LRS2: design, assembly, testing, and commissioning of the second-generation low-resolution spectrograph for the Hobby-Eberly Telescope

The second generation Low Resolution Spectrograph (LRS2) is a new facility instrument for the Hobby-Eberly Telescope (HET) at McDonald Observatory. Designed as a powerful spectroscopic follow-up platform, LRS2 is based on the design of the HETs new Visible Integral-field Replicable Unit Spectrograph (VIRUS) and provides integral field spectroscopy for two seeing-limited fields of 6”×12” with unity fill factor. The replicable design of VIRUS has been leveraged for LRS2 to gain broad wavelength coverage from 370 nm to 1.0 μm, spread between two fiber-fed dual-channel spectrographs that operate in unison but observe independent fields that are separated by 100”. The blue spectrograph pair, LRS2-B, covers 364≤λ (nm) ≤ 467 and 454 ≤ λ (nm)≤700 at fixed resolving powers of R =λ/δλ≈2500 and 1400, respectively, while the red spectrograph pair, LRS2-R, covers 643≤λ (nm)≤845 and 823≤λ (nm)≤1056 with both of its channels having R≈2500. In this paper, a detailed description of the instrument's design, assembly, and laboratory testing is provided in which the focus is placed on the departures from the basic framework of the design and processes previously established for VIRUS. Both LRS2 spectrograph pairs have been successfully deployed on the HET, and commissioning efforts are ongoing. Using on-sky data, the performance of the spectrograph is compared to models of the instrumental sensitivity. The measured performance of LRS2 indicates that the instrument will provide efficient spectroscopic follow-up observations of individual targets, and will be especially powerful when combined with the extensive survey capabilities of VIRUS for HETDEX.