Target of Opportunity Policies


Target of Opportunity Observations (ToOs) are used to follow-up transient phenomena:

  • Unpredictable sudden astronomical events which require urgent or immediate observations. They are submitted as Director Discretionary Time (DDT) proposals, following the procedure described in the DDT policy page. An emergency procedure exists for extremely urgent observations.
  • 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. Telescope time to observe these events is obtained using the normal proposals for observing time that go through the OPC review. In this document, we refer to these as OPC ToOs.

OB Preparation

The user completed the Phase 2 material to prepare the ToO observations, following the User Support Department ToO procedure. USD receives the material (template OB that define the instrument configuration and README), reviews, and checks the OBs.

In the case of an extremely urgent DDT ToO, the La Silla and Paranal SciOps Departments can give some assistance to the PI in preparing the OBs. The PI must prepare the OBs with finding charts using p2demo and use the option 'export OB to OBX'. The downloaded folder has to be send to 

Activation of a ToO

A request for a ToO observation has to be triggered within p2, following the Target of Opportunity procedure. For approved urgent DDT ToOs (runs not yet available in p2), the observations can be triggered via email to with the exported OBs attached. Rapid Response Mode users should refer to the specific RRM page.

Upon successful submission, requesters will recieve a confirmation by email that acknowledges the receipt of the trigger. In case the submission fails (e.g., in case of network problems), the requester is asked to contact the Observatory directly, either via email (shift coordinator:, or via phone (Paranal: +56 55 243 5312). Requesters will receive information on the progress of their program by email. Notifications are sent each time an update of the status of the trigger is made.

Restrictions and Override Policies

ToO observations should ideally be executed only during service mode. ToOs cannot be imposed over time-critical scheduled observations as established in the OPC-approved proposal (e.g., occultations). This restriction can only be over-ridden by the Director General or the Observatory Director, and this approval will be granted only in exceptional cases.

Generally, ToO observation can only interrupt a Visitor Mode run if the visiting astronomer explicitly agrees to it. The Observatory Director can grant an interruption in special cases, in which case compensation time in service mode can be offered to the visiting astronomer.

In the case of RRM observations (cf RRM page), ongoing visitor mode programs are generally interrupted, with override protection for special visitor mode runs defined at the time of scheduling. The time lost will be fully compensated in service mode, with weather conditions specified in the Phase 1 proposal. In this context, also RRM follow-up observations (ToO hard trigger within 4 hours of the RRM trigger on the same target and the same program) override visitor mode observations. 

There is no restriction as to how much time a ToO trigger can use other than that it has to be within the allocated time for the run. 

Time Accounting

The time accounting for ToO programmes is made on the basis of the actual time invested in executing the observations, taking into account all operational overheads, which may include the change of focal station of the telescope.