As described above, the preparation of XMM-Newton observations starts with a technical
feasibility calculation, using primarily the information provided in the
XMM-Newton Users Handbook and the tools introduced there.
The scientific goal of the proposal determines the choice of prime science instrument and the total integration time required. Users must note that at this stage, the integration time has already been fixed by the OTAC. It is expected that in all cases either EPIC or RGS will be the most important instrument for the proposed science, i.e., the instrument driving the feasibility calculations. Having determined the total integration time needed for the primary instrument, one must consider in which mode this instrument should be operated and how many exposures (possibly in different modes) should be taken during the intended observation. Then, the use of the other instruments, which will be operated in parallel, is planned.
It should not be forgotten that all instrument modes have the so-called
''overhead'' times associated which have to be subtracted from the total
observing time. These overheads can take a considerable percentage of the
total observing time, particularly for shorter observations, and vary
a lot depending on the instrument mode used. A summary of the different
overhead times can be found in the following link:
http://xmm-tools.cosmos.esa.int/external/xmm_user_support/documentation/rpsman/overheads_table.html
For a detailed description of the different instrument modes see also
§ 5.2 and the XMM-Newton Users Handbook.
The time given by OTAC to a specific proposal does include these overhead times. This means, that a proposal with one granted observation of 10000s and e.g. RGS as prime instrument will get effectively only 8906s of integration time using the ''Spectroscopy'' mode.
Note that the overhead times may change slightly throughout the mission if for some technical reason the activation and deactivation sequences for different instrument modes have to be adjusted.
One of the science goals of XMM-Newton is to conduct serendipitous surveys. To achieve this, all XMM-Newton science instruments should be active and defined whenever permitted by applicable constraints (such as, visibility constraints, target brightness, etc...). This implies that exposures should be defined for each instrument for the entire duration of an observation.