Performance of flight-model on-board calibration sources on Herschel-SPIRE

The SPIRE instrument for the Herschel Space Observatory has two on-board calibration sources. The photometer calibrator, PCAL, is an electrically-heated thermal source which can be seen by all detectors, including the spectrometer. It is not an absolute calibrator, but a repeatable source of sub-millimetre radiation which may be used in the overall calibration scheme. The purpose of the spectrometer calibrator is to null the background emission from the Herschel telescope, thereby improving the dynamic range of the spectrometer detectors. This paper details the final flight design of the calibration units, and presents results from SPIRE flight model ground testing.