Target of Opportunity Policies


Target of Opportunity Observations (ToOs) are needed to follow-up transient phenomena of great scientific interest:

  • Unpredictable sudden astronomical events which require urgent or immediate observations. Those should be submitted as Director Discretionary Time (DDT) proposals, following the procedure described in the DDT Proposal web page (
  • 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.

Time Allocation


The proponent sends a request to Director's Discretionary Time Committee (DDTC) following the instructions in the DDT Proposal Page. The Director General transmits the decision to the Observatory for execution. For urgent observations, a decision can be taken in about 48h; an emergency procedure exists for extremely urgent observations.


OPC ToOs are approved by the Director General, following the recommendations of OPC, together with all the other proposals.

OB Preparation

In both DDT and OPC ToOs, the user goes through a full Phase II in order to prepare the observations as a set of Observation Blocks (OBs) and README instruction file, following the User Support Department ToO procedures ( USD receives, reviews and checks the OBs. In the case of OPC ToOs, the coordinates and exposure times of ToO observations are not known. Generic OBs have to be submitted that define the instrument configuration (coordinates are left blank).

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 with p2demo and use the option 'export OB to OBX'. The downloaded folder has to be send to The PI must communicate as much as possible the observing strategy and constraints not specified in the OBs, and must be available for questions before and during the observations.

Activation of a ToO


As soon as a DDT ToO is approved, it is scheduled for execution.


The information in this section applies only to non-RRM ToOs. Rapid Response Mode users should refer to the specific RRM page (

A request for a ToO observation has to be triggered within p2 (Target of Opportunity Procedures). 

Upon successful submission, requesters will obtain a confirmation by email typically within 30 minutes 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 SciOps directly, either via email ( or ), or via phone (La Silla: +56 2 2464 4531, 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 by the SciOps staff.

For La Silla, execution of ToOs on a given night will be scheduled on a given night only if the activation reaches the Observatory no later than 16:00 LCT. Activations arriving after that time may be executed on a best effort basis during the current night provided that they do not require a setup change; if they cannot be executed on the activation night, they will automatically be rescheduled for the next night. Execution of activations received after 16:00 LCT for which the instrument setup needs to be modified will be postponed to the next night.

Restrictions and Override Policies

ToO observations should be executed during service mode. There is no restriction as to how much time a ToO can use during service mode other than the time limit and/or number of trigger imposed by the OPC or DDTC. 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.

If the request can only be done during a visitor night, the policies are different for La Silla and Paranal.

La Silla

  • ToO observations (both OPC and DDT) cannot exceed the following limits of telescope time (TT, is the total time spent for the observations. TT includes time to change configuration, point the telescope, acquire guide stars, expose, read-out, calibrate, etc. It is measured from the time the telescope is taken over by the ToO observer, until the time the scheduled observer begins the first exposure after the ToO).
    • OPC ToOs: 3 hours of TT per run or per 3 nights in the case of longer runs (within the limit of the total TT allocated by the OPC).
    • DDT ToOs: 2 hours of TT per run or per 3 nights for longer runs (within the limit of the total TT allocated by the DDTC)
  • No ToO observations can be scheduled during 1-night runs.
  • ToO observations requiring more time than specified above must be spread over consecutive observing runs, if justified.
  • Follow-up and monitoring ToOs must be scheduled in Service, Reserved, Technical or Idle nights. If not possible, the timing of the follow-up or monitoring observations must be adjusted in order to minimize the impact on regular programs.

Compensation. Observers of La Silla Visitor Mode programs interrupted by a ToO for more than 1 hour TT (per run or 3-night period) may request compensatory observations in "Queue Scheduling" Service Mode by the La Silla Science Operation Department, for a total telescope time less than or equal to the time they lost minus 1h (e.g. a program that has lost 2.5h can request 1.5h compensation). Whenever possible, SciOps will schedule and execute these observations during special nights reserved for this purpose, on a best effort basis. The affected visitor will have to submit a reduced "Phase 2" package using the ToO form, with the assistance of his Support Astronomer, with the Remedy System, while on mountain. This information shall contain

  • list of OBs to be executed, totaling to the TT lost minus 1h
  • finding charts,mandatory for spectroscopy
  • strategy instructions

Since in general it may not be feasible to compensate all the lost time, priority will be given to the observers who lost the largest fractions of their scheduled time. The pending compensations will be declared obsolete and removed from the queue at the end of the visibility period of the OBs.


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 Rapid Response Mode ToOs (RRM, cf RRM page), ongoing visitor mode programs are generally interrupted, with override protection for special visitor mode runs defined at the time of scheduling. In case a Visitor Mode run is interrupted by such an RRM, the time lost due to the RRM will be fully compensated in Service Mode, with top priority, 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. 

Common to La Silla and Paranal

ToOs cannot be imposed over time-critical scheduled observations as established in the OPC-approved proposal (e.g. occultations).

The above restrictions can only be over-ridden by the Director General or the Observatory Director, and this approval will be granted only in exceptional cases.

ToO during National Time

ToO observations on national telescopes during national time must be requested through the chairpersons of the corresponding committees, to be executed either by the scheduled observers, or by La Silla staff. In the latter case, the La Silla coordinator must be kept fully informed, and s/he will follow the general rules described above. (Note for SciOps: the details for the various national telescopes are kept in lasilla/ToO/ToO_Contacts).