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A typical and rather frequent example of such exposures are the EPIC closed filter calibration exposures. At least one of such exposures in each EPIC camera is normally taken at the beginning of each XMM-Newton orbit. A full orbit lasts about 48 hours. However, part of this time is not fully available for science, due to high particle radiation background. This background component is particularly intense during and immediately after the crossing of the Earth's radiation belts. The XMM-Newton instruments can be switched on only beyond a certain altitude (46000 km). However, the filters of the EPIC cameras are opened only at least seven hours after each perigee passage. This constraint is set to prevent soft protons from being focused onto the EPIC cameras focal plane. The impinging proton flux could in facts produce serious cumulative damages to the Charge Transfer Efficiency of the EPIC CCDs.
During the time span when the X-ray instruments are switched on, but the EPIC filters are not open, EPIC internal calibration exposures are performed. The EPIC optical filters are kept closed, and the cameras often illuminated by an internal calibration source. These exposures allow a regular monitoring of the instrumental gain. Generally, the same attitude is used as the first observation in the schedule, where EPIC science exposures are allowed. The RGS and OM runs in parallel, generally with the same instrumental configuration as the next "normal" science observation. These closed filter EPIC calibration exposures represent therefore additional science OM and RGS exposure time.
A similar situation occurs when the OM camera filter needs to remain closed due to the presence of a bright star in the field of view. The OM time is employed for internal calibration exposures. The relevant data are as shipped to be PI, but are not processed by the PPS.
The instrumental configuration of a performed XMM-Newton observation can be inquired using The XMM-Newton SOC Observation Database.
Of course, it may happen that science OM and EPIC exposures are erroneously not processed due to problems in the the data format or in the reduction software. In case of doubts, users are encouraged to contact the XMM-Newton HelpDesk.