Graham J. Murray

Prime Focus Spectrograph for the Subaru telescope: massively multiplexed optical and near-infrared fiber spectrograph

Abstract. The Prime Focus Spectrograph (PFS) is an optical/near-infrared multifiber spectrograph with 2394 science fibers distributed across a 1.3-deg diameter field of view at the Subaru 8.2-m telescope. The wide wavelength coverage from 0.38  μm to 1.26  μm, with a resolving power of 3000, simultaneously strengthens its ability to target three main survey programs: cosmology, galactic archaeology and galaxy/AGN evolution. A medium resolution mode with a resolving power of 5000 for 0.71  μm to 0.89  μm will also be available by simply exchanging dispersers. We highlight some of the technological aspects of the design. To transform the telescope focal ratio, a broad-band coated microlens is glued to each fiber tip. A higher transmission fiber is selected for the longest part of the cable system, optimizing overall throughput; a fiber with low focal ratio degradation is selected for the fiber-positioner and fiber-slit components, minimizing the effects of fiber movements and fiber bending. Fiber positioning will be performed by a positioner consisting of two stages of piezo-electric rotary motors. The positions of these motors are measured by taking an image of artificially back-illuminated fibers with the metrology camera located in the Cassegrain container; the fibers are placed in the proper location by iteratively measuring and then adjusting the positions of the motors. Target light reaches one of the four identical fast-Schmidt spectrograph modules, each with three arms. The PFS project has passed several project-wide design reviews and is now in the construction phase.

Modal noise prediction in fibre spectroscopy – I. Visibility and the coherent model

Fibre modal noise occurs in high spectral resolution, high signal-to-noise ratio applications. It imposes fundamental limits on the photometric accuracy of state-of-the-art fibre-spectrograph systems. In order to maximize the performance of current and future instruments it is therefore essential to predict fibre modal noise. To attain a predictive model we are using a dual approach, bringing theoretical assumptions in line with the experimental data obtained using a test-bench spectrograph. We show that the task of noise prediction can be reduced to determining the visibility of the modal pattern which can be measured at the detector plane. Subsequently, the visibility dependence of essential parameters is presented. This work will soon provide a basis for prediction of modal noise limitations in fibre-coupled spectrograph designs.

TEIFU: a high-resolution integral field unit for the William Herschel Telescope

In order to enhance the spectroscopic capabilities of the William Herschel Telescope (WHT) we have recently completed an integral field unit comprising 1000 elements. Integral field units maximize the efficiency of a spectrograph by re- formatting a 2D field in order to match the entrance slit of the camera. Such techniques enable high-resolution spectral data to be obtained over the whole field simultaneously, and are particularly suited for use with adaptive optics systems. TEIFU is an optical fiber system employing microlens arrays for input and output coupling. The field is divided into two halves, permitting object and background to be derived during the same exposure. In addition, the fields can be optically re-positioned to form a larger, single field for greater object coverage. Thus the observer can choose between different observing modes to emphasize background subtraction or contiguous field. The fore-optics can be changed to alter the image scale and to interface to the NAOMI adaptive optics system which is currently under construction. TEIFU in its present configuration as tested on the WHT, gives a spatial sampling of 0.25 arcsec with a total field of 7.8 by 7.0 arcsec, but a 0.125 arcsec sampling option may be provided. We are also considering an option to upgrade TEIFU for near IR operation. This paper will outline system design, operation and preliminary results.

Developments on the UK FMOS project for the Subaru Telescope

We describe the UK participation in the FMOS project to provide multi-object IR spectroscopy for the Subaru telescope. The UK is working on the design of an OH suppression IR spectrograph, this work comprises the optical design, the opto-mechanical layout, spectrograph thermal environment and cryogenics and detector control system. We give a progress report on the current design work.

