VIRUS2: IFU and mechanical interfaces to the 2.7 m Harlan J Smith telescope

The VIRUS2 instrument is a fiber-fed multiplexed integral field spectrograph consisting of 6 units, each with 4 spectral channels, to provide large on-sky coverage coupled with broad spectral coverage. As a new NSF-funded facility instrument on the McDonald Observatory, 2.7m Harlan J Smith Telescope (HJST), VIRUS2 will cover a 2.3 square arcminute field of view, with 1536, 2.5 arc-second spatial elements. An additional 192 sky fibers are deployed in a separate Integral Field Unit (IFU) for sky subtraction. This paper addresses those elements which connect VIRUS2 to the focal plane of the telescope, as well as the facility infrastructure required to support its functionality. A novel IFU Catenary Strain-Relief ( CSR) system has been designed, prototyped, and validated by field-testing, which converts the 6-axis motion of the straight-Cassegrain focal surface, to a fully-constrained location on the telescope pier, with no twisting of the IFU bundle. This approach prevents Focal Ratio Degradation (FRD) due to rotation-induced stresses at the Input Head, while enabling a 52% reduction in fiber length compared to that required for conventional routing around the declination and polar axes. A design for IFU deployment, and storage are also described which enables a single person to quickly and safely mount & detach the IFU at the focal surface. The IFU input, acquisition & guide camera, focus camera, and calibration system, as well as the instrument location, thermal management, and other critical support systems are also covered.