24 September 2001ATNF Imaging Workshop1 The Sydney University Stellar Interferometer (SUSI) John Davis School of Physics University of Sydney 24 September.

24 September 2001

ATNF Imaging Workshop 1

The Sydney University Stellar Interferometer (SUSI)

John DavisSchool of PhysicsUniversity of Sydney

24 September 2001

ATNF Imaging Workshop 1

24 September 2001

ATNF Imaging Workshop 2

Outline• Background

What SUSI is and is not! What SUSI is designed to do

• SUSI The instrument The problems faced by optical stellar

interferometry Solutions adopted/developed for SUSI

• Example SUSI observations & results

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ATNF Imaging Workshop 3

SUSI is not an imaging instrument!

• Atmospheric turbulence introduces phase fluctuations - the phase of the interference fringes is corrupted

24 September 2001

ATNF Imaging Workshop 3

• It uses 2 apertures at a time (i.e. a single baseline) so phase closure and image reconstruction is not possible - SUSI measures “correlation” C = V2

12() = |12()| exp i12()V12() = |12()|

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ATNF Imaging Workshop 4

Some Programs for SUSI• Measurement of the angular diameters of

single stars leading to the determination of fundamental stellar properties:

Emergent fluxes Effective Temperatures Radii Luminosities

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ATNF Imaging Workshop 4

• Measurement of orbits of close binary stars leading to:

Stellar Masses Accurate distances

M, L & R for individual stars

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ATNF Imaging Workshop 5

SUSI from the air

Photo: D. McConnell

Baselines5 < b < 640m

Spectral Range440 < < 900nm

Aperture Diameter14cm

Resolution75as - 20mas

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ATNF Imaging Workshop 6

The Basic Problems to be SolvedI. Wavefront distortion due to

atmospheric turbulence

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ATNF Imaging Workshop 7

The Effects of Atmospheric Turbulence

• Wavefront distortion in the form of: Wavefront curvature Wavefront tips and tilts Phase fluctuations

Starlight

Wavefront distorted by atmospheric turbulence

Portions of wavefront accepted by interferometer apertures

Phase difference on arrival at apertures = 2/

24 September 2001

ATNF Imaging Workshop 8

How SUSI overcomes Wavefront Distortion

• Wavefront curvature - small apertures (< r0)

• Wavefront tilt - “tip-tilt” correction (first order adaptive optics)

• Phase fluctuations - rapid signal sampling and processing

24 September 2001

ATNF Imaging Workshop 9

The Effect of Atmospherically Induced Wavefront Tilts

0.1

1.0

0.1 1.0 10.0

d/r0

Cor

rela

tion

Loss

Fac

tor

With no wavefront-tilt correction

With wavefront-tilt correction

24 September 2001

ATNF Imaging Workshop 10

The Effect of the Atmospheric Coherence Time

24 September 2001

ATNF Imaging Workshop 11

SUSI Simultaneous Observations of Atmospheric Coherence Time and

Coherence Diameter

t0 = 0.314r0/V

24 September 2001

ATNF Imaging Workshop 12

The Basic Problems to be Solved

I. Wavefront distortion due to atmospheric turbulence

II. Need for extreme mechanical stability

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ATNF Imaging Workshop 13

Mechanical Stability

Average 5.5 m deep & 1.2 m diameter

Example: Siderostat support pier

24 September 2001

ATNF Imaging Workshop 14

A SUSI Siderostat & Housing

Siderostat & Relay Mirrors

Siderostat Housing with Roll-off Roof

24 September 2001

ATNF Imaging Workshop 15

The Basic Problems to be Solved

I. Wavefront distortion due to atmospheric turbulence

II. Need for extreme mechanical stability

III. Need to match the optical paths

24 September 2001

ATNF Imaging Workshop 16

The Variation of V and C with OPD

24 September 2001

ATNF Imaging Workshop 17

Optical Path Length Compensation

• The individual sections of the blue paths are all equal in the two arms

• The internal red path compensates the external red path

24 September 2001

ATNF Imaging Workshop 18

Delay Curve for Sirius

0.00

0.05

0.10

0.15

0.20

0.25

0.30

-250 -200 -150 -100 -50 0 50 100 150 200 250

Path Compensator Offset (m)

