A Slit Mask Integral Field Unit for the Robert Stobie Spectrograph on the Southern African Large Telescope: I. Instrument Development

Abstract: Integral field spectroscopy (IFS) has been added as a new observation mode to the Robert Stobie Spectrograph (RSS), the workhorse multi-mode instrument on the Southern African Large Telescope. RSS operates as an imaging spectrograph covering 320-900 nm with a spectral resolution--slit-width product of 6600 arcsec. Using fiber optics and prismatic fold mirrors, we have been able to construct compact integral field units (IFUs) that fit within the same volume as the long-slit cassettes (134 mm x 130 mm x 8 mm). These `slit mask' IFUs (SMIs) direct the telescope beam into a 2D sky-facing fiber array routed in the focal plane dimension into an 8$\arcmin$ 1D pseudo-slit, with fiber output redirected back into the spectrograph collimator. The first completed unit, SMI-200, features 303 object fibers and 24 sky fibers, providing a spatial resolution of 0.8$\arcsec$ (200 $\upmu$m core diameter) over a field of view (FOV) of 22.5$\arcsec$ x 17.6$\arcsec$. This paper describes the specific design considerations and design and fabrication strategies to maximize performance and minimize risk during construction, given the demanding and highly constrained cassette geometry. We also detail mapping and laboratory characterization of the IFU. Laboratory measurements demonstrate a total throughput of 77\%, but an effective throughput of only 55-60\% within the RSS collimator acceptance beam of f/4.2 due to losses dominated by focal ratio degradation (FRD) induced by sharp bend radii imposed by the tight cassette volume.