The Sydney University Stellar Interferometer SUSI John Davis

The Sydney University Stellar Interferometer (SUSI) John Davis School of Physics University of Sydney 10/26/2021 24 September 2001 ATNF Imaging Workshop 1

Outline • Background Ø What SUSI is and is not! Ø What SUSI is designed to do • SUSI Ø The instrument Ø The problems faced by optical stellar interferometry Ø Solutions adopted/developed for SUSI • Example SUSI observations & results 24 September 2001 ATNF Imaging Workshop 2

SUSI is not an imaging instrument! g 12(t) = |g 12(t)| exp if 12(t) V 12(0) = |g 12(0)| • Atmospheric turbulence introduces phase fluctuations - the phase of the interference fringes is corrupted • 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 = V 2 10/26/2021 24 September 2001 ATNF Imaging Workshop 3

Some Programs for SUSI • Measurement of the angular diameters of single stars leading to the determination of fundamental stellar properties: Ø Ø Emergent fluxes Effective Temperatures Radii Luminosities • Measurement of orbits of close binary stars leading to: Ø Stellar Masses Ø Accurate distances Ø M, L & R for individual stars 10/26/2021 24 September 2001 ATNF Imaging Workshop 4

SUSI from the air Baselines 5 < b < 640 m Aperture Diameter 14 cm Spectral Range 440 < l < 900 nm Resolution 75 mas - 20 mas Photo: D. Mc. Connell 24 September 2001 ATNF Imaging Workshop 5

The Basic Problems to be Solved I. Wavefront distortion due to atmospheric turbulence 24 September 2001 ATNF Imaging Workshop 6

The Effects of Atmospheric Turbulence • Wavefront distortion in the form of: Ø Wavefront curvature Ø Wavefront tips and tilts Ø Phase fluctuations 24 September 2001 ATNF Imaging Workshop 7

How SUSI overcomes Wavefront Distortion • Wavefront curvature - small apertures (< r 0) • Wavefront tilt - “tip-tilt” correction (first order adaptive optics) • Phase fluctuations - rapid signal sampling and processing 24 September 2001 ATNF Imaging Workshop 8

The Effect of Atmospherically Induced Wavefront Tilts 24 September 2001 ATNF Imaging Workshop 9

The Effect of the Atmospheric Coherence Time 24 September 2001 ATNF Imaging Workshop 10

SUSI Simultaneous Observations of Atmospheric Coherence Time and Coherence Diameter t 0 = 0. 314 r 0/V 24 September 2001 ATNF Imaging Workshop 11

The Basic Problems to be Solved I. Wavefront distortion due to atmospheric turbulence II. Need for extreme mechanical stability 24 September 2001 ATNF Imaging Workshop 12

Mechanical Stability Example: Siderostat support pier Average 5. 5 m deep & 1. 2 m diameter 24 September 2001 ATNF Imaging Workshop 13

A SUSI Siderostat & Housing Siderostat Housing with Roll-off Roof Siderostat & Relay Mirrors 24 September 2001 ATNF Imaging Workshop 14

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 15

The Variation of V and C with OPD 24 September 2001 ATNF Imaging Workshop 16

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 17

Delay Curve for Sirius 24 September 2001 ATNF Imaging Workshop 18

Beam Combination in SUSI Photon counts n 1 and n 2 in sampling time t n 1 µ [1 + |g 12(0)| cos f] n 2 µ [1 – |g 12(0)| cos f] n 1 – n 2 µ 2|g| cos f < (n 1 – n 2)2 > µ 4|g|2 < cos 2 f > < (n 1 – n 2)2 > µ 2|g|2 24 September 2001 ATNF Imaging Workshop 19

Interferometer Response to a Uniform Disk Source 24 September 2001 ATNF Imaging Workshop 20

Observations of Delta Canis Majoris SUSI (e CMa as calibrator) Stellar Intensity Interferometer SUSI (h CMa as calibrator) 24 September 2001 ATNF Imaging Workshop 21

Response of an Interferometer to a Binary Star 24 September 2001 ATNF Imaging Workshop 22

Examples of SUSI Observations of Beta Centauri 24 September 2001 ATNF Imaging Workshop 23

The Orbit of Beta Centauri determined from SUSI Observations 24 September 2001 ATNF Imaging Workshop 24

The SUSI Programme • Observing Programme Ø Single stars for Te, R and L Ø Spectroscopic binaries for M and d, as well as Te, R and L Ø Pulsating stars (e. g. Cepheids) for d • Technical Programme Ø Installation and commissioning of “red” beam combination system 24 September 2001 ATNF Imaging Workshop 25

24 September 2001 ATNF Imaging Workshop 26

Ground-Based Long-Baseline Optical/IR Interferometers KEY: C - Closed; W - Working; UC - Undergoing commissioning or under construction 24 September 2001 ATNF Imaging Workshop 27