Obs

erve

d C

orre

latio

n

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ATNF Imaging Workshop 19

Beam Combination in SUSIPhoton counts n1 and n2 in sampling time t

n1 [1 + |12(0)| cos ] n2 [1 – |12(0)| cos ]

n1 – n2 2|| cos

< (n1 – n2)2 > 4||< cos2 >

< (n1 – n2)2 > 2||

24 September 2001

ATNF Imaging Workshop 20

Interferometer Response to a Uniform Disk Source

0.0

0.2

0.4

0.6

0.8

1.0

0 1 2 3 4 5 6 7 8

x = b/

V a

nd C

x

Visibility (V = 2J1(x)/x)

Correlation (C = V2)

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ATNF Imaging Workshop 21

Observations of Delta Canis Majoris

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40

Baseline (m)

Corre

lation

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40

Baseline (m)

Corre

lation

0.0

0.2

0.4

0.6

0.8

1.0

0 10 20 30 40

Baseline (m)

Corre

lation

SUSI ( CMa as calibrator)

SUSI ( CMa as calibrator)

Stellar Intensity Interferometer

Instrument Calibration UD ± %Star (mas)

NSII n/a 3.29 ± 0.46 14

SUSI CMa 3.474 ± 0.091 2.6SUSI CMa 3.535 ± 0.090 2.5

SUSI Mean 3.505 ± 0.064 1.8

24 September 2001

ATNF Imaging Workshop 22

Response of an Interferometer to a Binary Star

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ATNF Imaging Workshop 23

Examples of SUSI Observations of Beta Centauri

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ATNF Imaging Workshop 24

The Orbit of Beta Centauri determined from SUSI Observations

-30

-25

-20

-15

-10

-5

0

5

10-15 -10 -5 0 5 10 15 20 25 30

y (mas)

x (m

as)

1995 MAPPIT Observation

1997 SUSI Observations

1998 SUSI Observations

1999 SUSI Observations

2000 SUSI Observations

Fitted Orbit

E

N

Period: 357.0±0.3 daysInclination: 67.5±0.4 degSemi-major axis: 25.3±0.2 mas

24 September 2001

ATNF Imaging Workshop 25

The SUSI Programme• Observing Programme

Single stars for Te, R and LSpectroscopic binaries for M and d, as well

as Te, R and LPulsating stars (e.g. Cepheids) for d

• Technical Programme Installation and commissioning of “red”

beam combination system

24 September 2001

ATNF Imaging Workshop 26

24 September 2001

ATNF Imaging Workshop 27

Ground-Based Long-Baseline Optical/IR Interferometers

KEY: C - Closed; W - Working; UC - Undergoing commissioning or under construction

Instrument Institution Location Aperture Maximum Wavelength StatusAcronym Diameter Baseline Range

(m) (m) (m)NSII Sydney U. Narrabri, Australia 26.8 188 0.44 CSUSI (P'type) Sydney U. Sydney, Australia 20.10 11 0.4-0.5 CMark III NRL/MIT/CfA Mt. Wilson, USA 20.05 32 0.45-0.8 CI2T CERGA Calern, France 20.26 144 Visible CGI2T CERGA Calern, France 21.5 65 Visible/IR WCOAST Cambridge U. Cambridge, UK 50.4 100 Red/near IR WSUSI Sydney U. Narrabri, Australia 20.14 640 0.4-0.9 WIOTA CfA Mt.Hopkins, USA 30.45 38 Visible/IR WISI UC Berkeley Mt. Wilson, USA 70 10 WNPOI USNO/NRL Anderson Mesa, USA 26.17 435 0.45-0.9 WPTI JPL/Caltech Mt. Palomar, USA 20.4 110 2.2 WCHARA Georgia St. U. Mt. Wilson, USA 61.0 350 0.45-2.4 UCKeck CARA Mauna Kea, USA 2(4)10(1.5) 165 2.2-10 UCVLTI ESO Cerro Paranal, Chile 4(3)8(1.8) 200 0.45-20 UC