John W. O'Byrne

GNOSIS: the first instrument to use fiber Bragg gratings for OH suppression

The near-infrared is an important part of the spectrum in astronomy, especially in cosmology because the light from objects in the early universe is redshifted to these wavelengths. However, deep near-infrared observations are extremely difficult to make from ground-based telescopes due to the bright background from the atmosphere. Nearly all of this background comes from the bright and narrow emission lines of atmospheric hydroxyl (OH) molecules. The atmospheric background cannot be easily removed from data because the brightness fluctuates unpredictably on short timescales. The sensitivity of ground-based optical astronomy far exceeds that of near-infrared astronomy because of this long-standing problem. GNOSIS is a prototype astrophotonic instrument that utilizes “OH suppression fibers” consisting of fiber Bragg gratings and photonic lanterns to suppress the 103 brightest atmospheric emission doublets between 1.47 and 1.7µm. GNOSIS was commissioned at the 3.9m Anglo-Australian Telescope with the IRIS2 spectrograph to demonstrate the potential of OH suppression fibers, but may be potentially used with any telescope and spectrograph combination. Unlike previous atmospheric suppression techniques GNOSIS suppresses the lines before dispersion and in a manner that depends purely on wavelength. We present the instrument design and report the results of laboratory and on-sky tests from commissioning. While these tests demonstrated high throughput (� 60%) and excellent suppression of the skylines by the OH suppression fibers, surprisingly GNOSIS produced no significant reduction in the interline background and the sensitivity of GNOSIS+IRIS2 is about the same as IRIS2. It is unclear whether the lack of reduction in the interline background is due to physical sources or systematic errors as the observations are detector noise dominated. OH suppression fibers could potentially impact ground-based astronomy at the level of adaptive optics or greater. However, until a clear reduction in the interline background and the corresponding increasing in sensitivity is demonstrated optimized OH suppression fibers paired with a fiber-fed spectrograph will at least provide a real benefit at low resolving powers. Subject headings: atmospheric effects – infrared: diffuse background – instrumentation: miscellaneous

Characterization of hexabundles: initial results

New multicore imaging fibre bundles – hexabundles – being developed at the University of Sydney will provide simultaneous integral field spectroscopy for hundreds of celestial sources across a wide angular field. These are a natural progression from the use of single fibres in existing galaxy surveys. Hexabundles will allow us to address fundamental questions in astronomy without the biases introduced by a fixed entrance aperture. We have begun to consider instrument concepts that exploit hundreds of hexabundles over the widest possible field of view. To this end, we have compared the performance of a 61-core fully fused hexabundle and five lightly fused bundles with seven cores each. All fibres in the bundles have 100-μm cores. In the fully fused bundle, the cores are distorted from a circular shape in order to achieve a higher fill fraction. The lightly fused bundles have circular cores and five different cladding thicknesses which affect the fill fraction. We compare the optical performance of all the six bundles and find that the advantage of smaller interstitial holes (higher fill fraction) is outweighed by the increase in modal coupling, cross-talk and the poor optical performance caused by the deformation of the fibre cores. Uniformly high throughput and low cross-talk are essential for imaging faint astronomical targets with sufficient resolution to disentangle the dynamical structure. Devices already under development will have between 100 and 200 lightly fused cores, although larger formats are feasible. The light-weight packaging of hexabundles is sufficiently flexible to allow existing robotic positioners to make use of them.

Orbital parameters, masses and distance to β Centauri determined with the Sydney University Stellar: Interferometer and high-resolution spectroscopy

The bright southern binary star β Centauri (HR 5267) has been observed with the Sydney University Stellar Interferometer (SUSI) and spectroscopically with the European Southern Observatory Coude Auxiliary Telescope and Swiss Euler telescope at La Silla. The interferometric observations have confirmed the binary nature of the primary component and have enabled the determination of the orbital parameters of the system. At the observing wavelength of 442 nm the two components of the primary system have a magnitude difference of 0.15 ± 0.02. The combination of interferometric and spectroscopic data gives the following results: orbital period 357.00 ± 0.07 d, semimajor axis 25.30 ± 0.19 mas, inclination 67. ◦ 4 ± 0. ◦ 3, eccentricity 0.821 ± 0.003, distance 102.3 ± 1.7 pc, primary and secondary masses M 1 = ( ◦ )( ◦ ) .

Suppression of the near-infrared OH night sky lines with fibre Bragg gratings - first results

