GMTIFS is a first generation instrument for the Giant Magellan Telescope (GMT). It is a combined Imager and Integral Field Spectrograph (IFS) designed to work with the Adaptive Optics (AO) Systems of the GMT. Working at the diffraction limit of the GMT and satisfying the challenging AO interface requirements and constraints results in unique optical challenges. We describe two of these challenges and how we have addressed them. The GMT has a direct feed architecture that maximizes transmission and reduces emissivity. This means that the cryostat window is tilted to reflect visual wavelengths to the external Visual Wave Front Subsystem (VWS). For a plane-parallel window, this tilt causes astigmatism in the transmitted beam that must be corrected. A corrective system using two plates, tilted and slightly wedged in opposite directions, is used. Geometry and performance of the system is described. Another challenging problem is the optical design of the anamorphic field projector. The Integral Field Unit of GMTIFS requires that a small field delivered to it be projected onto an Image Slicer at much larger scale, with the magnification in the spectral direction being twice that in the spatial direction so that the spaxels are square when referred to the sky. Output images must be coincident in the spectral and spatial projections in both the field and pupil domains. Additionally, field and pupil image locations must be independently controllable so that they can be made coincident for interchangeable units that provide a range of output field scales. A two-mirror system that satisfies these requirements is described.
GMTIFS is the first-generation adaptive optics integral-field spectrograph for the GMT, having been selected through a competitive review process in 2011. The GMTIFS concept is for a workhorse single-object integral-field spectrograph, operating at intermediate resolution (R~5,000 and 10,000) with a parallel imaging channel. The IFS offers variable spaxel scales to Nyquist sample the diffraction limited GMT PSF from λ ~ 1-2.5 μm as well as a 50 mas scale to provide high sensitivity for low surface brightness objects. The GMTIFS will operate with all AO modes of the GMT (Natural guide star - NGSAO, Laser Tomography – LTAO, and, Ground Layer - GLAO) with an emphasis on achieving high sky coverage for LTAO observations. We summarize the principle science drivers for GMTIFS and the major design concepts that allow these goals to be achieved.
Instrument development for the 24m Giant Magellan Telescope (GMT) is described: current activities, progress, status, and schedule. One instrument team has completed its preliminary design and is currently beginning its final design (GCLEF, an optical 350-950 nm, high-resolution and precision radial velocity echelle spectrograph). A second instrument team is in its conceptual design phase (GMACS, an optical 350-950 nm, medium resolution, 6-10 arcmin field, multi-object spectrograph). A third instrument team is midway through its preliminary design phase (GMTIFS, a near-IR YJHK diffraction-limited imager/integral-field-spectrograph), focused on risk reduction prototyping and design optimization. A fourth instrument team is currently fabricating the 5 silicon immersion gratings needed to begin its preliminary design phase (GMTNIRS, a simultaneous JHKLM high-resolution, AO-fed, echelle spectrograph). And, another instrument team is focusing on technical development and prototyping (MANIFEST, a facility robotic, multifiber feed, with a 20 arcmin field of view). In addition, a medium-field (6 arcmin, 0.06 arcsec/pix) optical imager will support telescope and AO commissioning activities, and will excel at narrow-band imaging. In the spirit of advancing synergies with other groups, the challenges of running an ELT instrument program and opportunities for cross-ELT collaborations are discussed.
ULTIMATE is an instrument concept under development at the AAO, for the Subaru Telescope, which will have the unique combination of ground layer adaptive optics feeding multiple deployable integral field units. This will allow ULTIMATE to probe unexplored parameter space, enabling science cases such as the evolution of galaxies at z ~ 0:5 to 1.5, and the dark matter content of the inner part of our Galaxy. ULTIMATE will use Starbugs to position between 7 and 13 IFUs over a 14 × 8 arcmin field-of-view, pro- vided by a new wide-field corrector. All Starbugs can be positioned simultaneously, to an accuracy of better than 5 milli-arcsec within the typical slew-time of the telescope, allowing for very efficient re-configuration between observations. The IFUs will feed either the near-infrared nuMOIRCS or the visible/ near-infrared PFS spectrographs, or both. Future possible upgrades include the possibility of purpose built spectrographs and incorporating OH suppression using fibre Bragg gratings. We describe the science case and resulting design requirements, the baseline instrument concept, and the expected performance of the instrument.
