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Target of Opportunity (ToO)

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The Target of Opportunity (ToO) observing mode allows observation of targets that cannot be specified in advance but which have a well defined external trigger. Examples include following up newly detected supernovae or Gamma Ray burst afterglows or observing a particular class of objects from an ongoing imaging survey. All facility instruments that are offered for queue observing are available for ToO programs, except for GMOS MOS. Proposals for the Target of Opportunity mode must be made via the normal, fast turnaround, or large and long proposal processes, must select the TOO Activation mode in PIT Time Requests tab, and must summarise the trigger event (e.g. identification of a target brighter than a pre-determined threshold) in the proposal abstract (additional details can be included in the technical justification).  Phase II templates must also be provided by the regular Phase II deadlines, even if the specific targets are unknown and will be completed at a later date. For other unforeseen events, see Director's Discretionary Time.

ToO Types

There are two types of ToO triggers.

  • Rapid ToO (RToO) observations must be executed within 24 hours of the trigger. 
  • Standard ToO (SToO) observations can be executed at any time after the trigger and are usually done more than 24 hours after.

All proposals for (RToO) followup are required to submit a separate proposal for (SToO) followup when the required weather conditions are better than SB/CC/IQ=Any, if such followup is planned. Upgrades to good conditions will not be approved for RToO programs, and the SToO proposal is required if such conditions are necessary for later followup.

Observing Conditions

All Target of Opportunity observations are executed in the queue mode by Gemini staff. Therefore, all such proposals must specify the poorest acceptable conditions under which the observations should be executed, as if these conditions are too restrictive then the observations might not be carried out. The conditions must however be appropriate for the targets and desired science. Observations which are more flexible with respect to instrument configuration are also more likely to be executed (e.g., for choice of GMOS grating). After recommendation of the proposal by the NTAC and approval by the ITAC and Director, the observing conditions and configurations can only be changed via the queue change request process.

Proposals that require later followup of an RToO must be accompanied by a separate proposal for SToO in conditions better than SB/CC/IQ=Any. Upgrades to good conditions will not be approved for RToO programs, and the SToO proposal is required if such conditions are necessary for later followup. 

Triggering and Execution

It is the responsibility of the PI to trigger to trigger the observation request for their program. Details of the ToO request process are given here.

Target of Opportunity programs are merged into the telescope queue, occupying one of the scientific ranking bands. Note that the override provided by the trigger event normally acts only on its own band. Thus if a ToO program falls in the topmost science ranking band it will be executed, observing conditions permitting, whereas a program in a lower band will not be executed if there is a higher-ranked program that is also suited to the conditions. Rapid Response programs are only accepted if the relevant NTAC places them in Science Band 1. ToO transients requiring followup on timescales shorter than a month are not appropriate for Band 3.

The instruments currently available for tonight, details of the GMOS North and South configurations, current open status and site conditions are kept up to date and are available for both the North and South telescopes (more information here) to help with deciding to trigger. Standard ToO Gemini North laser guide star (LGS) observations are allowed; however, targets must be defined at least 4 business days in advance of the intended observation date. The observation must coincide with a scheduled laser run, and use a scheduled instrument. We do offer limited LGS observations of Band 1 and 2 ToOs that are triggered less than 4 days before, or during, an LGS run. Only two such targets (for all programs) can be observed during any typically week-long LGS run, and only one such target (for all programs) can be observed on any given night. All effort will be made to obtain approval and observe a target within 24 hours, however this cannot be guaranteed, and the observation may occur two or three nights after the trigger is made.

For the purpose of optimizing the queue planning as well as the use of ToO time,

  • PIs who know the RA(s) of their target triggers should include this information in their Phase I, so that it can be taken into account for the queue filling;
  • PIs of programs with triggers expected throughout the semester are requested to use no more than half the time in the last 2 months of the semester, and the Observatory may take late triggering into account when deciding on queue priorities

Competitive ToOs

In the event that multiple teams (including those seeking DD time) activate ToOs on the same target and on the same or subsequent nights, the Observatory has a clearly-defined policy for program approval and for data access

Data Access

Data files are accessible from the Gemini Science Archive shortly after each observation has ended and the file has been written to disk. The ingestion time is less than a minute.


All questions concerning Target of Opportunity proposals should be made using the Gemini HelpDesk. This web-based system will send the request to your National Gemini Office staff in the first instance who will escalate it to Gemini staff if necessary.

Target of Opportunity (ToO) | Gemini Observatory


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