Our goal is to maximize the scientific return of HST. In particular, with UV sensitivity, a substantial field of regard, and moderately quick response time, HST provides important capabilities to support Time-Domain and Multi-Messenger (TDAMM) science. There are a variety of opportunities available to request observations of both expected and unexpected transient phenomena, and to request observations with exceptional scientific urgency. 

Targets of Opportunity (ToOs)

ToOs are intended for observations of expected transient phenomena. That is, when a proposer has a high degree of confidence that a certain event of class of event will happen over the coming cycle, but the exact timing and location of that event are unknown so cannot be stated at the time of proposing. These responses can be rapid or slow, depending on the scientific need. Observations of supernovae are a good example of a classic ToO proposal, it's expected that one or more supernovae matching a certain set of criteria will happen during a cycle, but where and when is not known at the time of submitting the proposal.

HST offers several types of ToO:

• Non-Disruptive ToOs:
  • These are for observations that can accommodate turnaround times (from the submission of the trigger) of more than 21 days.
  • There is no limit on the number of non-disruptive ToOs within a given cycle.
• Disruptive ToOs:
  • These are for sources that require turnaround times between 2 days and 21 days.
  • Disruptive ToOs are limited to 8 activations per cycle.
• Ultra-Disruptive ToOs:
  • These are for sources that require turnaround times of ≤2 days. In general, the fastest turnaround time anticipated is ~36 to 48 hours from the activation of the trigger.
  • Ultra-disruptive ToOs are limited to one per cycle.
• FlexDay ToOs:
  • These are intended for programs where proposers expect a large pool of events requiring rapid turnaround, from which scientifically appropriate targets can be selected to minimize response time and disruption of Hubble operations. This relatively recent avenue for transient science expands the opportunities for rapid response, with turnaround times as short as ~2 days possible. Activations will be observed on a Thursday, and triggers can be submitted until 10:00 UT on the preceding Tuesday.
  • FlexDay ToOs are limited: STScI currently identifies one Flexible Thursday per month for these observations, with up to two FlexDay activations per Flexible Thursday.
• Carry-Over ToOs:
  • Standard ToO programs terminate at the end of each cycle. Carry-over ToOs are valid for 2 cycles. They are designed to accommodate rare phenomena, where the likely frequency is <1/cycle.
  • Carry-over ToOs may be disruptive or non-disruptive, but not ultra-disruptive.
• Future-Cycle ToOs:
  • Proposers may request ToO activations in the 2 cycles following the current cycle, but those requests are limited to non-disruptive activations. This category is designed to accommodate phenomena that are relatively frequent, where more than one activation can be expected on an annual basis.

ToOs may only be submitted in response to the main cycle call. As much as possible, we encourage observations of expected transient phenomena to be submitted as ToOs in the main call. This ensures that the proposals can be evaluated fairly alongside the other science being proposed for the cycle.

Director's Discretionary (DD) Programs

DD programs are designed for observations of newly-discovered unexpected transient phenomena or when developments since the last cycle make time-critical observations or timely follow-up necessary. That is, for observations of events or objects that were not anticipated to occur in the cycle (or at all!) so could not have been submitted at the last main call, or for which it is critical that they are observed on a rapid timescale. Interstellar interlopers are a classic example of a DD program, these are unexpected phenomena that are only observable for a short period of time, so their presence was unanticipated and requires quick follow-up.

In general, pilot studies are not appropriate for any of these opportunities.