Mechanical Design of VIRUS-P for the McDonald 2.7m Harlan J Smith Telescope

We present the mechanical and opto-mechanical design for the Prototype Visible Integral-Field Unit Spectrograph (VIRUS-P). The VIRUS-P instrument is the single unit prototype for the planned VIRUS instrument which consists of 192 spectrographs for the Hobby Eberly Telescope Dark Energy Experiment (HETDEX). The VIRUS Prototype is a test bed for the design and will be used for a survey on the McDonald 2.7m Harlan J. Smith Telescope. The mechanical design is driven by the need for high stability. The structure of the instrument is aluminum but the internal optical elements of the collimator and the camera are held in alignment with respect to each other using Invar metering rods. The spectrograph is fiber fed with 246 fibers in a hexagonal packing pattern at the telescope focal plane Integral Field Unit (IFU) and arranged in a slit at the input to the spectrograph. The reverse-Schmidt collimator articulates, and the Volume Phase Holographic (VPH) grating rotates independently relative to the fixed Schmidt camera to allow for versatile grating configurations during the prototype testing. Since the VIRUS spectrograph units will be mounted in a gravity neutral configuration on the HET, the prototype instrument is mounted on a gimbal at the folded cassegrain port of the 2.7m Smith Telescope to negate gravity vector changes.