Optical throughput and sensitivity of JWST NIRSpec

TL;DR

This paper reports on the in-flight measurements of JWST NIRSpec's photon-conversion efficiency, showing it meets or exceeds expectations in most modes, with some discrepancies in integral-field spectroscopy, confirming its high sensitivity.

Contribution

First in-flight PCE measurements of JWST NIRSpec, updated light-loss modeling, and validation of its high sensitivity and performance across different modes.

Findings

PCE meets or exceeds pre-launch models in most modes

Integral-field mode shows up to -20% or +30% deviation from models

High-quality PSF reduces slit-losses at shorter wavelengths

Abstract

To achieve its ambitious scientific goals, the Near-Infrared Spectrograph, NIRSpec, on board the Webb Space Telescope, needs to meet very demanding throughput requirements, here quantified in terms of photon-conversion efficiency (PCE). During the calibration activities performed for the instrument commissioning, we have obtained the first in-flight measurements of its PCE and also updated the modeling of the light losses occurring in the NIRSpec slit devices. The measured PCE of NIRSpec fixed-slit and multi-object spectroscopy modes overall meets or exceeds the pre-launch model predictions. The results are more contrasted for the integral-field spectroscopy mode, where the differences with the model can reach -20%, above 4 micron, and exceed +30%, below 2 micron. Additionally, thanks to the high quality of the JWST point-spread function, our slit-losses, at the shorter wavelength, are significantly decreased with respect to the pre-flight modeling. These results, combined with the confirmed low noise performance of the detectors, make of NIRSpec an exceptionally sensitive spectrograph.