3.6. Astrometric accuracy

The absolute pointing accuracy for Herschel is measured to be 2 arc-seconds at the 1-sigma level on pointed observations. However, some larger deviations, as large as 5 to 8 arc-seconds (solid shift) in some rare cases, have been reported for some scan map observations. Deviations of the actual telescope pointing from the scan legs (great circles in the sky) that is reported in the pointing product cause a smearing of the PACS PSF in the reconstructed scanmap, in particular in the blue/green bands. The jittering along scan legs is estimated to be usually around 1 arcsec level, therefore the effect is rather small even in the blue band.

The scan speed profile exhibited sometimes some significant bumps above the average speed, before OD 320 (April 2010), due to warm pixels in the star-tracker CCD. This caused also some deviation of the parallelism of scan legs for some observations, blurring the PSF. Since star-tracker CCD temperature was lowered no such anomaly has been observed since.