8.2 Introduction to Patterns

Rarely does an observer ask to look at precisely one point on the sky. Instead, small-scale repositionings of the aperture may be made to fully sample a region spatially, or larger-scale movements may be made to mosaic a region of the sky. These movements are known as patterns.

In the past we have defined patterns on an case-by-case basis in a manner that seemed appropriate to each instrument. In rewriting the software that transforms your Phase II program into detailed observations, we realized that these different patterns were sometimes contradictory and incompatible, and that a common basis for describing patterns was needed for the current and future instruments. At the same time, we do not wish to further confuse observers by abandoning terms they may be familiar with, especially if they have continuing programs that could be affected by a change at an arbitrary time.

Here we describe the patterns syntax in APT for both the User Interface and the Text Proposal File; these descriptions provide both general capabilities and pre-defined “convenience” patterns.

8.2.1 APT User Interface

Creating Patterns

1.	First, you specify the pattern or patterns you want to use by using the “Patterns” form; then you select the type of pattern (or patterns) you want from a list, filling in the fields (where you can) with the values you want, or leave them to their assigned default values.

2.	Then you can create a Pattern Container (see Section 5.19 on page 100) in a visit, and then place your exposures in this container.

If the exposures in the Pattern Container contain other expansion constructs (such as Number_Of_Iterations > 1 or one of the CR-SPLIT Optional Parameters), the multiple exposures will be taken at each pattern point.

General Pattern Container Rules

If the exposures contain coordinated parallels, it must contain the entire parallel sequence (both parallel and primary exposures in the Coordinated Parallel Container; see Section 5.19 on page 100), and only those exposures. All parallel exposures will be taken at each pattern point. Only one pattern is allowed within any sequence.

All primary exposures in the exposure list (container) must have the same initial pointing: i.e., the same instrument configuration, aperture, target, and POS TARG offsets (if any).

Patterns may be specified for a moving target. In this case the pattern is taken while the target is being tracked and the offsets obtained will be a combination of target motion and motion imposed by the pattern relative to the target.

The following exposure-level Special Requirement is not allowed on an exposure that is part of a pattern:

SAME POSition AS <exposure>

Patterns are also not allowed with FGS TRANS mode (which specifies a separate scan with the FGS star selectors).

8.2.2 Text Proposal File

In the Text Proposal File, a pattern is specified as an exposure-level Special Requirement.

PATTERN <#> [<exposure-list>]

This specifies that each exposure in <exposure-list> (or the current exposure if no <exposure-list> is provided) should be repeated at each point in a pattern of discrete pointing offsets from the target. The <#> should be an integer between 1 and 999 that corresponds to the Pattern_Number of a pattern defined in the Pattern_Data block of the Text Proposal File. Details for providing Pattern_Data are given in the next section.

If the exposures in the <exposure-list> contain other expansion constructs (such as Number_of_Iterations > 1 or one of the CR-SPLIT Optional Parameters), the multiple exposures will be taken at each pattern point.

If the <exposure-list> contains coordinated parallels, it must contain the entire parallel sequence (both <parallel-exp-list> and <primary-exp-list> from the PARALLEL WITH Special Requirement; see PARallel <parallel-exp-list> WITH <primary-exp-list> (replaced by Coordinated Parallel Containers in the APT User Interface)), and only those exposures. All parallel exposures will be taken at each pattern point. Only one PATTERN Special Requirement is allowed within any sequence.

All primary exposures in <exposure-list> must have the same initial pointing: i.e., the same instrument configuration, aperture, target, and POS TARG offsets (if any).

Patterns may be specified for a moving target. In this case the pattern is taken while the target is being tracked and the offsets obtained will be a combination of target motion and motion imposed by the pattern relative to the target.

Notes: The exposure-level Special Requirement, SAME POSition AS <exposure>, is not allowed on an exposure that is part of a pattern: Also, Patterns are not allowed with FGS TRANS mode (which specifies a separate scan with the FGS star selectors).