3.3 Snapshot (SNAP) Proposals

3.3.1 Characteristics of SNAPs

3.3.2 Calibration SNAP Programs

3.3.3 Guidelines for SNAP Programs

Snapshot (SNAP) Programs consist of separate, relatively short observations with typical durations of 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 of objects selected from a large list of available candidates.

3.3.1 Characteristics of SNAPs

Accepted SNAP programs are allocated a specific number of Snapshot targets. However, there is no guarantee that any individual target will be observed, because 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 allocated targets will be observed.

There is no commitment on the part of STScI to obtain any specific completion factor for Snapshot Programs.

The Cycle 19 primary GO scheduling rate was nearly identical to the rate from Cycle 18, providing a similar number of Snapshot scheduling opportunities. However, as of the end of the nominal cycle, the completion rate for Cycle 19 Snapshot Programs was ~36% compared to ~46% for Cycle 18 Snapshot Programs at the end of that cycle. SNAP Programs are scheduled at high priority during their allocated cycle and, unlike GO Programs (see Section 3.2.5), SNAP Programs cannot request observing time in future cycles. However, they are kept active for one additional cycle at decreased priority to supplement the SNAP pool.

Given the two-stage scheduling priorities for SNAPs, the results of the past two cycles highlight the effect of visit duration and target distribution on completion rates. For example, there were 2.4 times as many Cycle 18 Snapshot visits with durations of 21-30 minutes executed as compared to Cycle 19 SNAPs of that duration. The numbers of scheduling opportunities did not change significantly between these two cycles. However, the candidate pool in this size category was 41% of the total candidates for Cycle 18 compared to just 23% for Cycle 19. So even though the total SNAP candidate pools for both cycles were about the same, Cycle 18 had a much larger selection of small-duration visits to choose from to fill the available SNAP scheduling opportunities of this size.

Likewise, a review of the target lists showed that Cycle 18 had a better target distribution on the sky compared to the Cycle 19 distribution. Of the 422 total Snapshot observations scheduled during the nominal 52 weeks of Cycle 19 GO science observing, 361 were from Cycle 19 programs. The remainder were from Cycle 18 programs. For comparison, there were 459 Snapshot visits scheduled during the nominal 52 weeks of Cycle 18, very few of which were from the prior cycle allocation.

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, 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 Calibration 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.

Users submitting Calibration Proposals are required 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).

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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 8.16) you should provide a typical example of a Snapshot exposure.

•	SNAP Programs cannot request observation times longer than 45 minutes, including guide star acquisition and target acquisition. In general, 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 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.

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SNAP Programs 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., monitoring or follow-up observations). 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.

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Moving-target Snapshot Programs are acceptable only if the timing requirements are of at least one month duration. Solar system targets interior to the orbit of Jupiter are not permitted. 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.

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Spectroscopic COS and STIS/MAMA SNAPs (other than those using the NUV-PRISM) are allowed, but the total number of targets accepted from all SNAP programs for COS and STIS/MAMA will be limited to 150. Imaging and moving target SNAPs with COS or STIS/MAMA modes are not allowed, due to the target and field bright-object checking requirements. Variable STIS/MAMA and COS SNAP 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.

•	STIS/CCD SNAPs are allowed for both imaging and spectroscopic modes.

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In addition, STIS/MAMA SNAP 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).

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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).

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Because SNAP targets are added to the observing schedule at a late stage of the schedule building process, moving-target SNAP programs may not use any detector that requires bright object screening (e.g. STIS/MAMA or COS). It is simply not practical to screen the field for any background objects that might violate bright object screening limits.