Pointing the Telescope

Acquiring an astronomical target and keeping it fixed in an aperture is the heart of observing with HST. Properly specifying target coordinates and motions is essential, of course, as is finding and acquiring suitable guide stars. Earlier chapters in this document have provided detailed instructions on how to specify coordinates and acquisitions for both fixed and moving targets; see 3.1 Fixed Targets and 3.2 Solar System Targets.

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.

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.

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 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), 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.

SAME POSition AS <exposure> is not allowed on an exposure that is part of a pattern. Patterns are also not allowed with FGS TRANS mode (which specifies a separate scan with the FGS star selectors).

How to Fill out the Pattern Parameters Form

The Pattern Parameters Form allows a pattern of pointing offsets to be defined. Patterns are useful for several different purposes: dithering to remove the effects of detector artifacts or to increase spatial resolution, mosaicing to cover a larger area on the sky, or moving away from the target to sample the background.

It is also possible to specify two nested patterns for an exposure or set of exposures. In this case, instead of taking the exposures at each point of the first pattern, the entire Sub-Pattern (or Secondary_Pattern in the Text Proposal File) will be executed at each point, creating a two-dimensional matrix of pointing offsets. 

Pattern Parameters

The Pattern Parameters Form has the following fields:

Pattern_Number

A unique identifier from 1 to 999; this is assigned in APT.

Pattern_Type

This specifies the type (and shape) of a pattern (generally an instrument-specific name; see Convenience Patterns), and, depending on the type, may constrain or completely determine the values of the other parameters. Many pattern types have been defined for the different instruments, as given below. Pattern parameters that are determined by the pattern type, or have default values, need not (or cannot) be changed in the form.

Some pattern types are intended for use with a combination of two patterns (Primary_Pattern and Sub-Pattern). With such pattern types the Sub-Pattern is mandatory. The Pattern_Type may specify that a parameter of the Sub-Pattern is constrained or determined by the value of the corresponding parameter of the Primary_Pattern. Whenever two patterns are used, the Pattern_Type of the Sub-Pattern defaults to the pattern type of the Primary_Pattern.

A Pattern_Type defined for a particular instrument is legal only if the first primary exposure in the pattern uses that instrument. Generic pattern types are valid with any instrument. Patterns specific to individual instruments are described in Convenience Patterns. The following are generic pattern types.

Pattern_Type: LINE

This specifies a linear pattern of offsets; all other parameters are open.

Pattern_Type: SPIRAL

This specifies a spiral pattern: all other parameters are open.

Pattern_Type: BOX

This specifies a box pattern; the Number_Of_Points = 4 and all other parameters are open.

Pattern_Purpose

This is a required text field which describes the scientific purpose of the pattern, but it is treated as a comment to help evaluate the Phase II program. There are four legal values: DITHER, MOSAIC, BACKGROUND, and OTHER:

DITHER means small-scale motions used to improve spatial resolution and data quality.

MOSAIC means movement of the aperture to sample different parts of an object or regions of sky.

BACKGROUND means a movement away from a source to sample the sky background, ordinarily used with infrared instruments.

OTHER means that the reason for the pattern should be provided in the Comments field. Unless a default is determined via the Pattern_Type, this parameter is required, although you may edit it from the default value to another valid value.

Number_of_Points

This specifies the number of points in the pattern; allowed values are from 2 to 50. Unless a default is determined via the Pattern_Type, this parameter is required.

Point_Spacing

This specifies the spacing between adjacent points in a pattern, in arcsec. For parallelogram patterns Point_Spacing will specify the length of the segment between the first two pattern points. The value of Point_Spacing ranges from 0 to 1440 arcsec. Unless a default is determined via the Pattern_Type, this parameter is required.

Note: patterns larger than about 130 arcsec (defined by the maximum pointing change between any two points in the pattern) may not be done on a single set of guide stars. For such patterns, unless the entire visit uses gyro guiding (PCS MODE Gyro), the visit-level Special Requirement DROP TO GYRO IF NECESSARY (see "DROP TO GYRO [NO REACQuisition]") must be specified to allow the farther pattern points to execute under gyro guiding.

