The Visit and Exposure Specifications are used to define the proposed exposures for all the Scientific Instruments. While the number of parameters needed to define all possible instrument configurations is large, the Visit and Exposure Specifications has been simplified by using standard Instrument Configurations and Operating Modes to set most of the instrument parameters to default values. The rest of the exposure keywords are used to define parameters that usually change from one exposure to the next, such as filters, exposure times, and special scheduling requirements.
Before proceeding further, it is useful to define more carefully what is meant by an exposure, a subexposure, and by a visit. (Note: APT also uses Exposure Groups or Containers; see Section 5.19, "Exposure Containers: Exposure Groups, Coordinated Parallels and Patterns" )
An exposure consists of the events and data corresponding to a unique Exposure_Number within a given visit. The full description of an exposure is called an Exposure Specification. Although many data samples (see APT subexposures) may result from a single execution of an Exposure Specification (due to the Instrument Configuration, Operating Mode, and Optional Parameters chosen), they are considered to be one exposure. Also, you may specify multiple exposures in an Exposure Specification by entering an integer greater than 1 for the Number_of_Iterations keyword (see 5.14 Number of Times to Iterate the Exposure [Number_of_Iterations]); additional exposures will be obtained consecutively (except for possible interruptions by Earth occultations, guide star acquisitions, and SAA passages).
Exposures defined within a visit will be obtained consecutively and in the order specified within a visit.
Within an Exposure Specification, each data sample that will be taken onboard HST is represented in APT by a separate entity called a subexposure. Subexposures are used to track the duration of the sample (actual_duration) and the orbit number in which it occurs (orbit_number).
A visit is an exposure or series of consecutive exposures, with overheads, on a given target, and may consist of the following parts:
If the visit lasts more than one orbit, it will continue with the following for each subsequent orbit:
Whenever one of the following occurs, a new visit must be defined:
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