1. 20 April 2001AAO Workshop1 Optical & IR Interferometry Bill Tango School of Physics University of Sydney

2. 20 April 2001AAO Workshop2 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

3. 20 April 2001AAO Workshop3 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).

4. 20 April 2001AAO Workshop4 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...

5. 20 April 2001AAO Workshop5 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

6. 20 April 2001AAO Workshop6 A simple stellar interferometer b d d = aperture diameter b = baseline d  r 0  min  b

7. 20 April 2001AAO Workshop7 Long baseline interferometry  x = b. s b s Added path =  x(tolerance:  « 2 / 

8. 20 April 2001AAO Workshop8 The fringe visibility I V = (I max – I min )/ (I max + I min ) Phase  : fringes are shifted wrt “phase centre” The van Cittert-Zernike theorem:

9. 20 April 2001AAO Workshop9 An example of fringes Image courtesy of P. Tuthill

10. 20 April 2001AAO Workshop10 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

11. 20 April 2001AAO Workshop11 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

12. 20 April 2001AAO Workshop12 Science goals Angular diameters can be used to find effective temperature: F =  T 4 = 4f bol   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

13. 20 April 2001AAO Workshop13 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.

14. 20 April 2001AAO Workshop14 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)

15. 20 April 2001AAO Workshop15 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.)

16. 20 April 2001AAO Workshop16 Techniques Intensity interferometry (obsolete) Heterodyne interferometry (far IR) Speckle interferometry (visual binaries) Masked aperture or “Fizeau” interferometry Modern Michelson interferometry

17. 20 April 2001AAO Workshop17 Masked Aperture Instruments MAPPIT (Sydney University/AAO) –Host telescope: AAT –Used primarily for imaging cool supergiants Keck Interferometer (UC Berkeley, Sydney University)

18. 20 April 2001AAO Workshop18 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

19. 20 April 2001AAO Workshop19 SUSI 0 < b < 640m 440< <900nm Tip-tilt wave- front correc- tion Location: Paul Wild Observatory, Narrabri, NSW Photo credit: D. McConnell

20. 20 April 2001AAO Workshop20 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

21. 20 April 2001AAO Workshop21 Palomar Testbed Interf. (PTI) 3x0.5m siderostats 110 m baseline Dual beam for dif- ferential astrometry Testbed for Keck Interferometer Photo credit: JPL

22. 20 April 2001AAO Workshop22 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)

23. 20 April 2001AAO Workshop23 CHARA Array, Mt Wilson, CA 6x1m tele- scopes 350 m max baseline tip-tilt correc- tion visible & K band

24. 20 April 2001AAO Workshop24 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

25. 20 April 2001AAO Workshop25 IOTA, Mt Hopkins, AZ 2 (soon 3) x 0.45m telescopes Maximum b = 38 m Visible & IR FLUOR fibre beam combiner Photo credit: IOTA

26. 20 April 2001AAO Workshop26 Where it all started: Photo credit: CHARA