Unless PCS MODE Gyro is specified on the visit, all patterns must be contained within a circle of radius 24 arcmin (1440 arcsec), with the first pattern point at the center.

Line_Spacing

This parameter is relevant only for parallelogram (box) patterns; it specifies the length of the second pattern segment of the parallelogram in arcsec. The allowed range is from 0 to 1440.0 arcsec.

Angle_Between_Sides

This parameter is relevant only for parallelogram (box) patterns; it specifies the angle (from 0 to 360) between sides of the parallelogram in degrees measured clockwise from the first pattern segment to the second.

Coordinate_Frame

This field has two legal values, POS-TARG and CELESTIAL. It specifies whether the pattern is being done in the POS TARG (spacecraft) frame or the celestial frame (by specifying offsets from the original target position on the sky). For patterns with a Pattern_Purpose of DITHER, POS-TARG is the only legal value.

Pattern_Orient

This field specifies the orientation of the first segment of the pattern with respect to the chosen frame, in degrees. If Coordinate_Frame: POS-TARG, this angle will be measured from the POS TARG +X axis toward the +Y axis. If Coordinate_Frame: CELESTIAL, it will be measured North through East, and will specify the direction of an offset from the target on the sky.

A value of 0 will cause the first pattern segment to be oriented along the POS TARG +X axis if Coordinate_Frame: POS-TARG, and along the North vector on the sky if Coordinate_Frame: CELESTIAL.

Center_Pattern

This indicates whether the pattern should be centered relative to the pointing (hereafter referred to as the “default pointing”) that the exposures would have had in the absence of a pattern. For a primary pattern (Primary_Pattern), the default pointing is the target position, unless the POS TARG Special Requirement (see "POSition TARGet <X-value>,<Y-value>") is used to shift the target relative to the aperture. For a secondary pattern (Sub-Pattern), the default pointing is the pointing offset from the target determined by the given pattern point in Primary_Pattern.

Legal values for Center_Pattern are YES (check in box provided) and NO (no check). The default is NO, unless overridden via the Pattern_Type. If Center_Pattern: NO, the first pattern point is placed at the default pointing. If Center_Pattern: YES, the first pattern point is offset so that the default pointing is placed at the geometric center of the pattern.

Sub-Pattern (or Secondary_Pattern in the Text Proposal File) Information

If applicable, the Sub-Pattern information must be provided (Type, Shape, Number of Points, etc.).

Note that if you are using the Text Proposal File, a Sub-Pattern is labeled as a Secondary_Pattern.

Convenience Patterns

The following patterns use the new syntax but have been constructed to duplicate the earlier pattern forms (if applicable). Their nomenclature is meant to be obvious; for example, “STIS-CCD-BOX” means the former "box" pattern for the STIS CCD. After each description we show what the information would look like.

A specific entry for a parameter (such as 4 for Number_of_Points with STIS-CCD-BOX) means that value may not be changed and doing so will cause an error. An indicated range (such as 0.0275-2.75 for Point_Spacing in STIS-MAMA-BOX) means you may select from within that range, and a “?” means any numeric value may be entered.

As previously noted, you may also nest patterns, but not all combinations make sense. If you use both a Primary_Pattern and a Sub-Pattern (or Secondary_Pattern in the Text Proposal File), they should be for the same instrument or at least one should be generic. Note that any convenience pattern can be added as a Sub-Pattern when  a  generic  pattern is selected  as the Primary_Pattern. If a pattern with large steps is to be combined with a pattern with small steps, the Primary_Pattern should contain the large steps, to minimize the time spent on pointing maneuvers. In the following sections, we show which patterns may be selected as a Sub-Pattern when the indicated convenience pattern is the Primary_Pattern. The observer should consult the references given within the instrument-specific subsections for detailed advice on selecting patterns and modifying pattern parameters. Do not mix instrument-specific patterns.

See 7.1 STIS Patterns, 7.2 ACS Patterns, and 7.3 WFC3 Patterns for the pattern details for each instrument. Note that COS does not have any predefined patterns.

Related Links

7.1 STIS Patterns
7.2 ACS Patterns
7.3 WFC3 Patterns

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