The XMM-ESAS software package offers users an opportunity to mosaic multiple observations of a region regardless of their pointing directions and orientations. By mosaics we mean multiple observations that are relatively close or overlapping on the sky. Mosaics may include mosiac-mode observations, but need not do so. The peculiarities of mosaic-mode observations are addressed in the next section.

The programs described in the previous sections produce components (counts, exposure time, quiescent particle background, soft proton flare background, and SWCX background) for a instrument of an observation. The mosaic merging programs can be run only after all contributing observations have been processed to the point that all the individual images can be produced (i.e., you can't merge what you haven't created). Of course, for some components (e.g., SPF and SWCX) that component may exist for some observations and not for others; this is not a problem.