Hubble Space Telescope Call for Proposals for Cycle 17

4.2 Parallel Observations

Since the scientific instruments are located at fixed positions in the telescope focal plane, it is possible to increase the productivity of HST by observing simultaneously with one or more instruments in addition to the prime instrument. Those additional observations are called parallel observations.

Since each instrument samples a different portion of the HST focal plane (see Figure 2.2 of the HST Primer), an instrument used in parallel mode will normally be pointing at a "random" area of sky several minutes of arc away from the primary target. Thus parallel observations are usually of a survey nature. However, many HST targets lie within extended objects such as star clusters or galaxies, making it possible to conduct parallel observations of nearby portions of, or even specific targets within, these objects.

Depending on whether a parallel observation is related to any specific primary observation, it is defined either as a coordinated parallel or pure parallel. Coordinated Parallel Observations are observations related to a particular primary observation in the same proposal. Pure Parallel Observations are unrelated to any particular primary observation (i.e., the primary observation is in another program). Investigators interested in proposing for parallels must consult the Parallel Observations User Information Report, which provides further details on how coordinated and pure parallels are defined, implemented and scheduled.

Parallel observations are rarely permitted to interfere significantly with primary observations; this restriction applies both to concurrent and subsequent observations. Specifically,

• A parallel observation cannot dictate how the primary observation will be structured (e.g. it cannot cause the adjustment of primary exposures). This is particularly directed toward pure parallels where the definition of the observations is independent of and subordinate to a primary observation.
• Parallel observations will not be made if the stored command capacity or data volume limits would be exceeded.
• Pure Parallel observations may not explicitly constrain the scheduling of a primary observations, that is, they may not specify orientation or timing constraints.
• Coordinated Parallel observations may include orientation or timing constraints as requested and justified in the accepted HST Phase I proposal.
• Pure Parallel observations are subject to the availability of parallel observing opportunities as identified by STScI (see Section 4.2.2).

4.2.1 Coordinated Parallel Observations

Coordinated Parallel Observations must be marked in the `Observation Summary' section of the proposal (see Section 8.16).

Coordinated Parallels use one or more instruments, in addition to and simultaneously with the prime instrument in the same proposal, e.g., to observe several adjacent targets or regions within an extended object. Proposals that include Coordinated Parallel Observations should provide a scientific justification for and description of the parallel observations. It should be clearly indicated whether the parallel observations are essential to the interpretation of the primary observations or the science program as a whole, or whether they address partly or completely unrelated issues. The parallel observations are subject to scientific review, and can be rejected even if the primary observations are approved.

Proposers are generally not allowed to add Coordinated Parallel Observations in Phase II that were not explicitly included and approved in Phase I. Any such requests will be adjudicated by the Telescope Time Review Board (TTRB).

4.2.2 Pure Parallel Observations

Pure Parallel Observations must be marked in the `Observation Summary' section of the proposal (see Section 8.16).

A new process for obtaining Pure Parallel Observations is being implemented in Cycle 17. It is designed to solve some of the problems that occurred with our earlier Pure Parallel implementation, and also take full advantage of the instruments being installed in SM4. This new process will provide a more reliable estimate, in advance of observations, of the number of orbits that will be executed for the accepted parallel programs.

The Parallel Observing User Information Report provides a complete description of this observing mode and is required reading if you are considering submitting a Pure Parallel observing program.

Restrictions

Pure Parallel observations will be restricted to orbits where COS is the primary instrument. Consequently, parallel opportunities will be limited by the actual number of COS orbits that are allocated in the Cycle 17 prime science program, and to the regions of sky being observed by the prime COS programs. The actual allocation will depend on the results of the Cycle 17 peer review, but STScI is anticipating that as many as 600 HST orbits may be available for parallel observing. We expect that the bulk of these COS visits will support multi-orbit (up to ~5 orbits) Pure Parallel visits.

Pure Parallel programs will be restricted to using ACS/WFC, ACS/HRC, and WFC3/UVIS or WFC3/IR for parallel observing. Multiple parallel science instrument observing may carried out using ACS and WFC3 simultaneously on the same prime COS observation (see the Parallel Observing User Information Report for details on the use of multiple parallel science instruments in a Pure Parallel Program). COS prime science programs with Coordinated Parallels that use ACS or WFC3 are not eligible for Pure Parallel programs.

