LRS2: the new facility low resolution integral field spectrograph for the Hobby-Eberly Telescope

摘要

The second generation Low Resolution Spectrograph (LRS2) is a new facility instrument for the Hobby-Eberly Telescope (HET). Based on the design of the Visible Integral-field Replicable Unit Spectrograph (VIRUS), which is the new flagship instrument for carrying out the HET Dark Energy Experiment (HETDEX), LRS2 provides integral field spectroscopy for a seeing-limited field of 12"×6". For LRS2, the replicable design of VIRUS has been leveraged to gain broad wavelength coverage from 370 nm to 1.0 μm, spread between two fiber-fed dual-channel spectrographs, each of which can operate as an independent instrument. The blue spectrograph, LRS2-B, covers 370 ≤ λ (nm) ≤ 470 and 460 ≤ λ (nm) ≤ 700 at fixed resolving powers of R = λ/δλ ≈ 1900 and 1100, respectively, while the red spectrograph, LRS2-R, covers 650 ≤ λ (nm) ≤ 842 and 818 ≤ λ (nm) ≤ 1050 with both of its channels having R ≈ 1800. In this paper, we present a detailed description of the instrument's design in which we focus on the departures from the basic VIRUS framework. The primary modifications include the fore-optics that are used to feed the fiber integral field units at unity fill-factor, the cameras' correcting optics and detectors, and the volume phase holographic grisms. We also present a model of the instrument's sensitivity and a description of specific science cases that have driven the design of LRS2, including systematically studying the spatially resolved properties of extended Lyα blobs at 2 ＜ z ＜ 3. LRS2 will provide a powerful spectroscopic follow-up platform for large surveys such as HETDEX.