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.

Figures (21)

• Figure 1: Different SMI variations to serve different spatial coverage, sampling, and spectral resolution. Top row: effective fiber sizes. Middle row: effective IFU footprint. Numbers inside the IFUs give the ideal IFU and total fiber count (including sky) for each SMI. In the case of SMI-200, the usable number of fibers is 327 (303 for the object IFU and 24 for sky). Bottom row: Spectral resolution as a function of wavelength delivered by RSS using the six available Littrow VPH gratings (PG700, PG900, PG1300, PG1800, PG2300, and PG3000, where the number refers to the line-density in mm$^{-1}$) with different SMIs (200, 300, 400, left to right).
• Figure 2: The observational efficiency advantage of using SMI-200 over a 1 arcsec wide longslit. SMI-200 can observe objects (such as the NGC 1309, at $z\sim0.007$, shown here) with $\sim$20-30$\arcsec$ diameter in a single exposure (right panel), which would take $\sim$20 separate exposures to be covered by a 1 arcsec longslit (left panel) at slightly lower spatial and spectral resolution. The optical image (F435W, F555W, and F814W bands) of NGC1309 is obtained from the Hubble archive (NASA, ESA, STScI/AURA).
• Figure 3: SMI-200 object (center) sampling array and sky (on the two sides) sampling array footprint over a blue DSS image of NGC 5468, a nearly face-on spiral galaxy at $z\sim0.01$. Each red circle denotes a 0.88$\arcsec$ diameter SMI-200 fiber core.
• Figure 4: Mechanical layout and routing of fibers inside SMI-200 cassette, with top lid removed. Telescope light, coming up to the cassette bottom as oriented in this figure, is reflected 90 deg just before focus via a fold ('IFU') prism into the 2D object IFU and sky fiber arrays housed horizontally in a single stainless steel mount (grey block). Fibers are routed around both edges of the cassette and fed into two 1D slit blocks (stainless steel V-grooves, but color-coded red and blue), each of which is abutted to its own fold ('Slit') prism, that reflects the fiber output 90 deg back into the spectrograph collimator. The optical arrangement of prisms places the apparent fiber output focal plane parfocal with the telescope focal plane in air. For later reference, the blue slit block is referred to as the 'right' block (closest to the insertion/retraction notch on the top right) with blue fibers routed around the left side of the cassette; the red slit block is referred to as the 'left' block, etc. The yellow portion of the structure is machined from a single block of Uddeholm Stavax ESR soft-annealed steel. One of two guide rails for mechanism insertion can be seen at the lower right edge.
• Figure 5: Coss-section schematic view of the SMI-200 central region. Telescope light from the bottom passes through the IFU prism (bottom red triangle) and is reflected via its protected Ag-coated hypotenuse to enter the 2D object fiber array sitting horizontally at focus on the right. Fibers are then routed and fed into a one-dimensional pseudo-slit (left) at the slit prism (top red triangle), which folds the light into the spectrograph collimator above. Prism catheti are coated with broadband anti-reflection coatings to reduce losses due to Fresnel reflection. For reference, the vertical dimension of the object fiber array (3.98 mm) and single fiber (0.2 mm) of SMI-200 are shown in grey at the right and left, respectively. Other critical dimensions are labeled: 8 mm outer vertical height of the SMI cassette; 7.8 mm internal height of the nested IFU and slit prisms; 5.8 mm prism cathetus; and 5.59 mm prism clear aperture.