3.3 Snapshot (SNAP) Proposals

Snapshot (SNAP) Programs consist of separate, relatively short observations whose durations should be limited to 40 minutes or less (including all overheads). During the process of optimizing the HST observing schedule, the scheduling algorithm occasionally finds short time intervals where it is impossible to schedule any exposures from the pool of accepted GO Programs. In order to make the HST schedule more efficient, STScI has developed the capability to insert Snapshot exposures of objects selected from a large list of available candidates.

3.3.1 Characteristics of SNAPs

Proposers request a specific number of Snapshot targets. If the proposal is approved, a specific number of targets is allocated. However, there is no guarantee that any individual target will actually be observed. SNAPs are placed on the schedule only after the observing sequence has been determined for the higher-priority GO targets. The number of observations actually executed depends on the availability of appropriate schedule gaps. In general, only a fraction of the sample targets will actually be observed. There is no commitment on the part of STScI to obtain any specific completion factor for Snapshot Programs. Increased scheduling efficiency has led to fewer Snapshot opportunities in recent cycles, and the restricted sky coverage offered by two-gyro observations has limited the number of targets available at any given time. As a result, the typical completion rates for Snapshot programs in Cycle 14 were in the range 10-40%, significantly lower than the average value of ~50% attained in earlier cycles. Proposers who require higher completion rates should consider submitting their programs as Survey GO Proposals (see Section 3.2.3).

The duration of a Snapshot visit, including guide-star acquisition, should not exceed 40 minutes. Proposals that require longer on-target times should be submitted as GO Survey programs.

Nominally, SNAP Programs terminate at the end of each cycle. However, they may be kept active, for scheduling efficiency reasons, for up to one additional cycle at decreased priority. In contrast to GO Programs (see Section 3.2.5), SNAP Programs cannot request observing time in future cycles.

Approximately 400 Snapshot observations were scheduled in Cycle 14. This number is expected to rise to some extent for Cycle 15. The number of Snapshot opportunities available in Cycle 16 will depend on the duration of the cycle and other factors that are unknown at this time.

Investigators interested in proposing for SNAPs are encouraged to consult the SNAP User Information Report, which contains details on how SNAPs are scheduled, the rules pertaining to them, completion rates for recent cycles, and other useful information.

3.3.2 Calibration SNAP Programs

Calibration proposals (see Section 3.2.4) may also be submitted as a Snapshot Program. As with GO Programs, all data obtained will be non-proprietary unless proposers specifically request a proprietary period. Successful proposers will be required to deliver documentation, and data products and/or software to STScI to support future observing or archival programs

Note that a specific science program that has special calibration requirements is not a Calibration Proposal; such a proposal should be submitted as a normal SNAP proposal and the necessary calibration observations should be added to the science program as described in Section 4.3.

All proposers are strongly encouraged to contact the appropriate instrument group to discuss their program prior to submission.

Calibration Proposals must be identified in the 'Special Proposal Types' section of the proposal (see Section 8.10).

3.3.3 Guidelines for SNAP Programs

Please consider the following when developing your SNAP proposal:

• Your willingness to waive part, or all, of the proprietary data-rights period is included in the selection criteria (see Section 6.1).
• You need not give a complete list of all targets and their coordinates in your Phase I proposal. However, you must specify the number of targets, and unambiguously identify the targets (e.g., reference to target lists in papers, or give a detailed description of the target characteristics). SNAP exposures may not be used for targets of opportunity (see also Section 4.1.2).
• In the 'Observation Summary' section of the proposal (see Section 8.15) you should provide a typical example of a snapshot exposure.
• SNAP programs cannot request observation times longer than 40 minutes, including target acquisition. Shorter duration snapshot observations have more scheduling opportunities than longer ones.
• SNAP observations should not be proposed with any special scheduling constraints (e.g., CVZ, timing requirements, or telescope orientation requirements). However, the special requirement BETWEEN may be used in the Phase II Program in some circumstances; for details see the SNAP User Information Report.
• A snapshot must not have any links to other snapshots (e.g., relative timing or orientation constraints), even if the snapshots are of the same source.
• A SNAP Program may not contain identical observations of the same source in different visits, unless there is a scientific motivation for obtaining observations of the same source at different times (e.g., science programs that require monitoring or follow-up). In the latter case, multiple identical visits of the same source may be requested; they should be counted as multiple targets (e.g., 10 different snapshot visits of the same galaxy count as 10 targets). Due to the nature of snapshot programs, repeated observations are not guaranteed.
• Moving-target Snapshot Programs are acceptable only if the timing requirements are at least one month duration. Solar system targets interior to the orbit of Jupiter are not permitted. Please note that timing constraints will reduce the chance of a target being scheduled. Due to the amount of effort required in implementing moving target SNAP programs, these observations ordinarily cannot be revised during the observing cycle, once the initial processing has been completed.
• SNAP Programs with the ACS/SBC are not allowed.
• Programs that require both GO orbits and SNAP targets should be submitted as two separate proposals. The proposals should refer to each other so that the reviewers will be aware that the proposals are part of the same project. This allows you to ensure that some essential targets are observed (the GO Program) with the rest of the targets being sampled statistically (the SNAP Program).
• It is no longer possible to assign priorities to snapshot targets.