Introduction

The instruments onboard HST have small apertures that must be accurately pointed to achieve useful scientific results. However, the value of  “parallel” observations was recognized during the earliest stages of HST’s design because by turning on other instruments one could discover new objects and new phenomena, or just acquire highly detailed images of random regions of the sky that are the equivalent of a survey. This parallel capability was present to a limited degree at launch, but it has become especially important since the fourth servicing mission. Now WFC3 and ACS can work at the same time with COS or STIS.

Parallel science uses two or more Science Instruments at the same time. The primary observation determines the pointing of HST and always takes precedence. The parallel observation is the one enabled to take place because the primary observation permits suitable conditions, but the parallel observation must never interfere with the primary. Although parallel observations are conceptually simple (e.g., “Turn on ACS while COS is in use”), they are, in fact, complex to execute. Commanding the instruments for parallels and primaries at the same time requires strict observance of complex timing rules in order to avoid damage to instruments. Some special restrictions must be applied in order to effectively schedule and execute parallel observations. For additional information see Parallel Observations with HST.

From the point of view of the HST ground systems there are two types of parallel exposures: pure and coordinated parallels.

• Coordinated parallels are written to be part of the same proposal, with the intention of scheduling the primary and the parallels together. Coordinated parallels are inextricably linked with their primaries: both must be scheduled in order for either to be scheduled.
• Pure parallels are defined at the beginning of the observing cycle based on the structure of available primary visits. They are scheduled simultaneously with and according to the plan windows of those primary visits. Pure parallels may not affect the scheduling of primary visits. This type of parallel must be used whenever the primary and parallel exposures come from different proposals.

The HST pointing control system will automatically correct for differential aberration effects for the Primary observations. There will be a small smearing effect in the parallel SI which cannot be corrected on-board. The magnitude of the effect will vary with the primary target location and aperture, but will be a maximum of ±0.020 arcsec.

Parallel Science Limitations

These restrictions are to ensure that parallels can be scheduled and executed safely and efficiently:

• Parallel observations have some special restrictions on the targets that may be specified; see Pure Parallels and Coordinated Parallels for details.
• All scheduling constraints (pointing, orientation, and relative timing) that apply to coordinated primaries and parallels must be specified only on the primary exposure.
• No visit-level special requirements, and only a few exposure-level special requirements, are applicable to pure parallel visits.
• Exposures parallel with any instrument mode which permits motion of HST (e.g., interactive or onboard acquisitions) are not supported.
• Neither parallel exposures nor the primary of a coordinated parallel may have real-time (i.e. TDRS contact) requirements of any kind.
• In order to protect the COS and STIS/MAMA detectors from inadvertent over-illumination, these configurations may be used for coordinated parallels only if an exact ORIENT (e.g., ORIENT 20D TO 20D) is specified (ORIENT can have a range). Also, the coordinates of the parallel field must be determined and the parallel target or field must pass the same bright-object  screening applied to COS and  STIS/MAMA primary observations.
• ACS/SBC may not be used for coordinated parallels.
• There is a limit of one COS time-tag exposure in a parallel exposure or as the primary exposure of a coordinated parallel.
• Parallel observations may not be made during STIS MAMA TIME-TAG observations.

These limitations are discussed further in the following sections.

Pure Parallels

Pure parallel proposals are identified by the Proposal Category. All visits in such proposals will be interpreted as pure parallel visits.

Instrument Configuration

Pure parallel exposures are limited to the following instrument configurations and modes:

Every exposure in a pure parallel proposal must use the same instrument.

Special Requirements

No visit-level or exposure special requirements are allowed in pure-parallel visits.

Exposure Containers

In pure-parallel visits you can use the following containers (see Exposure Containers: Exposure Sequences, Coordinated Parallels and Patterns):

• Exposure Group Container (Type: Sequence)
• Coordinated Parallel Container

For example, a pure parallel Visit may include an exposure using WFC3/IR and a coordinated exposure using ACS/WFC.

Coordinated Parallel Containers

The APT User Interface

This special requirement Parallel has been replaced in the APT GUI with the use of Coordinated Parallel Exposure Containers. See Exposure Containers: Exposure Sequences, Coordinated Parallels and Patterns.

