The duration of each requested observation has to be estimated and entered into RPS by the proposer. Instructions for calculating exposure times are contained in the XMM-Newton Users' Handbook. The XMM-Newton SOC recommends that proposers base their estimation of exposure times on previous X-ray measurements whenever possible, especially on previous XMM-Newton observations. After correction for the different energy bands and effective areas (for example with PIMMS, see below), these give reasonable estimates of the needed exposure times. It is mandatory that observers describe in their proposals how they calculated the exposure times. A realistic estimate of the observing time is a major selection criterion for the OTAC.
PIMMS (Portable, Interactive, Multi-Mission Simulator) allows users to estimate the count rate, hence the exposure time necessary to achieve the scientific objectives of an observation, based on the count rate measured with another instrument, or a theoretically calculated flux, and an approximate spectral shape. Although it cannot substitute a full spectral or timing simulation, PIMMS nevertheless provides a useful first-order estimate of the count rate when a proposal is being considered. ``First-order estimate'' in this context means that the uncertainties of the PIMMS estimates are often (though not always) dominated by the scientific uncertainties regarding the source.
PIMMS for XMM-Newton is officially supported by NASA and is available from HEASARC via the WWW:
Experience from previous AOs has demonstrated that the requested exposure time is often significantly underestimated. An estimate of the exposure time based on a ``3'' signal to noise ratio implies a significant (3) detection only in 50% of the cases. Many estimates of exposure times are based on a comparison with physically similar targets already observed in the X-ray band. Proposers should be aware that the literature is biased against detections and therefore many reported fluxes are not good representatives for a particular object class, as a whole.