Performance evaluation of the Herschel/SPIRE imaging Fourier transform spectrometer through ground-based measurements

Abstract

The Spectral and Photometric Imaging Receiver (SPIRE) is one of three scientific instruments onboard the European Space Agency (ESA)'s Herschel Space Observatory. Herschel was successfully launched on 14 May 2009; routine science observations commenced in late 2009. Medium resolution spectroscopy with SPIRE is accomplished via an imaging Fourier transform spectrometer (IFTS) of the Mach–Zehnder configuration. Although pre-launch performance verification and calibration measurements were conducted with the SPIRE instrument mounted in an evacuated cryostat at cryogenic temperatures, it was not possible to simulate fully the expected in-flight conditions. This paper compares the performance of the SPIRE IFTS, as measured during ground-based tests, with theoretical simulations. In turn, these results are used to provide an estimate of the in-flight instrument performance. This paper includes a discussion of key aspects of the SPIRE IFTS including the spectrometer dual-input compensation scheme, instrument line shape and the overall instrument sensitivity. As a case study, the derived instrument performance is used to investigate SPIRE's utility in observing astronomical line emission from the starburst galaxy M82.