|Space Telescope Science Institute|
|HST CP and Primer for Cycle 23|
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. In Cycle 23, up to 1000 SNAP observations may be accepted to provide a sufficiently large pool of candidates.3.3.1 Characteristics of SNAPsAccepted SNAP Programs are allocated a specific number of 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. Unlike GO Programs (see Section 3.2.5), SNAP Programs cannot request observing time in future cycles. However, un-executed SNAPs remain active at decreased priority for a second cycle.There is no commitment on the part of STScI to obtain any specific completion factor for SNAP Programs.The average expected completion rate for SNAP Programs is ~33%. However, the actual completion rates for individual programs vary, depending on several factors including the number of targets and the average duration and sky distribution of the observations. In general, shorter-duration and well-distributed SNAP observations have a higher number of scheduling opportunities and a higher chance of being executed than longer duration and/or spatially clustered SNAP observations.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.Calibration Proposals (see Section 3.2.4) may also be submitted as SNAP Proposals. 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.
• 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 SNAP exposure.
• 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.
• 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 SNAP visits of the same galaxy count as 10 targets). Due to the nature of SNAPs, repeated observations are not guaranteed.
• Moving-target SNAP 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.
• The number of spectroscopic COS and STIS/MAMA SNAPs (other than those using the NUV-PRISM) is limited to 150, due to the target and field bright-object checking requirements. For the same reason, imaging and moving target SNAPs with COS or STIS/MAMA modes are not allowed. 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 SNAP 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 when possible.
• 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 SNAP 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).
• Programs that require both GO orbits and SNAP targets should be submitted as two separate proposals. The proposals should refer to each other so the reviewers will be aware 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).
• 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.