Prime Focus Spectrograph (PFS) for the Subaru telescope: overview, recent progress, and future perspectives

PFS (Prime Focus Spectrograph), a next generation facility instrument on the 8.2-meter Subaru Telescope, is a very wide-field, massively multiplexed, optical and near-infrared spectrograph. Exploiting the Subaru prime focus, 2394 reconfigurable fibers will be distributed over the 1.3 deg field of view. The spectrograph has been designed with 3 arms of blue, red, and near-infrared cameras to simultaneously observe spectra from 380nm to 1260nm in one exposure at a resolution of ~1.6 - 2.7Å. An international collaboration is developing this instrument under the initiative of Kavli IPMU. The project is now going into the construction phase aiming at undertaking system integration in 2017-2018 and subsequently carrying out engineering operations in 2018-2019. This article gives an overview of the instrument, current project status and future paths forward.

End effects in optical fibres

The performance of highly multiplexed spectrographs is limited by focal ratio degradation (FRD) in the optical fibres. It has already been shown that this is caused mainly by processes concentrated around the mounting points at the ends of the fibres. We use the thickness of rings produced in the far-field when a fibre is illuminated by a collimated beam, to estimate the size of the region where the FRD is generated. This requires the development of a new model, using features of existing ray-tracing and wave-based models, which fits existing data very well. The results suggest that the amount of FRD is primarily determined by the length of fibre bonded into the supporting ferrule. We point out the implications for the production of future fibre systems.

Gemini-north multiobject spectrograph: integral field unit

The Gemini-North Multiobject Spectrograph (GMOS) includes a powerful capability for integral field spectroscopy - the first to be installed and used on an 8-10m telescope. GMOS is switched to this mode by the remote insertion of an integral field unit (IFU) into the focal plane in place of the masks used for multiobject spectroscopy. With 1500 lenslet-coupled fibres, it provides a total field of view exceeding 50 square arcseconds, including a separate field dedicated to background subtraction. We describe the design, construction and testing of the IFU and present performance results obtained during commissioning.

Design and Construction of the Fibre System for FMOS

A consortium of Japanese, Australian and UK groups has developed a fibre-fed near IR (J & H band) multi-object spectrographic facility (FMOS) for the Subaru telescope. In this second-generation instrument, a novel prime focus 400-fibre multi-object positioning system, ECHIDNA, is optically linked via twin cables to dual IR spectrographs. The spectrographs are located some distance away, on a dedicated platform two levels above Nasmyth. The Centre for Advanced Instrumentation at Durham University oversaw the design and construction of the optical fibre system linking ECHIDNA to the spectrographs. A modularised connector within the cable scheme and an integral back illumination unit additionally featured as part of the Durham work-package. At the time of writing (mid 2008) FMOS, including the fibre system, is installed and functional on-telescope, with commissioning currently underway. This paper provides an overview of the design and construction of the optical fibre system.

Cryogenic tests of volume-phase holographic gratings: results at 100 K

We present results from cryogenic tests of volume-phase holographic (VPH) gratings at ∼100 K. The aims of these tests are to see whether the diffraction efficiency as a function of wavelength is significantly different at a low temperature from that at room temperature and to see how the performance of a VPH grating is affected by a number of thermal cycles. We have completed ten cycles between room temperature and 100 K and find no clear evidence that the diffraction efficiency changes with temperature or with a successive thermal cycle.

Cryogenic tests of volume-phase holographic gratings

We present results from cryogenic tests of a Volume-Phase Holographic (VPH) grating at 200 K measured at near-infrared wavelengths. The aims of these tests were to see whether the diffraction efficiency and angular dispersion of a VPH grating are significantly different at a low temperature from those at a room temperature, and to see how many cooling and heating cycles the grating can withstand. We have completed 5 cycles between room temperature and 200 K, and find that the performance is nearly independent of temperature, at least over the temperature range which we are investigating. In future, we will not only try more cycles between these temperatures but also perform measurements at a much lower temperature (e.g., ~80 K).