User's Guide – 2.2 Field selection for the catalogue
The catalogue of sources in overlapping observations is based on data used to compile the XMM catalogues from individual EPIC observations and their selection criteria: Observations enter the catalogues, if they have a minimum net exposure time of 1 ks, which is the sum of good-time intervals after filtering the event lists, and non-empty images in all five energy bands. This first release of a stacked catalogue comprised selected good-quality observations. In the following releases, almost all overlapping observations were included:
- Observations are included if they overlap by at least 1 arcminute in radius,
- exposures by an instrument are included if the mean background level of each CCD (pn: chip quadrant) lies below the defined thresholds,
- EPIC/pn exposures in small-window mode are excluded because of their small area,
- the central CCDs of EPIC/MOS exposures in small-window mode are exluded, while the outer CCDs are included.
The final 4XMM-DR9s sample includes 6 604 observations in 1 329 stacks, the majority of them having two or three members.