A Coordinated Parallel Container designates a set of one or more parallel exposures (<parallel-exp-list>) that will execute in parallel with a set of one or more primary exposures (<primary-exp-list) in the same visit. The SI used in the first exposure defines the primary SI (and therefore defines the <primary-exp-list>). All other exposures that use the same SI will also be considered primary.

All exposures using a different SI will be considered parallel. Parallel exposures using the same SI will be executed in the order specified in the container. The first parallel exposure which uses a given instrument will be executed as early as possible, but not before the first primary exposure.

Instrument Configuration

The exposures in <primary-exp-list> and <parallel-exp-list> are limited to the following instrument configurations and modes:

¹In order to protect the STIS MAMA detectors from inadvertent over-illumination, these configurations may be used as coordinated parallels only if an exact ORIENT (e.g., ORIENT 20D to 20D) is specified. Also, the coordinates of the parallel field must be determined and the parallel target or field must pass the same bright-object screening applied to MAMA primary observations.

²FGS may only be used as primary, never parallel.

³ACS exposures cannot be used in both the <primary-exp-list> and the <parallel-exp-list>, and all ACS exposures in the <parallel-exp-list> must use the same configuration.

⁴In order to protect the COS FUV and NUV detectors from inadvertent overillumination, these configurations may be used as the coordinated parallel observation only if an exact ORIENT (e.g., ORIENT 20D to 20D) is specified. Also, the coordinates of the parallel field must be determined and the parallel target or field must pass the same bright-object screening applied to COS primary observations.

⁵COS Time-tag exposures are limited to a single one in a parallel exposure or as the primary exposure of a coordinated parallel.

All exposures in <primary-exp-list> must have the same Instrument Configuration (Config) and the same Aperture or Field of View (Aperture). No exposure in <parallel-exp-list> may use the same SI as the primary exposures.

Targets

Coordinated parallels may specify fixed, generic, or solar system targets; the special target ANY; or internal targets. However, the pointing of HST will be determined only by the primary exposures. Any pointing conditions to be applied on the primary+parallel combination must be specified on the primary exposures via exposure-level special requirements, or on the visit as a whole via visit-level special requirements. All external exposures in a given <primary-exp-list> must have the same pointing. This generally means the same target, aperture, and POS TARG.

If a parallel exposure specifies a fixed target, it should be a different target from the primary ones and should appear in the Target List. In this case, an ORIENT special requirement is required to ensure that the parallel target is in the aperture. It is the observer’s responsibility to verify that the specified orientation will place the parallel target in the aperture; STScI will not check the geometry. Contact your Program Coordinator if you need assistance.

If the parallel target is diffuse and the orientation does not matter, or if there is no parallel target as such and the intent is just to sample whatever the parallel aperture happens to fall on, you should select the ANY target (which should not appear in your Target List).

Special Requirements

The RT ANALYSIS exposure-level special requirement is disallowed for both primary exposures (any exposure in a <primary-exp-list>) and parallel exposures. 

The following exposure-level special requirements are allowed for primary exposures, but not for parallels:

Allowed without restriction:

No parallel exposure may appear in the <exposure-list> of an RT ANALYSIS FOR Special Requirement, or be the <exposure> referenced by a SAME POS AS special requirement.

Number of Iterations

The Number_Of_Iterations must be set to 1 for any COS exposure in a coordinated parallel container.

Ordering Restrictions and Interpretations

If the exposures in the <parallel-exp-list> contain exposures that use different SIs, an attempt will be made to execute each set of exposures with the same SI in parallel with the other sets. All exposures within a given set must be contiguous in the container. Within a set, exposures will be executed in the order they appear.

Due to readout conflicts and limits on the number of SIs which may execute simultaneously, parallel exposures sometimes have to be delayed. If a parallel exposure conflicts with an exposure in its primary list, the parallel will be delayed. If two parallel exposures using different SIs conflict, the exposure which appears later in the container will be delayed.

Following a set of exposures in a Parallel Container, subsequent exposures not in the container will be delayed until after all the primary and parallel exposures have completed.

Efficiency Considerations

Within a set of coordinated-parallel exposures, the ground system will not break up sequences of exposures that are too long to fit in an orbit, as it does for non-coordinated-parallel exposures. Each group of exposures with the same SI must be short enough to fit in one orbit. If it is necessary to take coordinated-parallel data over multiple orbits, a separate Parallel Container—with a new set of exposures—should be specified for each orbit.

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