One may consider two categories of Target of Opportunity Proposals (ToOs).

1. Unpredictable ToOs are those concerning unpredictable sudden astronomical events which require urgent or immediate observations. Those should be submitted as DDT proposals, and are hence by definition DDT proposals and not ToOs in the ESO proposal terminology.
2. Genericly predictable ToOs are those concerning events predictable in a generic sense only. This is typically (but not limited to) transient phenomena, and follow-up or co-ordinated observations of targets of special interest. Such proposals are, in the ESO proposal terminology, ToO proposals.

ToO proposals must be submitted by the given deadline on the corresponding LaTeX template. Proposals should be for generic targets and/or times, but if accepted by the OPC the programme will not be executed until the PI contacts ESO after the predicted event has occurred to request activation of the programme.

The PI of an accepted ToO programme will have priority to obtain the observations provided that a specific request is submitted in time. The observing strategy should be the one approved by the OPC. The time granted by the OPC has to be regarded as a strict offer limit which might well be not totally used in the absence of enough triggers. The observations will be conducted in Service Mode and, if necessary, current programmes will be interrupted.

Any request with exactly the same scientific goal and aiming at observing the same object, presented by other groups at the time the event occurs, will be rejected by ESO, except if it is a co-ordinated project in collaboration with the PI of the accepted ToO programme.

Policy for Target of Opportunity Observations at the Observatory.