George N. Dodsworth

GMOS: the GEMINI Multiple Object Spectrographs

The two Gemini multiple object spectrographs (GMOS) are being designed and built for use with the Gemini telescopes on Mauna Kea and Cerro Pachon starting in 1999 and 2000 respectively. They have four operating modes: imaging, long slit spectroscopy, aperture plate multiple object spectroscopy and area (or integral field) spectroscopy. The spectrograph uses refracting optics for both the collimator and camera and uses grating dispersion. The image quality delivered to the spectrograph is anticipated to be excellent and the design is driven by the need to retain this acuity over a large wavelength range and the full 5.5 arcminute field of view. The spectrograph optics are required to perform from 0.36 to 1.8 microns although it is likely that the northern and southern versions of GMOS will use coatings optimized for the red and blue respectively. A stringent flexure specification is imposed by the scientific requirement to measure velocities to high precision (1 - 2 km/s). Here we present an overview of the design concentrating on the optical and mechanical aspects.

Design and construction of the IMACS-IFU: a 2000-element integral field unit

The IMACS-IFU is an Integral Field Unit built for the IMACS spectrograph at the Magellan-I-Telescope at Las Campanas Observatory. It consists of two rectangular fields of 5 by 7 square arec seconds, spearated by roughly one arc minute. With a total number of 2000 spatial elements it is the second largest fiber-lenslet based IFU worldwide, working in a wavelength range between 400 and 900 nm. Due to the equally sized fields classical background subtraction, beam switching and shuffling are possible observation techniques. One particular design challenge was the single, half a metre long curved slit in combination with a non telecentric output. Besides the construction some preliminary results are described.

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.

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.

An ultraprecision fiber connector for FMOS

A fibre-fed near IR (J & H band) multi-object spectrograph (FMOS) is being constructed by a consortium of UK, Australian and Japanese groups for the SUBARU telescope on Mauna Kea, Hawaii. The prime focus of the telescope will support ECHIDNA, a 400-fibre multi-object positioning system. However, the IR spectrographs are located close to the Nasmyth platform, so an optical feed is required to deliver light from ECHIDNA to the spectrographs. The Astronomical Instrumentation Group at the University of Durham is undertaking the design and construction of a suitable fibre-optic downlink. To allow the prime focus unit that houses ECHIDNA to be removed, the fibre cabling is to include a connectorised break. The optical design of the fibre system also calls for a change in focal ratio from that delivered by ECHIDNA in order to couple light to the spectrographs with the greatest efficiency. This will be achieved in a custom designed connector head by means of a tailored lens array. The connector design will in addition incorporate a back-illumination system for fibre position determination at the prime focus. This paper describes the overall design of the connector that is to be employed. The modularity of the scheme and various innovative features are highlighted. A more advanced connector concept capable of dual bandwidth (visible & NIR) operation is also shown. Such a system could significantly enhance the science return from future ECHIDNA-type fibre instruments.

Integral field spectroscopy with the GEMINI multiobject spectrographs

We describe the integral field unit (IFU) which converts the Gemini Multiobject Spectrograph (GMOS) installed on the Gemini-North telescope to an integral field spectrograph,which produces spectra over a contiguous field of view of 7 × 5 arcsec with spatial sampling of 0.2 arcsecover the wavelength range 0.4-1.0 μm.GMOS is converted to this mode by the remote insertion of the IFU into thebeam in place of the masks used for the multiobject mode. A separate fieldof half the area of the main field, but otherwise identical, is alsoprovided to improve background subtraction. The IFU contains 1500lenslet-coupled fibres and was the first facility of any type for integralfield spectroscopy employed on an 8/10 m telescope.We describe the design, construction and testing of the GMOS IFU and present measurements of the throughput both in the laboratory and at the telescope. We compare these with a theoretical prediction made before construction started. All are in good agreement with each other, with the on-telescope throughput exceeding 60% (averaged over wavelength). Finallywe show an example of data obtained during commissioning to illustrate the power of the device.

Autofib--2: an automated fiber positioner for the prime focus of the William Herschel Telescope

We are currently testing an automated fibre positioner for the 4.2 m William Herschel Telescope (WHT). This instrument, known as Autofib-2, operates at the prime focus where it is able to utilize the full 1 degree field provided by the prime focus corrector (PFC). The robotic positioner is able to place 160 optical fibres in the focal plane of the WHT which feeds the light to a dedicated spectrograph (WYFFOS) located on the Nasmyth platform. This paper contains a description of the instrument which highlights the new techniques demanded by the prime focus plate scale and the scale distortions due to the PFC and its atmospheric dispersion compensator. These include robot vision to help achieve the high positioning accuracy and the use of two sky viewing probes to accurately determine the time dependent transformation from celestial coordinates to instrumental Cartesian coordinates. Also presented are the initial technical results on the performance of the instrument and the operational results of particular interest to the astronomical observer.

Design and construction of a fiber bundle connector using microlenses

A prototype of a multiway fibre bundle connector for astronomical applications is described. The connector provides for connection and disconnection in the fiber trains that feed an astronomical spectrograph. It also provides the more important function of converting the focal ratio from f/2 to f/5 because f/2 is too fast either for good transmission of light along a substantial length of fiber or for the input to typical astronomical spectrographs. The fiber bundle connector consists of 100 coupling fibers. It works over the full 0.9- to 1.8-?m wavelength range, and the chromatic aberration is well corrected in the design. The design principle and the construction of the connector are discussed. The measured coupling efficiency is up to 88%. The coupling efficiency is compared with a theoretical estimate, and good agreement is achieved. Possible further improvement is discussed.

The Gemini Multi-Object Spectrographs

Each GEMINI telescope will be equipped with a versatile optical/near-IR spectrographs which will fully exploit its large aperture and excellent image quality. The basic parameters of the GEMINI Multi-object Spectrographs (GMOS) are summarised in the table below.