Herschel SPIRE FTS Spectral Mapping Calibration
  The Herschel SPIRE Fourier transform spectrometer (FTS) performs spectral
imaging in the 447-1546 GHz band. It can observe in three spatial sampling
modes: sparse mode, with a single pointing on sky, or intermediate or full
modes with 1 and 1/2 beam spacing, respectively. In this paper, we investigate
the uncertainty and repeatability for fully sampled FTS mapping observations.
The repeatability is characterised using nine observations of the Orion Bar.
Metrics are derived based on the ratio of the measured intensity in each
observation compared to that in the combined spectral cube from all
observations. The mean relative deviation is determined to be within 2%, and
the pixel-by-pixel scatter is ~7%. The scatter increases towards the edges of
the maps. The uncertainty in the frequency scale is also studied, and the
spread in the line centre velocity across the maps is found to be ~15 km/s.
Other causes of uncertainty are also discussed including the effect of pointing
and the additive uncertainty in the continuum.
