One of the standard procedures applied during rgsproc is a so-called offset correction which is designed to deal with an additive noise property of CCDs. While it is possible to use an identical value for every pixel in a CCD node, since SAS v6.0, an alternative was offered that takes advantage of offset values determined for each individual pixel from routine instrumental monitoring data. These offset data, which form part of the ODF, are accumulated every 3 revolutions thus also providing each pixel with its offset history. Experience of this alternative approach has confirmed the advantages described in trials (see Technical report XMM-SOC-CAL-TN-0046 ) and it has become the default method.
For comparison, the original offset method could be selected with the
withdiagoffset switch as in the following example :
rgsproc withsrc=yes srclabel=Mkn421 \ srcstyle=radec srcra=166.113808 srcdec=+38.208833 \ withdiagoffset=no