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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
20 April 2001 AAO Workshop 3
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).
20 April 2001 AAO Workshop 4
• 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...
20 April 2001 AAO Workshop 5
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
20 April 2001 AAO Workshop 6
A simple stellar interferometerb
d
d = aperturediameter
b = baseline
d r0
min b
20 April 2001 AAO Workshop 7
Long baseline interferometry
x = b.s
b
s
Added path = x
(tolerance: « 2/
20 April 2001 AAO Workshop 8
The fringe visibilityI
V = (Imax – Imin)/ (Imax + Imin)
Phase : fringes are shiftedwrt “phase centre”
sdbsisIiV
}.2exp{)(}exp{
The van Cittert-Zernike theorem:
20 April 2001 AAO Workshop 10
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
20 April 2001 AAO Workshop 11
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
20 April 2001 AAO Workshop 12
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
20 April 2001 AAO Workshop 13
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.
20 April 2001 AAO Workshop 14
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)
20 April 2001 AAO Workshop 15
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.)
20 April 2001 AAO Workshop 16
Techniques
• Intensity interferometry (obsolete)• Heterodyne interferometry (far IR)• Speckle interferometry (visual
binaries)• Masked aperture or “Fizeau”
interferometry• Modern Michelson interferometry
20 April 2001 AAO Workshop 17
Masked Aperture Instruments
• MAPPIT (Sydney University/AAO)– Host telescope: AAT– Used primarily for imaging cool
supergiants
• Keck Interferometer (UC Berkeley, Sydney University)
20 April 2001 AAO Workshop 18
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
20 April 2001 AAO Workshop 19
SUSI0 < b < 640m
440<<900nm
Tip-tilt wave-front correc-tion
Location: Paul WildObservatory,Narrabri, NSW
Photo credit:D. McConnell
20 April 2001 AAO Workshop 20
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
20 April 2001 AAO Workshop 21
Palomar Testbed Interf. (PTI)
• 3x0.5m siderostats
• 110 m baseline• Dual beam for dif-
ferential astrometry
• Testbed for Keck Interferometer
Photo credit: JPL
20 April 2001 AAO Workshop 22
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)
20 April 2001 AAO Workshop 23
CHARA Array, Mt Wilson, CA
• 6x1m tele-scopes
• 350 m max baseline
• tip-tilt correc-tion
• visible & Kband
20 April 2001 AAO Workshop 24
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
20 April 2001 AAO Workshop 25
IOTA, Mt Hopkins, AZ
• 2 (soon 3) x 0.45mtelescopes
• Maximum b = 38 m
• Visible & IR• FLUOR fibre beam
combinerPhoto credit: IOTA