6.2 Pure Parallels

Pure parallel proposals are identified by the Proposal Category (see 2.3.1 Proposal Information [Proposal_Information]). All visits in such proposals will be interpreted as pure parallel visits.

• Pure parallel proposals require additional information at the proposal level to specify pointing tolerances (see Parallel Pointing Tolerance).
• At the visit level, the Visit_Priority (see 5.2 Visit Priority [Visit_Priority]) must be specified to provide guidance for the scheduling system.

Parallel visits using the same target will be considered in order of Visit_Priority. A range of visits designed for opportunities of different lengths (say 1 to 10 orbits) is recommended, with the longer visits being given higher priority. At each opportunity on the calendar which matches the pointing of a set of parallel visits and whose pointing is stable within the proposal's Parallel_Pointing_Tolerance, STScI will schedule the highest-priority visit that fits in the opportunity. Exposures within a visit will be obtained consecutively and in the order specified, just as with primary visits.

How Pure Parallels Are Scheduled

Observer-defined visits correspond to scheduling units (SUs) in the HST ground system. An SU is a set of exposures which must be scheduled at the same time. An SU with coordinated parallel exposures is scheduled just like one without parallels. However, pure parallel SUs are scheduled through a separate process.

First the scheduling system builds a calendar from primary visits. Once the primary calendar is complete, a description of it is used to find appropriate parallel SUs to match the opportunities from the primary calendar. These parallels are then scheduled, subject to the constraints of the primary SUs already on the calendar.

While a primary visit is only scheduled once, a parallel visit may be scheduled many times (and generating a new SU each time) whenever it is the most appropriate use of an opportunity on the calendar. On the other hand, if a parallel visit never turns out to be the most appropriate, it will never be scheduled at all.

Instrument Configuration

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

Configuration	Operating Mode
WFPC2	IMAGE
NIC1 or NIC2 or NIC3	ACCUM, MULTIACCUM

Every exposure in a pure parallel proposal must use the same instrument. For NICMOS, coordinated parallel exposure containers using different detectors are not allowed in pure parallel visits.

Special Requirements

No visit-level special requirements are allowed in pure-parallel visits. Exposure-level special requirements are limited to the following:

• END ORBIT; EXPAND;
• MAX DUR; MIN DUR;
• NO-SPLIT;

We recommend that MIN DUR, plus either MAX DUR or EXPAND, be specified on all pure-parallel exposures, except with NICMOS, where no adjustment of exposure time is possible. If MIN DUR is omitted, the default minimum exposure time is set to 80% of Time_Per_Exposure. If MAX DUR is omitted, the default maximum exposure time is unlimited if EXPAND is specified, and is otherwise set to 120% of Time_Per_Exposure.

For visits intended to span multiple orbits, END ORBIT should be specified on the exposure that is intended to end each orbit in order to provide STScI with better guidance in matching parallel visits to the sizes of opportunities.

Exposure Containers

In pure-parallel visits you can use the following container (see Section 5.19, "Exposure Containers: Exposure Groups, Coordinated Parallels and Patterns" ):

• Exposure Group Container (Type: Sequence)

Optional Parameters

Dithering parameters and wavelength scans with the WFPC2 linear ramp filter are not permitted with pure parallels because the pointing is determined by the primary observation.