Visits, Exposures and Exposure Groups

Chapter 5: Visits, Exposures and Exposure Groups

Tables and Figures

Table 5.1: Special External Target Names

Table 5.2: Instrument Configurations and Operating Modes

Table 5.3: Aperture and Field of View Names

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,” on page 100)

Exposures

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 Section 5.14 on page 98); 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.

APT subexposures

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).

Visits

A visit is an exposure or series of consecutive exposures, with overheads, on a given target, and may consist of the following parts:

1.	Guide-star acquisition (to point HST at the target)

2.	Target acquisition (to place the target in an instrument aperture)

3.	Science exposure(s) (to obtain the data)

4.	Instrument overheads (to set up the instrument and read out the data)

5.	Instrument calibrations (if more than the standard calibration is required)

If the visit lasts more than one orbit, it will continue with the following for each subsequent orbit:

6.	Guide-star re-acquisition (to keep HST pointed and locked after Earth occultations)

7.	Science exposure(s)

8.	Instrument overheads

9.	Instrument calibrations

Whenever one of the following occurs, a new visit must be defined:

•	A change in target position of greater than 1 degree. (Contact your Program Coordinator for details regarding solar system objects that move more than about 30 arcsec during the observation and thus are likely to require multiple sets of guide stars).

•	Repeated, periodic, or other time-separated observations with an interval between exposures such that one or more empty visibility periods (orbits with no exposures) would otherwise be required.

•	Required change in spacecraft roll orientation.