Open access

ALADDIN performance model
Olivier Absil
Laboratoire d’Astrophysique de Grenoble
&
Université de Liège
13/10/2008
ARENA Interferometry Workshop II Garching
1
Nulling interferometry: principle
baseline
– Flux integrated on the
whole field-of-view
• Calibration and/or
modulation mandatory
13/10/2008
Constructive
0
π
Destructive
2λ/D ~ 0.1” to 1”
• Co-axial combination
• Interferometric response:
transmission map
• No image
ARENA Interferometry Workshop II Garching
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λ/b
Signals and noises
• Stellar residuals
– Geometric leakage
• 4/π2 × (λ/b/θ)2 ~ 10−4
– Instrumental leakage
• Due to turbulence
• Bias + instability noise
• Thermal background
– Sky and optical train
– Dominant from ground
• Exozodiacal disc
– 20 zodi: typically 10−4 to 10−5 of stellar signal (thermal IR)
13/10/2008
ARENA Interferometry Workshop II Garching
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The GENIEsim software
Absil et al. 2006 (A&A 448)
Initialise instrument
and generate
the scene
Simulate
atmospheric
turbulence
Compute output
fluxes and
estimate noises
Synthesize
instantaneous
transmission map
13/10/2008
ARENA Interferometry Workshop II Garching
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Input parameters: atmosphere
• Crude model for free air
seeing
• Seeing: 0.27”
Mauna Kea
L’
– Equivalent r0 = 38 cm
• Coh. time τ0 = 7.9 msec
– Equivalent vwind = 15 m/s
• Water vapour seeing
– ‹PWV› = 250 µm
– σPWV = 1 µm
L
Dome C
• Tsky = 230 K
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ARENA Interferometry Workshop II Garching
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Input parameters: instrument
• Baseline range: ~4 to 30 m
• Telescope diameter: 1 m
• Warm throughput: 80%
– 5 mirrors at 230 K
Siderostat
Siderostat
Afocal
Telescope
Afocal
Telescope
Tip-Tilt
Mirror
Tip-Tilt
Mirror
π
Phase -shift
π
Phase -shift
• Cold throughput: 10%
Passive
Delay Line
– 15 mirrors at 150 K
• Science band: 3.1 – 4.1 µm
• Control loops
Tip-Tilt
Sensor
Beam
Combiner
Tip-Tilt
Sensor
OPD
Sensor
– Fringe tracking (K band)
– Tip-tilt control (J band)
Modal Filter
Cold Enclosure
13/10/2008
Active
Delay Line
ARENA Interferometry Workshop II Garching
Science
Detection
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Sensitivity vs. baseline
• Four representative
targets
• Integration: 30 min
• Competition
between
– Stellar leakage
– Exozodi
transmission
• Sensitivity in the
40-60 zodi range
Absil et al. 2007 (A&A 475)
13/10/2008
ARENA Interferometry Workshop II Garching
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Typical noise budget
Signals
Noises
1E+5
1E+9
1E+8
RMS photo‐electrons in 30 min
1.6E+8
Photo‐electrons in 30 min
1E+7
1E+6
1E+5
1.5E+6
1.3E+6
1.3E+5
1E+4
1E+3
1E+2
1E+4
1.8E+4
2.9E+4
3.5E+4
Geom. leak calib.
Instr. leak calib.
5.3E+3
1E+3
1.2E+3
1E+2
1E+1
1E+1
1E+0
1E+0
20‐zodi disc Geometric Instrumental Background
leakage
leakage
13/10/2008
Background Instability (shot + cal)
noise
ARENA Interferometry Workshop II Garching
Detector noise
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Comparison with GENIE
700
GENIE‐UT
GENIE‐AT
ALADDIN
604
600
500
455
400
300
278
200
154
125
100
117
55
38
50
37
67
59
0
K0V ‐ 5pc
13/10/2008
G5V ‐ 10pc
G0V ‐ 20pc
ARENA Interferometry Workshop II Garching
G0V ‐ 30pc
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Stellar leakage calibration
• Imperfect knowledge of stellar diameter
– ∆θ~1%
– Self-calibration possible
K0V at 5 pc
13/10/2008
ARENA Interferometry Workshop II Garching
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Integr. time / telescope diam.
• No large improvement
– 30min integration on 1m telescopes is appropriate
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ARENA Interferometry Workshop II Garching
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Influence of boundary layer
• At ground level: seeing 1.9”, τ0 ~ 2.9 msec
– 1st problem: wave front quality
• Multi-speckle Æ adaptive optics (20×20 actuators, 1 kHz)
– 2nd problem: piston
• Residual OPD ~25 nm @ 10 kHz (instead of 10 nm @ 4 kHz)
• Dispersion control might be required (TBC)
– Estimated sensitivity (G0V at 20 pc)
• ~50 / ~200 zodi with / without AO (instead of 37 zodi)
• Maximum tolerable seeing
– Around 0.8” before significant performance decrease
– Calibration may become difficult
13/10/2008
ARENA Interferometry Workshop II Garching
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ALADDIN at Paranal?
• Atmosphere: seeing 0.9”, τ0 ~ 3 msec
• 1st problem: background
– 8-m class telescopes would be required
• 2nd problem: water vapour seeing
– Dispersion control mandatory (fluctuation ~ 1 rad)
• 3rd problem: wave front quality
– A few speckles Æ intensity control (or AO)
• Tip-tilt control (1 kHz) Æ intensity variation of 7%
• Estimated sensitivity (G0V at 20 pc)
– ~250 / ~3000 zodi with / without dispersion control
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ARENA Interferometry Workshop II Garching
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Final benchmark: space missions
> 40 m
Defrère et al. 2008 (A&A 490)
Best performance with FKSI, but compare price (~700M$)
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ARENA Interferometry Workshop II Garching
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