Optical & IR Interferometry

1. Optical & IR Interferometry Bill Tango School of Physics University of Sydney 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 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 a Ori. • 1950s: Discovered by radioastronomers! • Intensity interferometry discovered by Hanbury Brown & Twiss (1956). 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... 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 AAO Workshop

6. A simple stellar interferometer b d = aperture diameter d b = baseline d @ r0qmin» l/b AAO Workshop

7. Long baseline interferometry s Dx = b.s b Added path = Dx (tolerance: d«l2/dl) AAO Workshop

8. The fringe visibility I V = (Imax – Imin)/ (Imax + Imin) Phase f : fringes are shifted wrt “phase centre” The van Cittert-Zernike theorem: AAO Workshop

9. An example of fringes Image courtesy of P. Tuthill 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 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 AAO Workshop

12. Science goals • Angular diameters can be used to find effective temperature:F= sT4 = 4fbolq-2 • Spectroscopic binaries: interferometry yields inclination hence masses can be determined • Variation of qwith lgives information about stellar atmospheres • Pulsating stars: radial velocity & dq/dt give distance independent of parallax • Imaging: morphology of complex objects 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. 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) 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.) AAO Workshop

16. Techniques • Intensity interferometry(obsolete) • Heterodyne interferometry (far IR) • Speckle interferometry (visual binaries) • Masked aperture or “Fizeau” interferometry • Modern Michelson interferometry 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) AAO Workshop

18. Examples of masked apertureinterferometry with Keck The binary WR 104 at 2.27 mm Dusty torus around LkHa 101 Images courtesy of P. Tuthill, Sydney University AAO Workshop

19. SUSI 0 < b < 640m 440<l<900nm Tip-tilt wave- front correc- tion Location: Paul Wild Observatory, Narrabri, NSW Photo credit: D. McConnell 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 Keck 1 & Keck 2 on Mauna Kea, Hawaii Photo credit: Keck Observatory 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 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) AAO Workshop

23. CHARA Array, Mt Wilson, CA • 6x1m tele-scopes • 350 m max baseline • tip-tilt correc-tion • visible & Kband 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 between USNO, NRL & Lowell Observatory AAO Workshop

25. IOTA, Mt Hopkins, AZ • 2 (soon 3) x 0.45mtelescopes • Maximum b = 38 m • Visible & IR • FLUOR fibre beam combiner Photo credit: IOTA AAO Workshop

26. Where it all started: Photo credit: CHARA AAO Workshop