Anthony Horton

The Sydney‐AAO Multi‐object Integral field spectrograph

We demonstrate a novel technology that combines the power of the multi-object spectrograph with the spatial multiplex advantage of an integral field spectrograph (IFS). The SydneyAAO (Australian Astronomical Observatory) Multi-object IFS (SAMI) is a prototype widefield system at the Anglo-Australian Telescope (AAT) that allows 13 imaging fibre bundles (‘hexabundles’) to be deployed over a 1-degree diameter field of view. Each hexabundle comprises 61 lightly fused multi-mode fibres with reduced cladding and yields a 75 per cent filling factor. Each fibre core diameter subtends 1.6 arcsec on the sky and each hexabundle has a field of view of 15 arcsec diameter. The fibres are fed to the flexible AAOmega double-beam spectrograph, which can be used at a range of spectral resolutions (R = λ/δλ ≈ 1700–13 000) over the optical spectrum (3700–9500 A). We present the first spectroscopic results obtained with SAMI for a sample of galaxies at z ≈ 0.05. We discuss the prospects of implementing hexabundles at a much higher multiplex over wider fields of view in order to carry out spatially resolved spectroscopic surveys of 10 4 –10 5 galaxies.

A complex multi-notch astronomical filter to suppress the bright infrared sky

A long-standing and profound problem in astronomy is the difficulty in obtaining deep near-infrared observations due to the extreme brightness and variability of the night sky at these wavelengths. A solution to this problem is crucial if we are to obtain the deepest possible observations of the early Universe, as redshifted starlight from distant galaxies appears at these wavelengths. The atmospheric emission between 1,000 and 1,800 nm arises almost entirely from a forest of extremely bright, very narrow hydroxyl emission lines that varies on timescales of minutes. The astronomical community has long envisaged the prospect of selectively removing these lines, while retaining high throughput between them. Here we demonstrate such a filter for the first time, presenting results from the first on-sky tests. Its use on current 8 m telescopes and future 30 m telescopes will open up many new research avenues in the years to come.

Characterization and on-sky demonstration of an integrated photonic spectrograph for astronomy

We present results from the first on-sky demonstration of a prototype astronomical integrated photonic spectrograph (IPS) using the Anglo-Australian Telescope near-infrared imaging spectrometer (IRIS2) at Siding Spring Observatory to observe atmospheric molecular OH emission lines. We have succeeded in detecting upwards of 27 lines, and demonstrated the practicality of the IPS device for astronomy. Furthermore, we present a laboratory characterization of the device, which is a modified version of a commercial arrayed-waveguide grating multiplexer. We measure the spectral resolution full-width-half-maximum to be 0.75 +/- 0.05 nm (giving R = lambda/deltalambda = 2100 +/- 150 at 1500 nm). We find the free spectral range to be 57.4 +/- 0.6 nm and the peak total efficiency to be approximately 65%. Finally, we briefly discuss the future steps required to realize an astronomical instrument based on this technology concept.

Instruments without optics: an integrated photonic spectrograph

In recent years, a great deal of emphasis has been placed on achieving the diffraction limit with large aperture telescopes. For a well matched focal-plane instrument, the diffraction limit provides the highest possible angular resolution and sensitivity per pixel. But it offers another key advantage as we now show. Conventionally, as the telescope aperture D grows, the instrument size grows in proportion to D, and the cost increases as D2 or faster. However, an instrument that operates at the diffraction limit can break the trend of spiralling costs. In traditional instruments, the light must pass through a succession of large lenses, mirrors and gratings, making it difficult to conserve the integrity of such a small psf. An alternative approach, as we now show, is to couple the diffraction-limited beam directly into an integrated photonic spectrograph operating in low-order modes.

Coupling light into few-mode optical fibres I: The diffraction limit.

Multimode fibres are widely used in astronomy because of the ease of coupling light into them at a telescope focus. The photonics industry has given rise to a broad range of products but these are almost exclusively restricted to single-mode fibres, although some can be adapted for use in fibres that allow several modes to propagate. Now that astronomical telescopes are moving toward diffraction-limited performance through the use of adaptive optics (AO), we address the problem of coupling light into a few-mode fibre (FMF). We find that fibres with as few as ~5 guided modes share important characterisitcs with multimode fibres, in particular high coupling efficiency.We anticipate that future astronomical instruments at an AO-corrected focus will be able to exploit a broad class of photonic devices.

First Science with SAMI: A Serendipitously Discovered Galactic Wind in ESO 185-G031

We present the first scientific results from the Sydney-AAO Multi-Object IFS (SAMI) at the Anglo-Australian Telescope. This unique instrument deploys 13 fused fiber bundles (hexabundles) across a one-degree field of view allowing simultaneous spatially resolved spectroscopy of 13 galaxies. During the first SAMI commissioning run, targeting a single galaxy field, one object (ESO 185-G031) was found to have extended minor axis emission with ionization and kinematic properties consistent with a large-scale galactic wind. The importance of this result is twofold: (1) fiber bundle spectrographs are able to identify low surface brightness emission arising from extranuclear activity and (2) such activity may be more common than presently assumed because conventional multi-object spectrographs use single-aperture fibers and spectra from these are nearly always dominated by nuclear emission. These early results demonstrate the extraordinary potential of multi-object hexabundle spectroscopy in future galaxy surveys.

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

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.

CIRPASS: description, performance, and astronomical results

The Cambridge Infra-red Panoramic Survey Spectrograph (CIRPASS) is described. This near-infrared (NIR) spectrograph has been used on the 8m Gemini-South Telescope, the 3.9m Anglo-Australian Telescope (AAT) and the 4.2m William Herschel Telescope (WHT). Its performance in both integral field mode and multi-object mode is discussed and some scientific highlights are presented. A multi-IFU system, which is currently under construction, is also described.

HERMES: revisions in the design for a high-resolution multi-element spectrograph for the AAT

The AAO is building an optical high resolution multi-object spectrograph for the AAT for Galactic Archaeology. The instrument has undergone significant design revision over that presented at the 2008 Marseilles SPIE meeting. The current design is a 4-channel VPH-grating based spectrograph providing a nominal spectral resolving power of 28,000 and a high-resolution mode of 45,000 with the use of a slit mask. The total spectral coverage is about 1000 Angstroms for up to 392 simultaneous targets within the 2 degree field of view. Major challenges in the design include the mechanical stability, grating and dichroic efficiencies, and fibre slit relay implementation. An overview of the current design and discussion of these challenges is presented.