VIRUS: comparison of lab characterization with on-sky performance for multiple spectrograph units

The Visible Integral Field Replicable Unit Spectrograph (VIRUS), the instrument for the Hobby Eberly Telescope Dark Energy Experiment (HETDEX), consists of 78 replicable units, each with two integral field spectrographs. The VIRUS design takes advantage of large-scale replication of simple units to significantly reduce engineering and production costs of building a facility instrument of this scale. With VIRUS being 156 realizations of the same spectrograph, this paper uncovers the statistical variations in production of these units. Lab relative throughput measures are compared with independently measured grating and optical element performance allowing for potential diagnosis for the cause of variation due to spectrograph elements. Based on variations in performance of individual optical components, throughput curves are simulated for 156 VIRUS spectrograph channels. Once delivered, each unit is paired with a fiber bundle and throughput measurements are made on sky using twilight flats. We compare throughput variance from on-sky measurements to the simulated throughputs. We find that the variation in throughput matches that predicted by modeling of the individual optics performance. This paper presents the results for the 40 VIRUS units now deployed.