Our goals are to maximise the scientific return of HST, and to ensure that the programs with the highest scientific merit are awarded time. With these goals in mind, there are a variety of different opportunities to request HST observations of both expected and unexpected transient phenomena, and to request observations with exceptional scientific urgency. But for what kind of programs is each opportunity best suited?

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.

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.

Mid-Cycle Programs

Mid-Cycle proposals are designed specifically for programs that, 1) for scientific reasons, could not have been proposed for at the time of the latest main cycle call, and, 2) for scientific reasons, must be observed before the next cycle, or for which there is a strong scientific urgency to observe them before the next cycle. Both criteria must be satisfied for a Mid-Cycle program. This is for observations of intermediate urgency where it is important that they are made before the next main call opportunity, but an extremely rapid response is not required. Classic examples include confirming a candidate redshift z~15 galaxy discovered in data taken since the last main cycle call, or follow-up of an Earth analog planet around a G-type star, discovered since the last proposal opportunity.

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.