Matching with Prime Programs in Phase II

The PI of an accepted Pure Parallel program will be asked to nominate parallel science opportunities for their program from a list of prime science observations provided by STScI. These opportunity matches are submitted to STScI where conflicts between parallel programs will be resolved.

The process of matching Pure Parallel observations to prime programs will occur during the planning and implementation phase (Phase II) for Cycle 17 so that it can be known in advance when and how the parallel observations can be executed. This is in contrast to previous cycles where PIs were asked to submit parallel observing templates which were not matched to prime science opportunities until an observing schedule was actually built. As in past cycles, proposals for Pure Parallel observations may specify either particular or generic targets, although the latter are more common and provide more flexibility for matching parallel observations to actual opportunities.

Review and Execution

The Review Panels and TAC will select the programs based on the proposed science. The TAC will consider all accepted programs and produce a ranked list as an aid for resolving potential conflicts. All GO Pure Parallel Programs will have a default proprietary period of 12 months. Pure Parallel observations are assigned to specific prime observations, and those parallel observations will be carried over to subsequent cycles if the prime observations are not executed in Cycle 17.

4.2.3 Restrictions and Limitations on Parallel Observations

Parallel Observations with ACS

The ACS/WFC and ACS/HRC may be used for Coordinated Parallel Observations with any other instrument as prime. One special case of this observing mode occurs when ACS itself is the prime instrument. In this case, ACS/WFC parallel observations are permitted with ACS/HRC in prime and vice versa (so called "auto-parallels"). Auto-parallels are inserted by software into the primary observation as Coordinated Parallels. The filter choice for auto-parallels is predetermined by the filter used in the primary observation (see the ACS Instrument Handbook for details). Whether or not auto-parallels can be created depends on the primary exposure and what the observer specifies in his/her proposal (see the ACS Instrument Handbook for details). Auto-parallel data have the same proprietary period as the primary data.

The ACS/SBC may be used for Coordinated Parallel Observations with any other instrument as primary, but only if the telescope orientation is exactly specified and the field passes bright-object checking. The ACS/SBC may not be used for pure parallel observations.

The ACS/WFC and ACS/HRC may be used for Pure Parallel Observations with the COS instrument as prime (see Section 4.2.2).

Parallel Observations with COS

The COS/FUV MCP detector may be used for Coordinated Parallel Observations with any other instrument as prime, provided that the telescope orientation is specified exactly and the parallel field passes bright object checking.

The COS/NUV MAMA detector may be used for Coordinated Parallel Observations with any other instrument as prime, provided that the telescope orientation is specified exactly and the parallel field passes bright object checking.

COS may not be used for Pure Parallel Observations in any detector mode.

Parallel Observations with FGS

The FGS cannot be used for either Pure or Coordinated Parallel Observations.

Parallel Observations with NICMOS

NICMOS may be used for Coordinated Parallel Observations with any other instrument as prime. Observations that use two or more NICMOS cameras at the same time are considered parallel observations and must be requested in Phase I.

NICMOS may not be used for Pure Parallel Observations in Cycle 17.

Parallel Observations with STIS

The STIS/CCD detector may be used for Coordinated Parallel Observations with any other instrument as primary.

The STIS/MAMA detectors may be used for Coordinated Parallel Observations with any other instrument as primary, but only if the telescope orientation is exactly specified and the field passes bright-object checking.

STIS may not be used for Pure Parallel Observations in any detector mode.

Due to some limitations of the HST scheduling system, when STIS is the prime instrument and another instrument is used for a Coordinated Parallel, STIS auto-wavecals will never be done during an occultation; instead these calibration exposures have to be scheduled when the external target is visible, leading to a reduction in the observing efficiency.

Parallel Observations with WFC3

WFC3 may be used for Coordinated Parallel Observations with any other instrument as prime. WFC3 may only be used for Pure Parallel Observations with COS as prime (see Section 4.2.2).

Pointing Accuracy for Parallel Observations

The spacecraft computers automatically correct the telescope pointing of the primary observing aperture for the effect of differential velocity aberration. This means that image shifts at the parallel aperture of 10 to 20 mas can occur during parallel exposures.