Phase-referenced Interferometry and Narrow-angle Astrometry with SUSI

This thesis describes the development of an astrometric facility at the Sydney University Stellar Interferometer (SUSI) with an aim to measure at high precision the relative astrometry of bright close binary stars and ultimately to detect the presence of exoplanets within those binary star systems through observations of the systems’ perturbed motion. At the core of the facility is a new beam combiner that is phase-referenced to an existing primary beam combiner in the visible wave- length regime. The latter provides post-processed fringe-tracking information to the former for fringe stabilization and coherent integration of pre-recorded stellar fringes using newly developed data reduction software. Interference fringe packets of a binary star are recorded alternately; first the fringe packet of the primary, then the secondary, finally back to the primary again. The measurement of the fringe packet separation is facilitated by an air-filled differential delay line and a network of interferometer-based metrology systems. Characterizations and initial astronomical observations carried out with the dual beam combiner setup demonstrated for the first time the success of the dual-star phase-referencing technique in visible (~1μm) wavelengths. The current astrometric precision is larger than 100μas while the long term astrometric accuracy is yet to be characterized. In a parallel development, a complementary observing method using only the primary beam combiner is also demonstrated in this thesis. Relative astrometry of binary stars up to ~0.8” separation with this technique has been demonstrated to have precision of better than 100μas. A simple detection limit analysis based on a list of target binary stars estimates up to two exoplanet detections can be achieved with SUSI if the new astrometric facility attains precision of 10μas while the primary beam combiner operates at its designed peak performance. Finally, one new stellar companion was resolved and a preliminary astrometry for another suspected companion was estimated from the astronomical observation data collected throughout the course of this thesis

Simulating a dual beam combiner at SUSI for narrow-angle astrometry

The Sydney University Stellar Interferometer (SUSI) has two beam combiners, i.e. the Precision Astronomical Visible Observations (PAVO) and the Microarcsecond University of Sydney Companion Astrometry (MUSCA). The primary beam combiner, PAVO, can be operated independently and is typically used to measure properties of binary stars of less than 50 milliarc- sec (mas) separation and the angular diameters of single stars. On the other hand, MUSCA was recently installed and must be used in tandem with the for- mer. It is dedicated for microarcsecond precision narrow-angle astrometry of close binary stars. The performance evaluation and development of the data reduction pipeline for the new setup was assisted by an in-house computer simulation tool developed for this and related purposes. This paper describes the framework of the simulation tool, simulations carried out to evaluate the performance of each beam combiner and the expected astrometric precision of the dual beam combiner setup, both at SUSI and possible future sites.Comment: 28 pages, 23 figures, accepted for publication in Experimental Astronomy. The final publication is available at http://link.springer.co

A low cost scheme for high precision dual-wavelength laser metrology

A novel method capable of delivering relative optical path length metrology with nanometer precision is demonstrated. Unlike conventional dual-wavelength metrology which employs heterodyne detection, the method developed in this work utilizes direct detection of interference fringes of two He-Ne lasers as well as a less precise stepper motor open-loop position control system to perform its measurement. Although the method may be applicable to a variety of circumstances, the specific application where this metrology is essential is in an astrometric optical long baseline stellar interferometer dedicated to precise measurement of stellar positions. In our example application of this metrology to a narrow-angle astrometric interferometer, measurement of nanometer precision could be achieved without frequency-stabilized lasers although the use of such lasers would extend the range of optical path length the metrology can accurately measure. Implementation of the method requires very little additional optics or electronics, thus minimizing cost and effort of implementation. Furthermore, the optical path traversed by the metrology lasers is identical with that of the starlight or science beams, even down to using the same photodetectors, thereby minimizing the non-common-path between metrology and science channels.Comment: 17 pages, 4 figures, accepted for publication in Applied Optic

Low-cost scheme for high-precision dual-wavelength laser metrology

A method capable of delivering relative optical path length metrology with nanometer precision is demonstrated. Unlike conventional dual-wavelength metrology, which employs heterodyne detection, the method developed in this work utilizes direct detection of interference fringes of two He-Ne lasers as well as a less precise stepper motor open-loop position control system to perform its measurement. Although the method may be applicable to a variety of circumstances, the specific application in which this metrology is essential is in an astrometric optical long baseline stellar interferometer dedicated to precise measurement of stellar positions. In our example application of this metrology to a narrow-angle astrometric interferometer, measurement of nanometer precision could be achieved without frequency-stabilized lasers, although the use of such lasers would extend the range of optical path length the metrology can accurately measure. Implementation of the method requires very little additional optics or electronics, thus minimizing the cost and effort of implementation. Furthermore, the optical path traversed by the metrology lasers is identical to that of the starlight or science beams, even down to using the same photodetectors, thereby minimizing the noncommon path between metrology and science channels.This research was supported under the Australian Research Council’s Discovery Project funding scheme. Y. K. was supported by the University of Sydney International Scholarship (USydIS)

