A near infrared integral field spectrograph for the Southern African Large Telescope (SALT)

Abstract

Washburn Astronomical Laboratories of the University of Wisconsin-Madison Astronomy Department is developing a near infrared (NIR) integral field spectrograph for the 11-meter Southern African Large Telescope (SALT). This instrument will extend SALT’s capabilities into the NIR, providing medium resolution spectroscopy over the wavelength range of 0.8 to 1.7 microns. The integral field unit (IFU) is optimized for sampling nearby galaxies with an on-sky hexagonal extent of 24 x 28 arcsec containing 217 fibers of 1.33 arcsec diameter (median SALT seeing is 1.5 arcsec). Two separate blocks of 15 sky fibers are adjustable to distances ranging 54 to 165 arcsec from the IFU. This spectrograph, formerly known as RSS-NIR, was originally designed to mount at prime focus coupled to an optical spectrograph through a dichroic beam-splitter. The need to simplify telescope operations at prime focus prompted its reconfiguration into a fiber-fed, cooled, bench spectrograph, resulting in lower instrumental thermal background with a separate cooled collimator, stabilization of the pupil illumination in the spectrograph due to the azimuthal scrambling properties of fibers, and higher throughput at short wavelengths. Field-flattening and sky subtraction with the existing slit spectrograph has been challenging due to SALT’s varying pupil as the instrument payload tracks across the fixed primary mirror during observations. Simulations show that fiber scrambling of the pupil will improve the achievable sky subtraction residuals by 1-2 orders of magnitude. In this paper we present an overview of the reconfigured spectrograph design, its improved expected performance, and the new science drivers for NIR integral field spectroscopy.