3.3 Snapshot (SNAP) Proposals

Snapshot (SNAP) programs consist of separate, relatively short observations whose durations are usually limited to 45 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 into these gaps on 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. Typical completion rates are in the range from 30-70%. For STIS/MAMA SNAPs, the inability to schedule these observations during SAA-impacted orbits reduces the completion rate to about 20%. However, there is no commitment on the part of STScI to obtain any completion factor for Snapshot 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 3.2.3), SNAP programs cannot request observing time in future cycles.

In recent years there have been approximately 500-1000 Snapshot observations scheduled per year, but this could be different in Cycle 13 and depends on 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 Guidelines for SNAP Proposals

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.
• Shorter duration snapshot observations have more scheduling opportunities than longer ones. While SNAP observations longer than 45 minutes (including all overhead times; e.g., guide-star acquisition, target acquisition and readout times) are not impossible, their probability of execution is small. The majority of executed SNAP observations are 30 minutes or less.
• 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. Moving target snapshots are not permitted with the MAMA detectors and targets interior to the orbit of Jupiter can be observed on gyro control only. Please note that timing constraints will reduce the chance of a target being scheduled.
• Both imaging and spectroscopic STIS/MAMA snapshots are allowed, but the combined total targets accepted from all programs will not exceed 300, with imaging targets limited to a total of 100, due to the target and field bright-object checking requirements. Variable STIS/MAMA snapshot targets must have well-defined MAXIMUM UV fluxes, which will be used for the bright-object checking. There are no restrictions on the numbers or variability of proposed STIS/CCD snapshot targets, which do not require bright-object checking and have a higher expected completion rate since they are not restricted to SAA-free orbits. Thus, use of the CCD NUV configurations should be considered instead of the MAMA NUV.
• In addition, STIS/MAMA snapshot proposals should be limited to one or a few straightforward configurations. Specifically, use of the NDQ filters is not allowed. Use of the 0.2X0.2 echelle aperture is recommended for first-order programs without a scientific long-slit requirement, in order to expedite the field screening process. Excessively complex STIS/MAMA snapshot targets, fields, or instrumental configurations may not be implemented in Phase II because of the limited resources available for bright-object checking, combined with the relatively low expected completion rate; if you are in doubt on this issue, contact the STScI Help Desk (see Section 1.5).
• 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 possible to assign relative priorities (high/medium/low) to approved snapshot targets within a program. Priorities can be assigned in the Phase II program (see Section 10.2), and need not be addressed in the Phase I proposal. See the SNAP User Information Report for details.