Design and Assembly of the Low Resolution Spectrograph 2 Integral Field Unit

The Low Resolution Spectrograph 2 (LRS2) was recently deployed on the Hobby-Eberly Telescope ( HET). LRS2 consists of four spectrographic channels, each covering adjacent wavelength bands from 360-1050nm which are fed with a fiber optic integral field unit (IFU). The integral field unit developed for this instrument represents a transformative approach to expanding the wavelength coverage of integral field spectrographs. The unique input feed of the IFU serves two functions; combining the wavelength coverage of two spectrographic channels to one spatial field on sky, and expanding the field of view of each individual fiber to eliminate the interstitial space between fibers on sky. The spectral multiplexing is achieved with dichroic beam splitter and collimator, while the focal reduction is achieved with a pair of micro lens arrays. The optical components required micron scale alignment precision in a compact mechanical package to allow integration on the telescope focal surface. Here we report on the design, assembly, and performance of the IFU.