The recent availability of large format near-infrared detectors with sub-election readout noise is revolutionizing our approach to wavefront sensing for adaptive optics. However, as with all near-infrared detector technologies, challenges exist in moving from the comfort of the laboratory test-bench into the harsh reality of the observatory environment. As part of the broader adaptive optics program for the GMT, we are developing a near-infrared Lucky Imaging camera for operational deployment at the ANU 2.3 m telescope at Siding Spring Observatory. The system provides an ideal test-bed for the rapidly evolving Selex/SAPHIRA eAPD technology while providing scientific imaging at angular resolution rivalling the Hubble Space Telescope at wavelengths λ = 1.3-2.5 μm.
Routine photometric monitoring at near-ultraviolet wavelengths (< 400 nm) is compromised from the ground due to highly variable atmospheric transmission and cloud cover. The GLUV project will mount a modest sized telescope (200 mm primary) on a series of long-duration high-altitude balloon flights. The wide field camera (~7 deg2) will perform high cadence (10-300 second rolling integrations) each night for campaign durations of three to six months. The principle science mission is the early-time detection of supernova shock-breakout at near-ultraviolet wavelengths. Additionally, early design analysis has shown the system is also able to probe the atmospheric composition of exoplanet atmospheres through the combination of UV transit measurements with ground-based measurements at longer wavelengths. In this presentation we consider the specifications for a long-duration balloon platform for such a mission, focusing on the necessary mission requirements (sensitivity, sky coverage, cadence etc.) and the available platform suitability. Particular attention is paid to platform flight altitude and atmospheric transmission.
GMTIFS requires a deformable mirror (DM) as part of its on-instrument wavefront sensor (OIWFS). The DM facilitates wavefront correction for the off-axis natural guide star, with the objective being to maximize the energy in the diffraction core and improve the signal-to-noise ratio of the guide star position measurement. It is essential that the OIWFS be positionally stable with respect to the science field. The use of J–K to observe the guide star, and thus the need to limit thermal background, essentially requires the DM in the OIWFS to be operated at or below −40°C. This is below the standard operating temperature range of currently available DMs. In cooperation with the manufacturers we are testing the performance of three DMs at temperatures from ambient to −45°C, or cooler. In the context of the OIWFS adequate stroke, open-loop positioning stability, hysteresis, interactuator surface figure and dynamic response are key performance criteria. A test system based around high spatial sampling of the DM aperture with a Shack-Hartmann wavefront sensor has been built. The opto-mechanical design permits a DM to be contained in a cryostat so that it may be cooled in isolation. We describe this test system and the test cases that are applied to the ALPAO DM-69, Boston MicroMachines 492DM and the IrisAO PTT111 deformable mirrors. Preliminary results at ambient temperatures are presented.
To achieve the high adaptive optics sky coverage necessary to allow the GMT Integral-Field Spectrograph (GMTIFS) to access key scientific targets, the on-instrument adaptive-optics wavefront-sensing (OIWFS) system must patrol the full 180 arcsecond diameter guide field passed to the instrument. The OIWFS uses a diffraction limited guide star as the fundamental pointing reference for the instrument. During an observation the offset between the science target and the guide star will change due to sources such as flexure, differential refraction and non-sidereal tracking rates. GMTIFS uses a beam steering mirror to set the initial offset between science target and guide star and also to correct for changes in offset. In order to reduce image motion from beam steering errors to those comparable to the AO system in the most stringent case, the beam steering mirror is set a requirement of less than 1 milliarcsecond RMS. This corresponds to a dynamic range for both actuators and sensors of better than 1/180,000.
The GMTIFS beam steering mirror uses piezo-walk actuators and a combination of eddy current sensors and interferometric sensors to achieve this dynamic range and control. While the sensors are rated for cryogenic operation, the actuators are not. We report on the results of prototype testing of single actuators, with the sensors, on the bench and in a cryogenic environment. Specific failures of the system are explained and suspected reasons for them. A modified test jig is used to investigate the option of heating the actuator and we report the improved results. In addition to individual component testing, we built and tested a complete beam steering mirror assembly. Testing was conducted with a point source microscope, however controlling environmental conditions to less than 1 micron was challenging. The assembly testing investigated acquisition accuracy and if there was any un-sensed hysteresis in the system. Finally we present the revised beam steering mirror design based on the outcomes and lessons learnt from this prototyping.
A representative range of the rotary mechanisms proposed for use in GMTIFS is described. All are driven by cryogenically rated stepper motors. For each mechanism, angular position is measured by means of eddy current sensors arranged to function as a resolver. These measure the linear displacement of a decentered aluminum alloy target in two orthogonal directions, from which angular position is determined as a function of the displacement ratio. Resolver function and performance is described. For each mechanism, the mechanical design is described and the adequacy of positioning repeatability assessed. Options for improvement are discussed.
