6.5 Constructing the Program

Coordinated Parallel Observations

The final step is to efficiently pack all science exposures and overheads into the visibility period of each orbit for all visits. For particularly complex programs, the APT Phase II orbit planner can be used for assessing the orbit layout. Please contact help@stsci.edu for assistance.

When placing observations into orbits and visits, note that exposures cannot be paused across orbits. For instance, if there is 20 minutes left in an orbit, only an exposure that takes 20 minutes or less (including overhead) can be placed in it. For a 30 minute exposure, all of it can be placed in the next orbit, or a 20 minute exposure can be placed at the end of the first orbit, and a second 10 minute exposure in the next orbit.

As an example, Table 6.12 shows the layout of a visit with two orbits of spectroscopic observations that require a target acquisition, but no SAMs and no special calibration observations. For simplicity, overheads are shown to occur after each exposure; in reality, some overheads occur before an exposure (e.g., a filter change) while others appear afterwards (e.g., readout time).

Table 6.12: Example Visit

Orbit 1	Guide star acquisition	Target acquisition	Overhead	Science exposure	Overhead	Earth occultation
Orbit 2	Guide star re-acquisition	Science exposure	Overhead	Science exposure	Overhead	Earth occultation

More detailed examples for each of the SIs are given in Appendix A. Those examples are for common, simple uses of the instruments. For more complex examples and observing strategies, please consult the HST Instrument Handbooks (see Section 1.3).

Coordinated Parallel Observations

For a program with coordinated parallel observations (see Section 4.2.1 of the Call for Proposals), laying out the parallel observations into orbits and visits is fairly straightforward. The primary observations determine the orbit and visit structure, and the coordinated parallels should conform to the visit structure of the primary observations.