4.13. Astrometric accuracy

The absolute pointing error (APE) for Herschel is specified and measured to be 2 arc-seconds at the 1-sigma level on pointed observations. However, if the roll-angle of the telescope does not change significantly between two consecutive observation - typically the case for concatenated spectroscopy AORs - then such AORs could have a systematic offset to a given direction as the guide-star distribution within the star-tracker field-of view remains pretty much unchanged. Such a systematic offset is meant to be the main error component contributing to the APE, therefore positions should be still within 2 arc-seconds at the 1-sigma level to the commanded position.

Nodding observations require a move of the telescope boresight as many times nodding cycles are defined. Therefore this observing mode requires spacecraft attitude change (re-pointing) even for pointed observations. The pointing uncertainty for such small displacements is defined by the spacecraft relative pointing error (SRPE) what is typically found around ~1-2 arc-second even for the largest 6 arcminutes chopper throw (and the equal length nod slew). This means, the observer could safely use nodding cycles without compromising data quality more than what is defined by the absolute flux calibration error.

Observations what suffer from a larger than 1-sigma pointing error are not considered being severely compromised, although, in case of a point-source measurement the source photocentre might be significantly off from the central spaxel's geometric center position. In such a case, flux can be recovered two ways: using a beam profile map available for the closest wavelength (Figure 4.11) the source offset from the reference aperture is established. After this offset-correction the full beam line fluxes and flux densities can be recovered as described in Section 4.10.1. An offset-correction task is currently being developed in the PACS ICC and will be provided in forthcoming HIPE versions. Alternatively, the full flux or flux density of a source can be recovered by co-adding the spectra obtained from several spaxels (“aperture photometry”) also described in Section 4.10.1.