To achieve the high adaptive optics sky coverage necessary to allow the GMT Integral-Field Spectrograph to
access key scientific targets, the on-instrument adaptive-optics wavefront-sensing system must patrol the full 180
arcsecond diameter guide field passed to the instrument. Starlight must be held stationary on the wavefront
sensor (accounting for flexure, differential refraction and non-sidereal tracking rates) to ~ 1 milliarcsecond to
provide the stable position reference signal for deep AO observations and avoid introducing image blur. Hence a
tight tolerance of 1/180,000 is placed on the positioning and encoding accuracy for the cryogenic On-Instrument
Wave-Front Sensor feed. GMTIFS will achieve this requirement using a beam-steering mirror system as an
optical relay for starlight from across the accessible guide field. The system avoids hysteresis and backlash by
eliminating friction and avoiding gearing while maintaining high setting speed and accuracy with a precision
feedback loop. Here we present the design of the relay system and the technical solution deployed to meet the
challenging specifications for drive rate, accuracy and positional encoding of the beam-steering system.
The Giant Magellan Telescope (GMT) Integral-Field Spectrograph (GMTIFS)c is one of six potential first-light
instruments for the 25m-diameter Giant Magellan Telescope. The Australian National University has completed a
Conceptual Design Study for GMTIFS. The science cases for GMTIFS are summarized, and the instrument capabilities
and design challenges are described. GMTIFS will be the work-horse adaptive-optics instrument for GMT. It contains an
integral-field spectrograph (IFS) and Imager accessing the science field, and an On-Instrument Wave-Front Sensor
(OIWFS) that patrols the 90 arcsec radius guide field. GMTIFS will address a wide range of science from epoch of
reionization studies to forming galaxies at high redshifts and star and planet formation in our Galaxy. It will fully exploit
the Laser Tomography Adaptive Optics (LTAO) system on the telescope. The tight image quality and positioning
stability requirements that this imposes drive the design complexity. Some cryogenic mechanisms in the IFS must set to
~ 1 μm precision. The Beam-Steering mechanism in the OIWFS must set to milli-arcsecond precision over the guide
field, corresponding to ~ 1 μm precision in the f/8 focal plane. Differential atmospheric dispersion must also be corrected
to milli-arcsecond precision. Conceptual design solutions addressing these and other issues are presented and discussed.
KOALA, the Kilofibre Optimised Astronomical Lenslet Array, is a wide-field, high efficiency integral field unit
being designed for use with the bench mounted AAOmega spectrograph on the AAT. KOALA will have 1000
fibres in a rectangular array with a selectable field of view of either 1390 or 430 sq. arcseconds with a spatial
sampling of 1.25" or 0.7" respectively. To achieve this KOALA will use a telecentric double lenslet array with
interchangeable fore-optics. The IFU will feed AAOmega via a 31m fibre run. The efficiency of KOALA is
expected to be ≈ 52% at 3700A and ≈ 66% at 6563°Å with a throughput of > 52% over the entire wavelength
AAOmega is the new spectrograph for the 2dF fibre-positioning system on the Anglo-Australian Telescope. It is a bench-mounted, double-beamed design, using volume phase holographic (VPH) gratings and articulating cameras. It is fed by 392 fibres from either of the two 2dF field plates, or by the 512 fibre SPIRAL integral field unit (IFU) at Cassegrain focus. Wavelength coverage is 370 to 950nm and spectral resolution 1,000-8,000 in multi-Object mode, or 1,500-10,000 in IFU mode. Multi-object mode was commissioned in January 2006 and the IFU system will be commissioned in June 2006.
The spectrograph is located off the telescope in a thermally isolated room and the 2dF fibres have been replaced by new 38m broadband fibres. Despite the increased fibre length, we have achieved a large increase in throughput by use of VPH gratings, more efficient coatings and new detectors - amounting to a factor of at least 2 in the red. The number of spectral resolution elements and the maximum resolution are both more than doubled, and the stability is an order of magnitude better.
The spectrograph comprises: an f/3.15 Schmidt collimator, incorporating a dichroic beam-splitter; interchangeable VPH gratings; and articulating red and blue f/1.3 Schmidt cameras. Pupil size is 190mm, determined by the competing demands of cost, obstruction losses, and maximum resolution. A full suite of VPH gratings has been provided to cover resolutions 1,000 to 7,500, and up to 10,000 at particular wavelengths.
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.