Normally, in all cases when OM fast mode is used, the small fast window is embedded into a larger image mode window. If the target in the fast window is moderately bright, or if more than one source are present in the fast window, it is advisable to obtain the background from the image data. Using the parameter bkgfromimage=yes within omfchain will produce a constant background extracted from the corresponding image data for the whole exposure that will be subtracted from the source fast mode count rate. This is the default option in omfchain since SAS 12.