10.17 PHT-S Pointings Not Centred in the Aperture

PHT-S exhibits a steep beam profile in particular in the cross-dispersion (spacecraft y-axis) direction, see Section4.5.1.3. As a consequence, a small pointing offset can cause both a significant absolute flux offset and a change of the relative spectral shape. The latter is also visible via flux jumps at the wavelength borders of the order sorting filters. These borders are situated at 3.7$\mu$m for the PHT-SS array and 8.8$\mu$m for the PHT-SL array.

Caveat:  The accuracies for PHT-S point source spectrophotometry are only valid for sources within $\pm$3arcsec of the centre of the aperture. Off-pointings can lead to significant flux losses and modifications of the spectral shape. Conspicuous artifacts are flux jumps at the wavelengths of the order sorting filter transitions at 3.7$\mu$m for the SS array and 8.8$\mu$m for the SL array.

It is therefore advisable to cross-check the PHT-S pointing with independent determinations of the source position.