The near infrared spectrograph for the Southern African Large Telescope has been developed at the University of Wisconsin. This spectrograph (see Wolf et al., these proceeding) is fiber-IFU fed, and the instrument placed in a -40C freezer in the spectrograph room at SALT. The rectangular 212 fiber object IFU and 38 fiber sky mini-IFU are placed in the fiber instrument feed (FIF) in the SALT payload. The IFUs patrol the telescope field and the separation between them can be adjusted. When the separation changes the bundles automatically tilt to maintain telecentricity. The 43m fibers are individually sheathed in Teflon tubes and placed in one of 4 rugged conduits which feed through the telescope payload, down the telescope truss, through the telescope pintle bearing and into the cooled Spectrograph room. Just outside of the instrument enclosure (freezer) the fibers split out of the conduit via break-out boxes into one of four strain relief boxes in which the fibers are individually layered in take-up loops and then fed through a rubber seal system in the freezer wall. Inside the freezer the fibers route down to the spectrograph slit where they are arrayed in one of 8 v-groove blocks which are individually adjustable in dovetail slots on the slit plate. In this paper we detail the design of the FIF, discuss the design and complex fabrication of the fiber cable; we also discuss the design of the breakout boxes, strain relief box and cold feedthrough. Finally, we will discuss the design and alignment of the fiber slit.
The optical fiber integral field unit (IFU) built to feed the near infrared (NIR) spectrograph for the 11-meter Southern African Large Telescope (SALT) has undergone prototyping and rigorous performance testing at Washburn Astronomical Laboratories of the University of Wisconsin-Madison Astronomy Department. The 43 m length of 256 fibers which make up the object and sky arrays and spares are routed from the SALT payload down into the spectrograph room in four separate cables. The IFU covers 344 arcsec2 on the sky, with the object array spanning a 552 arcsec2 near-rectangular area at roughly 56% fill-factor. Companion papers describe the mechanical design of the fiber cable that mitigates potential sources of mechanical strain on the optical fiber (Smith et al.) and details of the spectrograph (Wolf et al.). Here we present the results of the performance testing of various test cables as well as performance testing and end-to-end mapping of the fully-assembled science cable. The fiber optics experience an extreme temperature gradient at the ingress to the instrument enclosure held at -40 ◦C during operation. We find an increase in focal ratio degradation (FRD) when holding progressively longer lengths of test fiber at reduced temperature. However, we confirm that this temperature dependent FRD is negligible for our designed length of cold fiber. We also find negligible contributions to FRD from the rubber seal that breaches the room temperature strain relief box and the cold instrument enclosure. Our measurements characterize performance including the effects of internal fiber inhomogeneities, stress induced from fiber handling and termination, as well as any imperfections from end-polishing. We present the room-temperature laboratory performance measurements of the fully-assembled science cable; the effective total throughput the fiber cable delivers to the spectrograph collimator is 81±2.5% across all fibers accounting for all losses.
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