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Spitzer Observing Rules

5         Targets of Opportunity 

Targets of Opportunity (ToO) are transient phenomena whose timing and/or location on the sky are unpredictable.  They include objects that can be generically identified before the onset of such phenomena (e.g., recurrent novae, variable stars) and predictable phenomena that can be expected, although whose precise timing cannot be specified a priori (e.g., newly discovered comets, novae, supernovae, gamma-ray bursts, extra-solar planets).


Predictable phenomena whose exact timing may remain uncertain at the time of proposal submission should be submitted in response to a General Observer Call for Proposals (CP).  Observations of completely unanticipated phenomena can be requested through Director’s Discretionary Time (DDT) procedures.


Starting with Cycle-6 only low-impact targets of opportunity may be included as part of GO proposals.  High/medium impact ToOs must be requested via DDT proposals. At the time of proposal submission, investigators will classify each ToO request, based on the degree to which the execution of such an observation affects normal scheduling and observing procedures (see below).


A General Observer proposal must include a valid Astronomical Observation Request (AOR) for each predictable ToO observation, or representative AORs for proposals requesting > 50 hours.  The AORs should be completed in as much detail as possible, lacking perhaps the precise target positions (i.e., a “null target”) and refined integration times.  The proposal must present a detailed plan of observations that will be implemented if the specific event occurs.  Moreover, it must also provide an estimate of the probability of occurrence of the specified event during the relevant Spitzer observing cycle(s).


The SSC Director reserves the right to designate any ToO data for early release when such a release is deemed (by the Director) to be in the interest of the community.

5.1       Classification of Impact

At the time of proposal/AOR submission, investigators must classify each ToO observation into one of three categories based upon the impact that the observation will have on the normal scheduling and observing procedures (if approved).  The classification scheme is based solely on the time elapsed between the activation of a Target of Opportunity AOR (§5.2) and the execution of the corresponding observation:


High-Impact               < 1 week

Medium-Impact         1 – 8 weeks

Low-Impact               > 8 weeks


High/Medium impact ToO observations are not allowed in warm mission GO proposals.  These observations must be submitted via a DDT proposal.


Starting with Cycle-7 the activation-to-execution window for a low-impact ToOs has been increased from 5 weeks to 8 weeks.    There is no formal limit on the number of these low-impact ToO observations that can be approved.


Apart from the overhead burdens applied to all Spitzer observations (§1), the SSC will impose no additional overheads on low-impact ToO observations. The SSC has developed a separate calculation of the observatory overhead to be assessed against the high/medium-impact category of ToO observations.  These special overhead burdens are described in Appendix E of the Call for Proposals.  DDT proposals for high/medium impact ToOs must include the overhead in the total requested observation time.


An investigator will self-determine the appropriate category, based upon the maximum delay (in their judgment) that is scientifically acceptable between the activation of an approved AOR and the execution of the observation.  This information will be useful in permitting the SSC and the reviewers to scientifically assess the value of the ToO observation vis-à-vis other approved observations.

5.2       Activation of AORs

For an approved ToO, the Principal Investigator (PI) must electronically submit a request for AOR activation to the SSC Director via the Spitzer Helpdesk (  Following the request for activation, the SSC will ascertain the feasibility of conducting the ToO observations, taking into account sky visibility and the existing science schedule. The observer will also submit a revised AOR(s), with precise coordinates and integration time.  If the observation(s) cannot be conducted on a schedule requested by the investigator, the SSC Director may consult with the Principal Investigator on the scientific utility of later observations.  The SSC Director must issue final approval for any high/medium-impact ToO observations requiring an interruption of the onboard observing schedule.


An approved ToO observation will be executed only in the event that the specified phenomenon actually occurs within the relevant observing cycle.  If the triggering event for an approved ToO observation does not occur during the observing cycle, the AOR will be deactivated at the end of the cycle.  Cycle-6 Exploration Science ToOs are valid for two years.  In the event that a ToO observation expires without execution, the allotted observing time will be returned to the General Observer pool.


For a low-impact ToO, the fully specified AOR must be available for scheduling in the SSC operations database a minimum of eight weeks prior to its anticipated execution week.  

5.3       Regulation of Observations

The SSC Director will rely on the recommendations of the reviewers to assess the benefits of a proposed ToO observation against any disruptions to the efficient planning and scheduling of science observations with the Spitzer Space Telescope.  The SSC will support a small number of high/medium impact ToOs on an annual basis (expected to be one or two per year) during the warm mission.