ALADDIN performance model

(1)

ALADDIN performance model

Olivier Absil

Laboratoire d’Astrophysique de Grenoble

&

Université de Liège

(2)

Nulling interferometry: principle

• Co-axial combination

• Interferometric response:

transmission map

• No image

–

Flux integrated on the

whole field-of-view

• Calibration and/or

modulation mandatory

π

0

2λ

/D ~

0.1”

to

1”

Constructive

Destructive

baseline

(3)

Signals and noises

• Stellar residuals

–

Geometric leakage

• 4/π

2

_{× (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)}

(4)

The GENIEsim software

Initialise instrument

and generate

the scene

Simulate

atmospheric

turbulence

Synthesize

instantaneous

transmission map

Compute output

fluxes and

estimate noises

Absil et al. 2006 (A&A 448)

(5)

Input parameters: atmosphere

• Crude model for free air

seeing

• Seeing: 0.27”

–

Equivalent r

₀

= 38 cm

• Coh. time τ

₀

= 7.9 msec

–

Equivalent v

_wind

= 15 m/s

• Water vapour seeing

–

‹PWV› = 250 µm

–

σ

_PWV

= 1 µm

• T

_sky

= 230 K

13/10/2008 ARENA Interferometry Workshop II 5

Mauna Kea

Dome C

L’

L

(6)

Input parameters: instrument

• Baseline range: ~4 to 30 m

• Telescope diameter: 1 m

• Warm throughput: 80%

–

5 mirrors at 230 K

• Cold throughput: 10%

–

15 mirrors at 150 K

• Science band: 3.1 – 4.1 µm

• Control loops

–

Fringe tracking (K band)

–

Tip-tilt control (J band)

Cold Enclosure Beam Combiner Modal Filter OPD Sensor Afocal Telescope Tip-Tilt Mirror π Phase -shift Tip-Tilt Sensor Passive Delay Line Afocal Telescope Tip-Tilt Mirror π Phase -shift Tip-Tilt Sensor Active Delay Line Siderostat Siderostat Science Detection

(7)

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)

(8)

Typical noise budget

1.3E+5 1.5E+6 1.3E+6 1.6E+8 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 1E+6 1E+7 1E+8 1E+9 20‐zodi disc Geometric leakage Instrumental leakage Background Photo ‐electrons in 30 min

Signals

1.8E+4 5.3E+3 1.2E+3 2.9E+4 3.5E+4 1E+0 1E+1 1E+2 1E+3 1E+4 1E+5 Background (shot + cal) Instability noise Detector noise Geom. leak calib. Instr. leak calib. RMS photo ‐electrons in 30 min

Noises

(9)

Comparison with GENIE

13/10/2008 ARENA Interferometry Workshop II 9 455 117 50 59 125 154 278 604 55 38 37 67

0

100

200

300

400

500

600

700 K0V ‐ 5pc

G5V ‐ 10pc

G0V ‐ 20pc

G0V ‐ 30pc

GENIE‐UT

GENIE‐AT

ALADDIN

(10)

Stellar leakage calibration

• Imperfect knowledge of stellar diameter

–

∆θ

~1%

–

Self-calibration possible

(11)

Integr. time / telescope diam.

• No large improvement

(12)

Influence of boundary layer

• At ground level: seeing 1.9”, τ

₀

~ 2.9 msec

–

1 st

_{problem: wave front quality}

• Multi-speckle Æ adaptive optics (20×20 actuators, 1 kHz)

–

2 nd

_{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

(13)

ALADDIN at Paranal?

• Atmosphere: seeing 0.9”, τ

₀

~ 3 msec

•

1 st

_{problem: background}

–

8-m class telescopes would be required

•

2 nd

_{problem: water vapour seeing}

–

Dispersion control mandatory (fluctuation ~ 1 rad)

•

3 rd

_{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

(14)

Final benchmark: space missions

Best performance with FKSI, but compare price (~700M$)

Defrère et al. 2008 (A&A 490)