Download SSM hardware

Scanning sky monitor (SSM)
Technical Physics Division, ISAC
&
Astrophysics Group, RRI
Role of a monitor
• To detect, locate and monitor x-ray transients.
- all x-ray sources are variable.
factor of 100 increase in x-ray flux in few days is termed
as a transient.
- Nearly half of known x-ray binaries are
transients
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Monitor known bright sources
- sampling time few minutes
- several samples/day; monitor for many months.
• To alert other instruments for detailed studies
Types of transients
• Hard x-ray transients
- Be binaries with very long orbital period
• Soft x-ray transients or x-ray novae
- episodic x-ray outbursts
-spectra similar to LMXBs
- no fixed period of recurrence
(1year to 50 years)
Scientific objectives
• Provides unique opportunity to study these objects
over a large dynamic range.
L - 1033 erg/s to 10 38 erg/s; dM/dt
• Study of mass transfer in accretion discs and the
processes causing instabilities.
• Study of the compact object
• Precursors; low level variabilities
• optical counterparts- parameters of the system
Proposed sky monitor
1-D coded mask position-sensitive detector very
similar to ASM on RXTE.
• Proven design; minor modifications to improve
performance
- Detectors: proportional counters with resistive anodes;
Ratio of signals on either ends of anode gives position.
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Energy range
2-10 keV
Position resolution ~ 0.5 mm.
Field of view
~ 6° X 90° (FWHM)
Sensitivity
~10 mCrab (1 day integration)
Best time resolution 1ms
Coded mask
• Coded mask casts a
shadow on the detector
plane
• 63 element mask 6x
11cm2.made in 6 parts
• Hadamard transform used
for the design
• Cross correlation technique
used for image reconstruction.
Satisfactory on simulated data.
• Iterative removal of sources, to
improve dynamic range
implemented.
Scanning arrangement
• Most other experiments on ASTROSAT are
pointed to a specific object for relatively long
periods of time (~hours to days).
• Scanning mechanism necessary for monitors to
scan the sky multiple times per day.
• FOV of two monitors forms an ‘X’ in the sky.
Third detector views the perpendicular direction.
• Monitors to be mounted on a boom which can
have scanning capability in discrete steps.
Scanning arrangement
• Rotation of spacecraft not adopted because
– SXT and UVIT images have to be deconvolved
• Monitors to be mounted on a boom which can
have scanning capability in discrete steps.
– Integration time can be varied for studying transient
(nominal 5 minutes)
– Deconvolution of the image simpler
– Onboard flagging of new transient possible
ssm2
ssm1
ssm3
Sky Coverage
Rot.
axis
Status of Engg. Model (prev. meet)
Detector (ISAC)
• Fabricated and assembled
• testing done on all wires
with 16 preamps connected
• position resolution 0.38mm
Coded mask (RRI)
Design and fabrication completed; basic software for
image deconvolution completed;
Current status
• Front end logic was tested; this includes:
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Logic to veto simultaneous signals from any two
wires, or which cross ULD;
• Conversion of amplitude of each pulse to digital form
and tag it with the respective wire ID, left/right
identification;
• Threshold levels for all the wires;
• logic to take o/ps on either ends from same wire only;
• O/p of the front end logic fed to PC to test the
ADC o/p
Current status (contd)
• Pulser readings with the ADC o/p checked. Found to be
linear
• The detector o/p fed to the front end logic; The o/ps
measured for 2 wires;
• Processing electronics:
• FPGA based electronics designed and simulated using
VHDL
• Software simulation of different modes of operation
tested.
O/p voltage of CSPA
Channel no of ADC o/P
Total o/p(L+R) in channel no.
Position in mm along the wire
Ratio in channel no.
Position in mm along the wire
Plans for the next 3 months
• Detector
• The overall noise level to be estimated
• Appropriate adjustments in the threshold to be made to
cover the 2-10 keV range
• Test of all wires to be done; characteristic curves to be
established for each wire;
• Thermovac test for 1 week and post thermovac
monitoring.
Plans for the next 3 months (contd)
• Processing electronics:
• Design of hardware of FPGA electronics to start.
• Boom
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Requirements generated
Discussions regarding boom realisation undertaken
Length of boom
Fractional loss
Further Plans (3-6 months)
• Opening the detector for checking problem
on one wire
• Mounting the coded mask on the detector
• Tests of the detector with coded mask
• Vibration test
• Finalisation of design of all onboard
electronics
R&D issues
• Alternate gas mixtures; to improve
detection efficiency;
• Alternate wire testing under progress
• Maintenance of purity of counter gas
without onboard purification; coating
options
Critical areas
• Be foil for window material
• Calibration set up for integrated tests with
the coded mask ;
Summary
• Overall test set up for the detector is in place.
This can also be used to compare different types of
wires.
• The counter has been stable over the last year.
The slope of the ratio graph in Feb 2001 at CSPA
o/p was = 0.13; and the same in Jan 2002 at o/p of
logic is 0.132 + 0.001
• The tests have to be completed with coded mask
• The computer simulation of the deconvolution of
images has also been completed.
Mission details
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Payload Weight - 48 kg (excluding boom arrangement)
Onboard memory - 3X12 Mbyte
Power- 18W
Attitude - pointing : better than 3´ , preferred 1´
– Knowledge better than 1´ (at the end of boom,
inclusive of tilt/resolver errors).
– Proposed accuracy of position sensing ~ 5-10´
depending on intensity of the transient
– Automation of ground software for preprocessing of the
data