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

Abstract

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" x 6". For LRS2, the replicable design of VIRUS has been leveraged to gain broad wavelength coverage from 370 nm to 1.0 &mu;m, spread between two fiber-fed dual- channel spectrographs, each of which can operate as an independent instrument. The blue spectrograph, LRS2-B, covers 370 &lambda; (nm) &le; 470 and 460 &le; &lambda; (nm) &le; 700 at fixed resolving powers of R = &lambda;/&delta;&lambda; &asymp; 1900 and 1100, respectively, while the red spectrograph, LRS2-R, covers 650 &le; &lambda; (nm) &le; 842 and 818 &le; &lambda; (nm) &le; 1050 with both of its channels having R &asymp; 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 Ly&alpha; blobs at 2 &lt; <i>z</i> &lt; 3. LRS2 will provide a powerful spectroscopic follow-up platform for large surveys such as HETDEX.