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All of the proposals approved by the International ROSAT Users' Committee are converted to ROSAT Observation Requests (ROR) at MPE. A ROR, uniquely defined by its sequence number, requests a pointing in a specified direction for a specified amount of time; the data collected during this pointing will then be merged by the data processing system into one ``observation''. Proposals with more than one target or proposals requesting several observations of the same source are converted into several RORs. It is the responsibility of the German Space Operations Center (GSOC) to produce a mission time line out of all approved observation requests. The process of mission time line generation is split up into two parts: First, for the entire period covered by the AO a long term mission time line (LMTL) will be generated; second, about one week prior to the execution of the observations a short term mission time line (SMTL) will be produced on the basis of the LMTL. The SMTL is used for the automatic generation of the required spacecraft commands.
MPE and GSOC will make their best effort to schedule all requested proposals with assigned priorities A and B. Proposals with priorities A or B that were not scheduled or that were scheduled but not executed for some reason will automatically be carried over into the observing program of the next observing season.
Guest investigators should be aware of the fact that proposals with assigned priority C have only a percent chance of being scheduled. Priority C proposals originally scheduled in the long term mission time line but not executed for some reason will also be automatically carried over into the next observing season. However, priority C proposals not scheduled in the long term mission time line will not be carried over into the next observing season. The principal investigators of such proposals must therefore resubmit such proposals for the next observing season if they wish them to be reconsidered.
Guest investigators should also be aware of the fact that observations of their targets will (normally) not be carried out sequentially but rather interleaved with other observations; only in this fashion is it possible to satisfy all the observing constraints and achieve a satisfactory observing efficiency. As a consequence individual observations may extend over periods of days and possibly even weeks. An example of a ROSAT schedule for a period of four days is given in Figure 8.1 ; this figure clearly demonstrates that first, a significant number of observations are in progress at any given instant of time, and second, that at least for observations longer than about 2 ks the elapsed time will greatly exceed the actual observing time.