The background noise between 1 and 1.8 ?mu m in ground-based instruments is dominated by atmospheric emission from hydroxyl molecules. We have built and commissioned a new instrument, the Gemini Near-infrared OH Suppression Integral Field Unit (IFU) System (GNOSIS), which suppresses 103 OH doublets between 1.47 and 1.7?mu m by a factor of 1000 with a resolving power of 10?000. We present the first results from the commissioning of GNOSIS using the IRIS2 spectrograph at the Anglo-Australian Telescope. We present measurements of sensitivity, background and throughput. The combined throughput of the GNOSIS fore-optics, grating unit and relay optics is 36?per cent, but this could be improved to 46?per cent with a more optimal design. We measure strong suppression of the OH lines, confirming that OH suppression with fibre Bragg gratings will be a powerful technology for low-resolution spectroscopy. The integrated OH suppressed background between 1.5 and 1.7 mu m is reduced by a factor of 9 compared to a control spectrum using the same system without suppression. The potential of low-resolution OH-suppressed spectroscopy is illustrated with example observations of Seyfert galaxies and a low-mass star. The GNOSIS background is dominated by detector dark current below 1.67 mu m and by thermal emission above 1.67 mu m. After subtracting these, we detect an unidentified residual interline component of 860 +/- 210 photons s-1 m-2?arcsec-2?mu m-1, comparable to previous measurements. This component is equally bright in the suppressed and control spectra. We have investigated the possible source of the interline component, but were unable to discriminate between a possible instrumental artefact and intrinsic atmospheric emission. Resolving the source of this emission is crucial for the design of fully optimized OH suppression spectrographs. The next-generation OH suppression spectrograph will be focused on resolving the source of the interline component, taking advantage of better optimization for a fibre Bragg grating feed incorporating refinements of design based on our findings from GNOSIS. We quantify the necessary improvements for an optimal OH suppressing fibre spectrograph design.

The radius and mass of the subgiant star β Hyi from interferometry and asteroseismology

We have used the Sydney University Stellar Interferometer to measure the angular diameter of β Hydri. This star is a nearby G2 subgiant the mean density of which was recently measured with high precision using asteroseismology. We determine the radius and effective temperature of the star to be 1.814 ± 0.017 R⊙ (0.9 per cent) and 5872 ± 44 K (0.7 per cent) respectively. By combining the radius with the mean density, as estimated from asteroseismology, we make a direct estimate of the stellar mass. We find a value of 1.07 ± 0.03 M⊙ (2.8 per cent), which agrees with published estimates based on fitting in the Hertzsprung–Russell diagram, but has much higher precision. These results place valuable constraints on theoretical models of β Hyi and its oscillation frequencies.

A new determination of the orbit and masses of the Be binary system δ Scorpii

The binary starδ Sco (HD143275) underwent remarkable brightening in the visible in 2000, and continues to be irregularly variable. The system was observed with the Sydney University Stellar Interferometer (SUSI) in 1999, 2000, 2001, 2006 and 2007. The 1999 observations were consistent with predictions based on the previously published orbital elements. The subsequent observations can only be explained by assuming that an optically bright emission region with an angular size of 2 ± 1 mas formed around the primary in 2000. By 2006/2007 the size of this region grew to an estimated 4m as. We have determined a consistent set of orbital elements by simultaneously fitting all the published interferometric and spectroscopic data as well as the SUSI data reported here. The resulting elements and the brightness ratio for the system measured prior to the outburst in 2000 have been used to estimate the masses of the components. We find MA = 15 ± 7M � and MB = 8.0 ± 3.6 M� . The dynamical parallax is estimated to be 7.03 ± 0.15 mas, which is in good agreement with the revised Hipparcos parallax.

Sydney University Stellar Interferometer

The Sydney University Stellar Interferometer (SUSI) is a long baseline optical interferometer located at the Paul Wild Observatory in northern New South Wales, some 400km NNW of Sydney. SUSI has been designed to measure the angular sizes of stars of essentially all spectral types and luminosity classes and to measure the angular separations of close binary stars. In addition to the science programs planned for SUSI, the technical features of the instrument dictated by these programs are discussed. The current status of the instrument and science programs, and the plans for further development of the instrument are described.

Design of optically path-length-matched, three-dimensional photonic circuits comprising uniquely routed waveguides.

A method for designing physically path-length-matched, three-dimensional photonic circuits is described. We focus specifically on the case in which all the waveguides are uniquely routed from the input to output-a problem that has not been addressed to date and that allows for the waveguides to be used in interferometric measurements. Circuit elements were fabricated via the femtosecond laser direct-write technique. We demonstrate via interferometric methods that the fabricated circuits were indeed optically path-length matched to within 45 μm, which is within the coherence length required for many applications.

The radius and other fundamental parameters of the F9 V star β Virginis

We have used the Sydney University Stellar Interferometer (SUSI) to measure the angular diameter of the F9 V star β Virginis. After correcting for limb darkening and combining with the revised Hipparcos parallax, we derive a radius of 1.703 ± 0.022 R ⊙ (1.3 per cent). We have also calculated the bolometric flux from published meas urements which, combined with the angular diameter, implies an effective temperature of 6059 ± 49 K (0.8 per cent). We also derived the luminosity of β Vir to be L = 3.51 ± 0.08 L ⊙ (2.1 per cent). Solar-like oscillations were measured in this star by Carrier et al. (2005) an d using their value for the large frequency separation yields the mean stellar density with an uncertainty of about 2 per cent. Our constraints on the fundamental parameters of β Vir will be important to test theoretical models of this star and its oscillations.

What does a physics undergraduate education give you? A perspective from Australian physics

In a study to assess how effectively undergraduate physics studies have prepared students for the workplace, we attempted to locate and interview traditional 3-year or 4-year physics students who had graduated in the past five years (2000 to 2004), and the employers of these graduates. The study was limited to recent graduates who have majored in physics and not obtained further or concurrent degrees. Overseas studies of the destinations of physics graduates referred to in this paper have not isolated the group we interviewed as a distinct group. A major finding was that the number of these graduates was unexpectedly low. Indeed, most physics graduates have two degrees. Interviews with graduates and employers suggest that physics graduates have particular strengths in problem solving and are good at applying their skills at the workplace.