CORE ZEBRA SARAS The Zero Spacing Interferometers · 2018-04-20 · Ravi Subrahmanyan, RRI ....
Transcript of CORE ZEBRA SARAS The Zero Spacing Interferometers · 2018-04-20 · Ravi Subrahmanyan, RRI ....
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CORE ⇒ ZEBRA ⇒ SARAS The Zero Spacing Interferometers
Ron Ekers, CSIRO &
Ravi Subrahmanyan, RRI
CAASTRO Global EoR Workshop Sydney, 19 Nov 2012
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From CoRE I to ZEBRA
CoRE I (Chippendale)
⇓
CoRE II (Virtual)
⇓
ZEBRA ⇒ SARAS
⇓
ZEBRA II
Calibratable receiver
CSI
RO
R
RI
MR
O
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Global EoR system RRI Bangalore
Ravi Subrahmanyan
Ron Ekers
Peter Shaver
A. Raghunathan
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Zebra – fat dipole v1
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Zebra – receiver MkI
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First light 14 Sep 2010
Spectrum is the expected galactic power law.
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Log (Freq) Tsky
fro
m c
osm
olog
ical
HI
~10mK
~ 10-100 MHz
Signal in the 50-200MHz depends on the (i) spin temperature (ii) ionization fraction during z = 26 - 6
Global EoR Signal
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The challenge
Global ∆T 30mK in few MHz – S/N easy – can reach a few mK in one hour
∆T/T < 10-4 to10-5
Calibrate the gain Minimize the number of unknowns that can couple to EoR Remove the additive constant
– Correlation receiver » Eliminate LNA additive noise – but still many problems
– Position switching » ∆T now very small so large antenna and long integration times » Correlation interferometer » Arrays
• Statistical detection • Direct detection
Zero spacing interferometer Zero spacing
interferometer
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ZEBRA– interferometer
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1.5m
110-170MHz 1.5m
– 0.55 to 0.85 λ 3m
– 1.1 to 1.7 λ
3m
Uniform sky visibility
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ZEBRA Global EoR Experiment
ZEro-spacing measurement of the Background RAdio spectrum
Partially reflecting resistive screen
Virtual zero spacing interferometer
Removes all additive errors
Modulate screen ?
Subrahmanyan, Ekers Patra
Partial reflector/transmitter
X
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The space beam-splitter: a resistive wire mesh
Need a space beam-splitter before the antenna A lossless screen (e.g. a conducting grid)
– transmitted & reflected waves are orthogonal Resistive wire mesh
– Thickness of wire < skin depth – Frequency independent – Re-radiated fields no longer cancel the incident field on
the far side of the wire screen
Lumped resistance on scale << λ – Practical solution instead of resistance wire
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Building resistive screen
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The Resistive Screen
copper wire + lumped resistors
resistor value = free space impedance/2
3x4 metres
holes to reduce wind loading
Roll up for transport
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Measurements of propagation Amplitude & phase through the resistive mesh For normal and oblique incidence Measurements of E and H plane fields
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Modeling the transmission amplitude and phase for FINITE SCREEN Predictions based on WIPL-D model vs Predictions from analytic formulation of transmission thro’ a mesh plus physical optics
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ZEBRA – interferometer first CMB correlation 20 Jan 2011
3.4m
1.5m separation Max sky coverage at zero spacing
26% Contributions to correlated output
– Global sky signal – Screen radiating – 1.5m interferometer sky
correlation » One path through screen » Both paths miss screen
Ferrite absorber
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ZEBRA at Gauribidanur
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ZEBRA receiver schematic Raghunathan Fat dipole
– 87.5-175MHz Correlator
– 2x ACR's – 1x XCR, – 12bit, 250MHz
Correlator 100m away – Replace coax by
fibre
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Zebra correlated output
Baseline ripple – changes with LST – Repeats each day – Multipath scattering of galaxy foreground signal – Shifted location …….
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Try heating the screen
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Screen Model grid 1
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Screen Model grid 2
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Screen Model grid 3
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Screen Model grid 4
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Screen Model grid 5
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ASKAP Phased Array Feed
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SARAS receiver evolution
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Antenna Requirements for EoR measurements
Half-power beamwidth variation < ±2.5% – For a frequency range 87.5 to 175 MHz – Keeps un-modeled foreground residuals < 5mK
An electrically small antenna – No spurious high order frequency components
Return loss > 10dB and constant over band – antenna gain losses and internal reflections of the
receiver noise must be low and vary smoothly with frequency.
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Fat Dipole Antenna Prototype II
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Fat Dipole Antenna
A. Raghunathan, Udaya Shankar, Ravi Subrahmanyan – submitted to IEEE AP
A wide-band fat-dipole antenna – sinusoidal profile – frequency independent performance 87.5 to 175 MHz
Structure optimized using electromagnetic modelling – Adopt a sinusoidal profile
Design validated by constructing a prototype – The input return loss > 15 dB – Radiation power pattern is a frequency invariant (< 2.1% ) cosine
square over the octave bandwidth Now used in SARAS by Nipanjana Patra
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RRI gets some interesting visitors!
Composition by Ravi
Snake photo by Nipanjana
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Pulse calibration ?
Pulse injected at Parkes vertex
Pulse reflected from Parkes focus
Inject and integrate short (ηsec) pulses Calibrated noise spectrum Understand & calibrate reflections Nipanjana Patra, Paul Roberts