential due to various reasons. Therefore it is planned to continue the remote Xray fluorescence spectroscopy experiment on-board Chandrayaan-2 which includes both lunar X-ray observations and solar X-ray observations as two separate payloads. The lunar X-ray observations will be carried out by Chandra Large Area Soft x-ray Spectrometer (CLASS) experiment; whereas the solar X-ray observations will be carried out by a separate payload, Solar X-ray Monitor (XSM). Here we present the overall design of the XSM instrument, the present development status as well as preliminary results of the laboratory model testing. XSM instrument will have two packages namely - XSM sensor package and XSM electronics package. XSM will accurately measure spectrum of Solar X-rays in the energy range of 1-15 keV with energy resolution similar to 200 eV @ 5.9 keV. This will be achieved by using state-of-the-art Silicon Drift Detector (SDD), which has a unique capability of maintaining high energy resolution at very high incident count rate expected from Solar X-rays. XSM onboard Chandrayaan-2 will be the first experiment to use such detector for Solar X-ray monitoring.
Full-text available
We present the formulation of an analytical model which simulates charge transport in Swept Charge Devices (SCDs) to understand the nature of the spectral redistribution function (SRF). We attempt to construct the energy-dependent and position dependent SRF by modeling the photon interaction, charge cloud generation and various loss mechanisms viz., recombination, partial charge collection and split events. The model will help in optimizing event selection, maximize event recovery and improve spectral modeling for Chandrayaan-2 (slated for launch in 2014). A proto-type physical model is developed and the algorithm along with its results are discussed in this paper.
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