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Cycle 21 Phase II Proposal Instructions
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Chapter 8: Pointings and Patterns > 8.3 How to Fill Out the Pattern Parameters Form

8.3 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.
Text Proposal File
Patterns are identified by the Pattern_Number field under the Pattern_Data block in the Text Proposal File. The value of Pattern_Number must be an integer between 1 and 999. If an exposure anywhere in the proposal uses the PATTERN <#> [<exposure-list>] Special Requirement and <#> matches Pattern_Number, the specified exposures will be repeated at each point of the pattern. 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 Secondary_Pattern will be executed at each point, creating a two-dimensional matrix of pointing offsets. The parameters for the first pattern appear under the Primary_Pattern label in the Text Proposal File. If a second pattern is to be used, a similar Secondary_Pattern label followed by the same set of parameters should be added below the first.
The Pattern Parameters form looks like this in the Text Proposal File:
Pattern_Data
Pattern_Number: 11 !unique identifier
Primary_Pattern:
Pattern_Type: STIS-MAMA-BOX
Pattern_Purpose: DITHER
Number_of_Points: 4
Point_Spacing: 0.559
Line_Spacing: 0.559
Coordinate_Frame: POS-TARG
Pattern_Orient: 26.565
Angle_Between_Sides: 143.1
Center_Pattern: NO
 
Not all fields are required in all cases (see the examples below), and for some patterns both a Primary_Pattern and a Secondary_Pattern must be described. If none of the instrument-specific patterns will work, it is also possible to specify more generic lines and spirals.
8.3.1 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” on page 139), 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 Section 8.4 on page 139. 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; see 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.

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