Science and Technology Progress at the Sydney University Stellar Interferometer

This paper presents an overview of recent progress at the Sydney University Stellar Interferometer (SUSI). Development of the third-generation PAVO beam combiner has continued. The MUSCA beam combiner for high-precision differential astrometry using visible light phase referencing is under active development and will be the subject of a separate paper. Because SUSI was one of the pioneering interferometric instruments, some of its original systems are old and have become difficult to maintain. We are undertaking a campaign of modernization of systems: (1) an upgrade of the Optical Path Length Compensator IR laser metrology counter electronics from a custom system which uses an obsolete single-board computer to a modern one based on an FPGA interfaced to a Linux computer - in addition to improving maintainability, this upgrade should allow smoother motion and higher carriage speeds; (2) the replacement of the aged single-board computer local controllers for the siderostats and the longitudinal dispersion compensator has been completed; (3) the large beam reducing telescope has been replaced with a pair of smaller units with separate accessible foci. Examples of scientific results are also included.Comment: 10 pages, 9 Figure

The GRAVITY instrument software / High-level software

GRAVITY is the four-beam, near- infrared, AO-assisted, fringe tracking, astrometric and imaging instrument for the Very Large Telescope Interferometer (VLTI). It is requiring the development of one of the most complex instrument software systems ever built for an ESO instrument. Apart from its many interfaces and interdependencies, one of the most challenging aspects is the overall performance and stability of this complex system. The three infrared detectors and the fast reflective memory network (RMN) recorder contribute a total data rate of up to 20 MiB/s accumulating to a maximum of 250 GiB of data per night. The detectors, the two instrument Local Control Units (LCUs) as well as the five LCUs running applications under TAC (Tools for Advanced Control) architecture, are interconnected with fast Ethernet, RMN fibers and dedicated fiber connections as well as signals for the time synchronization. Here we give a simplified overview of all subsystems of GRAVITY and their interfaces and discuss two examples of high-level applications during observations: the acquisition procedure and the gathering and merging of data to the final FITS file.Comment: 8 pages, 7 figures, published in Proc. SPIE 9146, Optical and Infrared Interferometry IV, 91462

Self-phase-referencing interferometry with SUSI

The Sydney University Stellar Interferometer (SUSI) is being fitted with a new beam combiner, called the Micro-arcsecond University of Sydney Companion Astrometry instrument (MUSCA), for the purpose of high precision astrometry of bright binary stars. Operating in the visible wavelength regime where photon-counting and post-processing fringe tracking is possible, MUSCA will be used in tandem with SUSI's primary beam combiner, Precision Astronomical Visible Observations (PAVO), to record high spatial resolution fringes and thereby measure the separation of fringe packets of binary stars. With continued monitoring of stellar separation vectors at precisions in the tens of micro-arcseconds over timescales of years, it will be possible to search for the presence of gravitational perturbations in the orbital motion such as those expected from planetary mass objects in the system. This paper describes the first phase of the development, which includes the setup of the dual beam combiner system and the methodology applied to stabilize fringes of a star by means of self-phase-referencing.12 page(s

Alternative approach to precision narrow-angle astrometry for Antarctic long baseline interferometry

The conventional approach to high-precision narrow-angle astrometry using a long baseline interferometer is to directly measure the fringe packet separation of a target and a nearby reference star. This is done by means of a technique known as phase-referencing which requires a network of dual beam combiners and laser metrology systems. Using an alternative approach that does not rely on phase-referencing, the narrow-angle astrometry of several closed binary stars (with separation less than 2″), as described in this paper, was carried out by observing the fringe packet crossing event of the binary systems. Such an event occurs twice every sidereal day when the line joining the two stars of the binary is is perpendicular to the projected baseline of the interferometer. Observation of these events is well suited for an interferometer in Antarctica. Proof of concept observations were carried out at the Sydney University Stellar Interferometer (SUSI) with targets selected according to its geographical location. Narrow-angle astrometry using this indirect approach has achieved sub-100 micro-arcsecond precision

Alternative approach to precision narrow-angle astrometry for Antarctic long baseline interferometry

The conventional approach to high-precision narrow-angle astrometry using a long baseline interferometer is to directly measure the fringe packet separation of a target and a nearby reference star. This is done by means of a technique known as phase-referencing which requires a network of dual beam combiners and laser metrology systems. Using an alternative approach that does not rely on phase-referencing, the narrow-angle astrometry of several closed binary stars (with separation less than 2"), as described in this paper, was carried out by observing the fringe packet crossing event of the binary systems. Such an event occurs twice every sidereal day when the line joining the two stars of the binary is is perpendicular to the projected baseline of the interferometer. Observation of these events is well suited for an interferometer in Antarctica. Proof of concept observations were carried out at the Sydney University Stellar Interferometer (SUSI) with targets selected according to its geographical location. Narrow-angle astrometry using this indirect approach has achieved sub-100 micro-arcsecond precision.17 page(s

Imaging rapid rotators with the PAVO beam combiner at CHARA

Rotation plays a crucial role in the shaping and evolution of a star. Widely incorporated into early and late-stage stellar models, rotational effects remain poorly understood in main-sequence stars, mainly due to the absence of observations challenging contemporary models. The Precision Astronomical Visible Observations (PAVO) instrument, located at the Center for High Angular Resolution Astronomy (CHARA) array, provides the highest angular resolution yet achieved ( 0.3 mas) for stars V=8 magnitude and brighter. We describe instrumental techniques and advances implemented in PAVO@CHARA to observe heavily resolved targets and yield well calibrated closure phases which are key milestones on the pathway to delivery of the first-ever image in the visible of fast-rotating main-sequence star.15 page(s