SAS home . ... List of SAS threads . OM data processing from A to Z is performed by two "chains": omichain and omfchain , for image and fast mode data respectively. ... Then a source detection algorithm is used to identify the sources present in the image. ... A final source list is obtained from all exposures and filters. ... If the user has PPS products from the processing of his/her observation, or from the XSA, then the first thing to do is to overlay the image source list onto the sky image. ...