The Herschel's prime star tracker had a non-negligible focal length correction that produced, in some circumstances, relatively large pointing offsets.

Before OD#320 the correction was rather small (approximately 5 microns) but, after that OD (when the STR CCD reference temperature was lowered to -10degC), the
correction became important (approximately 14 microns) and large offsets, up to some 10 arcsec in extreme cases, appeared. 

In particular, the Taurus region was quite seriously affected due to the extremely uneven distribution of guide stars within the Startracker field of view. Later on, in OD#762, a simple, 1-D correction (i.e. a linear
correction factor in the Y and Z STR axes) was uploaded to the S/C ACMS and the pointing performance was, as a result, substantially improved. 

In OD#858 an enhanced 2-D correction (different linear factors in the Y and Z axes) was successfully tested on board; this change was made permanent in OD#866 giving further improvements in pointing performance. 

Finally, in OD#1011, a full STR focal plane (FP) distortion correction was made operational and, once the performance enhancement was verified, a final update was made in OD#1032 when the Startracker guide star catalogue was purged by 
removing 73 'dubious' stars from the list that had given consistently larger residuals in the past. This gave the final refinement of the telescope pointing.

If you think your Herschel data is affected by significant pointing errors and you want to explore whether your data is affected by the problems above
mentioned and the astrometry of your images can be improved, we recommend you that you first read the detailed information available in the twiki page below, 
where we also provide improved pointing products, including the procedure to use them within the reduction scripts:

http://herschel.esac.esa.int/twiki/bin/view/Public/HowToUseImprovedPointingProducts

In case of problems or doubts please do not hesitate to contact us through this Helpdesk