SixPak - a wide-field IFU for the William Herschel Telescope Lars B. Venema* a , Ton Schoenmaker a , Marc Verheijen c , Scott Trager c , René Rutten b , Matthew Bershady d , Søren Larsen e , Reynier Peletier c , and Marco Spaans c a ASTRON, Netherl. Foundation for Research in Astronomy, PO Box 2, 7900 AA Dwingeloo, NL; b ING, Apartado de Correos 321, E-38700 Santa Cruz de la Palma, Canary Islands, Spain c Kapteyn Astronomical Institute, University of Groningen, PO Box 800, 9700 AV Groningen, NL d University of Wisconsin, Dep. of Astronomy, 475 North Charter Street, Madison, WI 53706, USA e Astronomical Institute, Utrecht University, Princetonplein 5, 3584 CC Utrecht, NL ABSTRACT We intend to construct SixPak, a wide-field fibre-based IFU for the 4.2-meter William Herschel Telescope on La Palma. The fibre bundle will consist of 238 fibres, each 3.0 arcsec in diameter, piping light from the Nasmyth focal plane of the WHT to the existing WYFFOS bench spectrograph. A total of 217 fibres will be densely packed to span a hexagonal field of view of 64 x 55 arcsec. The remaining 21 fibres will collect light from the sky background. SixPak is optimized for 2-dimensional spectroscopy at intermediate resolutions of extended objects of low surface brightness. At Nasmyth focus, a focal reducer matches the f-ratio of the telescope (f/11) to the "optimal" f-ratio of the fibres (f/3) to reduce the losses due to focal ratio degradation in the fibres. Microlenses convert the output f-ratio of the fibres to the f-ratio of the WYFFOS collimator (f/8.2). By means of an exchangeable slit at the pupils of the microlenses, a spectral resolution of R = 10,000 can be achieved. The intention is that SixPak will be open for general use in order to allow easy access to the broadest possible astronomical community. Keywords: Fibre fed integral field spectrometer, WHT, WYFFOS 1. INTRODUCTION SixPak is an integral wide field fibre unit between the 4.2m William Herschel Telescope (WHT) on La Palma and its spectrometer WYFFOS. The fibre-bundle will span a hexagonal field-of-view, 1 arcminute in diameter, piping light through 238 fibres from the Nasmyth focal plane of the WHT to the existing WYFFOS bench spectrograph. It will allow for 2-dimensional wavelength-agile spectroscopy at intermediate resolutions (R=10,000) of extended objects of low surface brightness, thus enabling a plethora of scientific investigations that closely match several areas of astrophysical research in The Netherlands. In particular, SixPak will be used to study the stellar kinematics in the faint outskirts of spiral and elliptical galaxies, to study spatially resolved stellar populations and abundances, to perform crowded-field spectroscopy of dense star clusters and clusters of galaxies, and to investigate the interstellar medium in the Milky Way and external galaxies. The design of SixPak is strongly based on earlier experience with SparsePak on the WIYN telescope’s Bench Spectrograph (Bershady et al 2004a; 2004b) and P-PAK on the PMAS spectrograph on the 3.5m telescope on Calar Alto (Verheijen et al 2004; Kelz et al 2005). SixPak will be commissioned as a common user instrument to allow easy access to the broadest possible astronomical community. This paper first discusses the science drivers behind this instrument and subsequently discusses the instrument concept. The paper concludes with some the performance estimates and a comparison with P-PAK on the Calar Alto telescope. 2. SCIENCE CASE 2.1 The Disk-Mass Project: dark and luminous matter in spiral galaxies Scientific motivation A major roadblock in testing galaxy formation models is the disk-halo degeneracy; density profiles of dark matter haloes as inferred from oft-used rotation curve decompositions depend critically on the adopted M/L of the disk component. An often used refuge to circumvent this degeneracy is the adoption of the maximum-disk hypothesis (van Albada & Sancisi 1986). However, this hypothesis remains unproven. Bell & de Jong (2001) have shown that stellar population synthesis Ground-based and Airborne Instrumentation for Astronomy II, edited by Ian S. McLean, Mark M. Casali, Proc. of SPIE Vol. 7014, 70140L, (2008) · 0277-786X/08/$18 · doi: 10.1117/12.789130 Proc. of SPIE Vol. 7014 70140L-1