Target of Opportunity Policies

Revision History:
2009-05-03 isaviane removed note on compensation time
2007-08-29 akaufer/msterzik (ed ohainaut) added GROND (5.)
2007-06-07 ohainaut updated technical procedure to submit ToO compensation at La Silla (5.) + several broken links fixed
2006-02-23 ohainaut fixed broken link ddt_policy
2005-10-05 ohainaut clarify ToO accounting (5.0)
2005-07-27 ohainaut added note on national telescopes (5.4)
2005-06-14 ohainaut+fcomeron correction of activator list (4.2)
2005-06-05 jspyromilio released
2005-05-01 gmathys, jspyromilio, msterzik reviewed
2005-04-23 ohainaut, created from previous La Silla and Paranal policies



Target of Opportunity Observations (ToOs) are needed to follow-up transient phenomena of great scientific interest. There are basically two types of events that trigger ToO:

  • 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 ("/sci/observing/policies/ddt_policy.html).
  • 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 her 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 should go 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 OBs into the repository. In the case of OPC ToOs, the coordinates and exposure times of ToO observations are not known. Nevertheless, it is very important that OBs are submitted in the usual manner. The coordinates and the exposure times can be left blank or set to some default value. However, the instrument configurations and modes have to be fully defined and will not be altered by SciOps upon execution.

In the case of an extremely urgent DDT ToO, the La Silla SciOps Departments can give some assistance to the PI by preparing the OBs directly on-line at the telescope. This should be an exception. The PI must communicate as much as possible of the following information, and must be available for questions before and during the observations.

  • A description of the observing strategy
  • A set of OB descriptions:
    • Precise coordinates precessed to J2000 and integrated to the epoch of observations
    • Exposure times
    • Filters refered to by their unique ESO id
    • Wavelength range and resolution. Grism or grating refered to by their unique ESO ID and central wavelength when applicable.
    • Slit position angle if relevant.
    • Detector setup (readout speed, binning, windowing)
    • Any constraints such as time, seeing, photometric conditions, moon distance, airmass, etc., must be clearly specified. For example, for transient sources such as GRB it is critical to observe the sources as soon as possible after the alert. This and any other science constraints must be clearly specified in order to preserve the scientific value of the observations
  • Calibrations in addition to those provided by the calibration plans must be specified completely. Photometric and spectro-photometric standards will be selected by the SciOps observer unless indicated otherwise by the requester.
  • A finding chart must always be provided for spectroscopy. For imaging, it should be clearly indicated whether the field is to be checked, and a finding chart provided accordingly. The scale and field of the finding chart should be appropriate to the crowding and magnitude of the targets and correspond approximately to the wavelength of the ToO observations. The general guidelines for finding charts should be respected.

Activation of a ToO


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


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 submitted using the proper web form ( La Silla:, Paranal: This trigger can only be activated by the PI of the proposal or a person pre-authorized by the PI.. The list of authorized persons must be included in the section "ToO activator list" of the README file at the time of the Phase 2 submission. An observation triggered by anyone not included in that list will be rejected.

The trigger includes all essential information SciOps needs to successfully perform the observations. In particular target information (including a finding chart or an ephemeris file in case of moving targets), a list of the observing blocks (OB), instrument configuration files (if needed), constraints, calibration requirements, contact information and any other information that may help in the execution of the OB. Note that one request refers to one and only one target.

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 via phone (La Silla: +56 2 464 4531).

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 as a rule 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. Note that unlike for normal programs, the time accounting for ToO programmes is made on the basis of the actual time invested in executing the observations, taking into accound all operational overheads, which may include the change of focal station of the 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. Itis 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 at all possible, the timing of the follow-up or monitoring observations must be adjusted in order to minimize the impact on regular programs.

Special case GROND: Special note for visiting astronomers at the ESO/MPG 2.2m telescope: 15% of the observing time is allocated to observations of Gamma-Ray Bursts and X-ray transient afterglows with the GROND instrument owned by the Max Planck Institute for Extraterrestrial Physics in Rapid Response Mode. Therefore, visitor mode runs can lose up to 15% of their observing time, and no compensation shall be provided. In case of exceptional events, more time can be dedicated to GROND observations provided mutual agreement between the visitor and the GROND team.


Common to La Silla  

ToO 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 very 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).