4.2 Parallel Observations

4.2.1 Coordinated Parallel Observations

4.2.2 Pure Parallel Observations

4.2.3 Restrictions and Limitations on 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 the 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). Coordinated Parallel Observations will ordinarily be given the same proprietary period as their associated primary observations.

4.2.2

Pure Parallel Observations

 

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

The Pure Parallel Observing process is designed to take advantage of the full complement of instruments installed in SM4. Similar to prime science planning, the parallels process provides a reliable estimate, in advance of observations, of the number of orbits that will be executed on accepted parallel programs during the cycle. 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 are currently restricted to orbits where COS and STIS are the primary instruments. Consequently, parallel opportunities will be limited by the actual number of orbits allocated to these instruments and to the corresponding regions of sky being observed. Past experience shows that the final allocation of Pure Parallel orbits also depends on the science goals of the parallel programs (e.g. desired targets may not be available and multiple Pure Parallel programs can compete for the same prime opportunities.) STScI continues to investigate ways to expand the number of Pure Parallel observing opportunities.

For the purpose of Pure Parallel orbit allocation, an orbit is defined as having target visibility of at least 2500 seconds. The number and types of parallel observing opportunities will vary depending on the mix of prime GO programs each cycle. However, the total number of Pure Parallel orbits executed could be less than originally planned due to changes to the prime programs or on-board execution failures. The actual Pure Parallel allocation for Cycle 19 was 360 HST orbits. Parallel observers selected 64 1-orbit, 38 2-orbit, 16 3-orbit, 10 4-orbit and 13 5-orbit-and-larger opportunities that could satisfy the science requirements of the accepted programs. This was less than anticipated due to fundamental changes in the construction of COS prime observations this cycle. However, STScI is working on enhancements to the Pure Parallel opportunity identification and matching process which we expect will result in a much greater number of observing opportunities in Cycle 20.

Pure Parallel programs will be restricted to using ACS/WFC, 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 observation (see the Parallel Observing User Information Report for details on the use of multiple parallel science instruments in a Pure Parallel Program). 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

PIs with accepted Pure Parallel programs will be given a list of parallel science opportunities that STScI has identified as being suitable for their program. The PI then selects and submits a final list of opportunity matches to STScI in the Phase II Pure Parallel program submission.

The process of matching Pure Parallel observations to prime programs will occur during the planning and implementation phase (Phase II) so that it can be known in advance when and how the parallel observations can be executed. 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 20.

4.2.3

Restrictions and Limitations on Parallel Observations

Parallel Observations with ACS

The ACS/SBC may not be used for either Pure or Coordinated Parallel Observations in any mode.

The ACS/WFC detector may be used for Coordinated Parallel Observations with any other instrument as prime.

The ACS/WFC may be used for Pure Parallel Observations with the COS and STIS instruments 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 STIS

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

Neither the STIS/NUV-MAMA PRISM mode nor any STIS/MAMA imaging mode can be used for Coordinated Parallel Observations.

STIS/MAMA spectroscopic modes (other than the NUV/PRISM) may be used for coordinated parallel observations, but only if an exact ORIENT is specified.

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

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 slight 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 or STIS 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.