ESO recognises two categories of Targets of Opportunity:

1. Unpredictable ToOs are those concerning unpredictable astronomical events that require immediate observations. The occurrence of such events cannot be anticipated on a sufficient timeframe to allow them to be the subject of a proposal prepared by the regular proposal submission deadline. They qualify for application to DDT proposals.
2. Predictable ToOs are those concerning predictable events in a generic sense only. These are typically known transient phenomena and follow-up or coordinated observations of targets of special interest, triggered by the proposing team.

Target of Opportunity observations are requested strictly as ToO runs pertaining to any programme type. ToO runs are defined as runs for which the target and/or observation epoch are not known at proposal submission, typically by more than one week before the observation needs to be executed.

Three different types of ToO runs are defined:

• Rapid Response Mode (RRM), for observations to be triggered via the automated Rapid Response Mode system within 4 hours after an event;
• Hard ToO runs, for manually triggered observations that must be carried out as soon as possible or at most within 48 hours of receipt of the trigger by the Observatory, or that involve a strict time constraint (i.e., that must be executed during a specific night);
• Soft ToO runs, for manually triggered observations, which can be scheduled for execution within a time window of 7 days.

Targets that are unknown at the time of the proposal submission but can be observed more than one week after they have been identified can be observed as part of normal (non-ToO) runs.

ToO runs will be evaluated in the same way as normal (non-ToO) runs but must be amongst the highest ranked runs in order to be scheduled.

RRM observations have overriding priority over other observations, unless the latter are strictly time critical. This is assessed by ESO before the start of the Period (and at the approval of DDT Programmes during the Period), based on information provided in the proposal. Visitor Mode runs that have time critical observations that cannot be moved to other nights (typically planetary transit where only one window is available during the scheduling period or simultaneous observations with another facility) are protected against RRM triggers. The list of protected runs is available on the RRM-protected runs webpage. RRM triggers will be accepted during Service Mode and Visitor Mode runs. An RRM trigger cannot interrupt an already on-going RRM observation.

For all approved ToO runs, generic Observation Blocks (OBs) must be submitted at Phase 2 using the p2 tool. These OBs are updated by the PI once the target is known. Information on OB preparation can be found in the ToO and RRM procedures webpages.

The observing strategy at execution must be the same as what was requested and approved in the proposal, and the triggers may not exceed the allocated time. The observations will be conducted in Service Mode and, in exceptional cases, ongoing programmes may be interrupted.

Eventual follow-up observations of a ToO target should use a normal (non-ToO) run, possibly with specific time constraints, if the observations take place more than one week after the reception of the first trigger for that object. Follow-up runs are allocated time only if the respective ToO is also approved.

RRM runs must be exclusively used for triggering the RRM system. Follow-up observations of a target observed using the RRM system must be requested through the activation of a hard or soft ToO trigger, or a non-ToO run, depending on the classification described above.

ToO (and RRM) runs are not carried over to a following Period. In case the programme includes a normal run for ToO follow-up observations, such runs can be exceptionally carried over to enable the planned follow-up, provided that the related ToO is triggered within the originally allocated period.

The RRM is offered for specific instrument modes (see the Call for Proposals). Users interested in using RRM for modes not currently offered for RRM should contact ESO through the ESO Operations Helpdesk at least two weeks before the proposal submission deadline.

ToO execution

ToO observations will be executed after the proposing team will trigger them. OBs for hard and soft ToO runs will be scheduled for execution following the acceptance by ESO of an activation trigger.

Upon receiving an RRM alert indicating the coordinates of the target and the associated Observation Block (OB), any ongoing integration will automatically be terminated and the RRM OB will be executed, unless the procedure is aborted by the operator due to safety concerns. Following an RRM trigger the telescope will be at the target’s location within about 6 minutes, not including any change of focus, and depending on the instrument and the target position.

RRM triggers can require a change of focus. If the RRM trigger requires a change of focus, the telescope is expected to point at the location of the target within about 12 minutes. The time required for the change-of-focus is not counted as part of the RRM runs.

For practical guidelines on ToO runs execution please see the ToO and RRM procedures webpages.