20 April 2001 AAO Workshop 1

Optical & IR Interferometry

Bill TangoSchool of Physics

University of Sydney

20 April 2001 AAO Workshop 2

Outline

• Historical background• Basic theory• Science goals• Modern interferometric techniques• For further information see “Optical

Long Baseline Interferometry News” at http://olbin.jpl.nasa.gov

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A Brief History

• First proposed by Fizeau in 1867.• First successful measurements in 1891

(Galilean satellites, by Michelson).• In 1921 Michelson & Pease measured

angular diameter of Ori.• 1950s: Discovered by

radioastronomers!• Intensity interferometry discovered by

Hanbury Brown & Twiss (1956).

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• The Narrabri Stellar Intensity Inter-ferometer (NSII) commissioned in 1963.

• Speckle interferometry discovered by Labeyrie in 1970.

• Mid-1970s: Rapid developments in optical technology stimulated many groups to build prototype interfer-ometers.

• Today: Keck, IOTA, NPOI, SUSI, VLTI...

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Basic theory

• Small aperture size (diameter d) reduces seeing effects

• Large separation (the “baseline” b) provides high resolution

• Light from the separated apertures must be coherently combined

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A simple stellar interferometerb

d

d = aperturediameter

b = baseline

d r0

min b

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Long baseline interferometry

x = b.s

b

s

Added path = x

(tolerance: « 2/

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The fringe visibilityI

V = (Imax – Imin)/ (Imax + Imin)

Phase : fringes are shiftedwrt “phase centre”

sdbsisIiV

}.2exp{)(}exp{

The van Cittert-Zernike theorem:

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An example of fringes

Image courtesy of P. Tuthill

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Two vs multi-aperture interferometry:• Two apertures:

– Only one baseline at a time– No phase information– Simple (but not easy!)

• Multiple apertures:– Many baselines simultaneously– Some phase information (“closure

phases”)– Complicated, but can be used for imaging

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So why is it so @#!% difficult?

• Observed V always less than true visibility– Instrumental effects– The atmosphere

• One must calibrate the visibility scale by observing unresolved sources

• Calibrators must be “near” the target sources

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Science goals• Angular diameters can be used to find

effective temperature: F = T4 = 4fbol

• Spectroscopic binaries: interferometry yields inclination hence masses can be determined

• Variation of with gives information about stellar atmospheres

• Pulsating stars: radial velocity & d/dt give distance independent of parallax

• Imaging: morphology of complex objects

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Science with 1 m < b < 10 m

• Angular diameters of supergiants• Studies of Mira and other long-

period giant and supergiant variables

• Imaging of accretion disks, dust around Wolf-Rayet stars, etc.

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Science with 10m < b < 100m

• Angular diameters of main sequence stars (spectral class A and later)

• Double-lined spectroscopic binaries• Cepheid variables: interferometry

provides an independent calibration of Cepheid distance scale

• AGNs• Planet searches (differential

astrometry)

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Science with 100m < b < 1000m

• Angular diameters of hot main sequence stars (O and B stars)

• Studies of hot, active stars (e.g., Wolf-Rayet stars, Be stars, etc.)

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Techniques

• Intensity interferometry (obsolete)• Heterodyne interferometry (far IR)• Speckle interferometry (visual

binaries)• Masked aperture or “Fizeau”

interferometry• Modern Michelson interferometry

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Masked Aperture Instruments

• MAPPIT (Sydney University/AAO)– Host telescope: AAT– Used primarily for imaging cool

supergiants

• Keck Interferometer (UC Berkeley, Sydney University)

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Examples of masked aperture

interferometry with Keck

Dusty torus around LkHa 101 The binary WR 104 at 2.27 m

Images courtesy of P. Tuthill, Sydney University

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SUSI0 < b < 640m

440<<900nm

Tip-tilt wave-front correc-tion

Location: Paul WildObservatory,Narrabri, NSW

Photo credit:D. McConnell

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The Keck Interferometer

• 2x10m telescopes &4+ 1.8m outriggers

• Full AO on 10 m Kecks

• Baselines up to 140m • Fringes obtained with

full-aperture K1 & K2 on 12/03/01

• K band operation• Only 1% of interfer-

metry time will use K1 & K2

Photo credit: Keck Observatory

Keck 1 & Keck 2 on Mauna Kea, Hawaii

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Palomar Testbed Interf. (PTI)

• 3x0.5m siderostats

• 110 m baseline• Dual beam for dif-

ferential astrometry

• Testbed for Keck Interferometer

Photo credit: JPL

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VLTI (ESO, Paranal, Chile)

• 4x8.2m Unit Telescopes and 3x1.8 m auxiliary telescopes

• baselines up to 202 m

• fringes obtained on 17/03/01 (with sid-erostats)

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CHARA Array, Mt Wilson, CA

• 6x1m tele-scopes

• 350 m max baseline

• tip-tilt correc-tion

• visible & Kband

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NPOI, Anderson Mesa, NM• 6x0.5m sidero-

stats• baselines up to

~ 500m• visible & IR• principal

mission: astrometry

Photo credit: NPOI

NPOI is a collaboration betweenUSNO, NRL & Lowell Observatory

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IOTA, Mt Hopkins, AZ

• 2 (soon 3) x 0.45mtelescopes

• Maximum b = 38 m

• Visible & IR• FLUOR fibre beam

combinerPhoto credit: IOTA

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Where it all started:

Photo credit: CHARA