Chapter 6:
Parallel Science Exposures

In this chapter . . .

6.1 Parallel Science Limitations 6.2 Pure Parallels 6.3 Coordinated Parallel Containers

HST is, of course, a telescope with instruments that have relatively 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 because now WFC3 and STIS can work at the same time with ACS and NICMOS.

Parallel science refers to the use of two or more Science Instruments at the same time. The primary observation is the one that has determined the pointing of HST and it 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 NICMOS 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.

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 scheduled separately from primaries late in the scheduling process (essentially after the primary science schedule has been completely constructed). This type of parallel must be used whenever the primary and parallel exposures come from different proposals. The scheduling of pure parallels has no effect on the primary schedule: pure parallels will be placed on the schedule only when there are suitable opportunities available.

  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.