Target of Opportunity Procedures

This page describes the procedures to be followed in the preparation, activation, and data retrieval of Target of Opportunity (ToO) programmes both at the VLT and the La Silla telescopes. Please note that further details on policies and procedures are given in the Observatory's webpages.

Phase 2 Submission

The Phase 2 process for ToO programmes follows similar procedures as those of other, regular programmes as described in the Instructions for Phase 2 preparation of Service Mode runs. In particular, the following information must be submitted before the Phase 2 deadline:

  • A set of Observation Blocks (OBs) that completely describes the required observations. The target coordinates should be set to RA=00:00:00.0, Dec=00:00:00. Note that it is not necessary to submit OBs to fill up the entire allocated time: only one complete set of OBs is required

    Users can also submit multiple sets of OBs to cover different situations (e.g. faint and bright events). In this situation, the submitted OBs can over-subscribe the time allocated to the programme, which will be terminated once the actually executed time equals the allocate time. This is a unique exception in Service mode procedures applicable exclusively to ToO programmes. The set of OBs submitted for any other class of programmes must always keep within the allocated execution time.

    Starting from P104, ToO OBs can be organised in Group containers or Concatenations. The use of time-link containers is not allowed.  

    The time-critical nature of ToO observations supersedes the conditions specified in the Constraint Set: this means that, once triggered and started, a ToO observation will be considered to have been successfully executed independently of the prevailing seeing and sky transparency. The Constraint Set of the OB must therefore request only the least stringent constraints possible.

    Finally, please be reminded that, as explained in the OB naming conventions, all ToO OBs must have names starting with the prefix TOO_

  • A complete README file. Please pay special attention to the ToO Information section, where any particular instructions for the execution must be clearly explained.

The submission of finding charts is obviously not required, since the targets are not known in advance.

Follow-up observations of ToO targets

This note applies to programs proposing first to discover a given target and then to follow-up the newly discovered target(s) with a specific observing cadence in order to properly characterise it/them. In principle, while the discovering part needs for ToO triggers, the follow-up part does not require ToO triggers, as reported also in the Call for Proposal. Ideally, at Phase-1 and Phase-2 the ToO and non-ToO parts shall be split into separate runs. The non-ToO part shall be a normal A-class Service Mode run and therefore eligible for carry-over. If instead all the observations will be included in a single ToO run please be aware that the standard carryover policy for Target of Opportunity programmes will be applied, which foresees the termination of incomplete ToO runs at the end of the Period.

Activators must have an account on the ESO User Portal

Starting from P103, the trigger of the ToO observations with all instruments at Paranal is done using p2. Please notice that is possible for the PI of an observing run to allow (an)other user(s) to create and modify the OBs
using their own User Portal account by delegating Phase 2 preparation permission to them. Such users will also be able to act as activators and trigger the ToO observations.

Activating ToO Observations

The procedure to be used to activate the ToO will depend on whether the Run uses a instrument for which the preparation of the Phase 2 material is supported by (Paranal) or (La Silla). Here a summary of the specific processes:

  • For Paranal Observations, the ToO trigger must be activated using p2.

In this case, the following steps must be followed:

1- At the moment you want to trigger a ToO observation finalize the OB preparation with the web-based p2 tool. In detail, this means that you will have to log-in into p2 at

2- Go to the run valid for the observing semester and create a new folder. Please edit the folder name keeping in mind that the name will be used as the name of the trigger.

3- You can now copy the approved template OB(s) that you want to use for the trigger and paste them into the newly created folder. Make sure to fill in all important parameters (exposure time, filters, Finding Charts, etc.) into the copied template OB(s).

4- Any other configuration file must be prepared using the instrument-specific preparation tools (FIMS, NAOS-PS, FPOSS, KARMA, SADT) and included when needed.

5- All "ready-to-go" OBs must be certified by you (click "certify" from the action bar menu in p2). It is not necessary to press "Notify ESO" at this point.

6- Once all the OBs to be used for the trigger are certified, click on the folder and press the button Trigger ToO. You will be then guided trough the completion of a ToO form and asked to include all the relevant information (e.g. contact phone number and email addresses, general observing description) that are critical for the trigger and execution of the ToO.

7- Once the form is properly filled and the trigger is sent, you (and those persons behind the email addresses indicated in the form) will receive a confirmation email that a new trigger was sent to Paranal. You should expect to receive further information related to the ToO activation within 30min. Should this not happen, please contact Paranal at sciops(at)

A video tutorial showing the above described procedure is available here.

  • For La Silla ToO observations, please use the dedicated web form.

This form is designed to make sure that the Observatory receives the complete information package needed to perform the observations. Alerting the Observatory via this Web entry is the first contact, and guarantees a direct line to the support astronomer. The Observatory may contact the activator for further iterations, and will keep the activator up-to-date during the progress of his/her request. In case of submission or network problems, an e-mail message should be sent to La Silla Science Operations ( activation e-mail message must contain:

  • ESO Programme ID
  • List of available OBs
  • Target coordinates
  • Exposure times (if necessary)
  • A few words about the target and any special requirement (e.g. timing) as well as any other information which is felt to be important for a successful execution of the observations.
  • The name, e-mail, and phone number of a contact person who is prepared to discuss this ToO programme and make decisions about its execution. This person should be available via telephone.
  • One or more finding charts should also be provided, in PostScript format, attached to the activation email. Finding charts should be prepared following the conventions indicated in the instructions for the preparation of Finding Charts for Service Mode Programmes, although in this case they must be submitted separately as PostScript files rather than as a part of the OB.

Time accounting

Unlike for normal programmes, the time accounting for ToO programmes is made on the basis of the actual time invested in executing the observation, taking into account all operational overheads, which may include the change of focal station of the telescope. For all other classes of programmes, the overheads charged are those published in the Call for Proposals and the instrument manuals, and computed by p2 or p2ls.

Based on this accounting, once a program has used up all its hours, irrespective of the number of triggers executed, no more triggers will be accepted.


Post-execution Process

After ToO OBs have been executed, ToO data will be made available immediately to the PIs and their data delegates through the ESO archive via the User Portal. ToO data delivery via dedicated FTP accounts is discontinued since Period 90 (October 1, 2012).


Instrument selector

This page is valid for all instruments