Phase 2 Proposal Instructions for Cycle 10

8.3 How to Fill Out the Pattern Parameters Form

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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.

Patterns are identified by the Pattern_Number field under the Pattern_Data block in the RPS2 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 RPS2 template. 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.

You do not need to specify a Secondary_Pattern unless you wish to, and you may delete it from your template

 

The Pattern Parameters form looks like this in your RPS2 file:

• Pattern_Data
• Pattern_Number:11!uniqueidentifier
• Primary_Pattern:
• Pattern_Type:WFPC2-BOX
• Pattern_Purpose:DITHER
• Number_of_Points:4
• Point_Spacing:0.559
• Coordinate_Frame:POS-TARG
• Pattern_Orient:26.565
• Center_Pattern:?

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.

Pattern_Type

This specifies the type of pattern (generally an instrument-specific name), 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 be entered in the RPS2 file.

Some pattern types are intended for use with a combination of two patterns (Primary_Pattern and Secondary_Pattern). With such pattern types the Secondary_Pattern is mandatory. The Pattern_Type may specify that a parameter of the Secondary_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 Secondary_Pattern defaults to the pattern type of the Primary_Pattern, so it is not necessary to enter it under Secondary_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 the next section; see 8.4Convenience Patterns. The following are generic pattern types.

• Pattern_Type: LINE

This specifies a linear pattern of offsets, with all other parameters open.

• Pattern_Type: SPIRAL

This specifies a spiral pattern, with all other parameters 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 2 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.
• If OTHER is used, the reason for the pattern should be provided in the visit or exposure comments. 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, from 1 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 such as STIS-CCD-BOX, Point_Spacing will specify the length of the segment between the first two pattern points (the spacing between adjacent lines is determined by the Line_Spacing and is not user-specifiable). 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 IF NECESSARY [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.

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). The default is POS-TARG, unless overridden via the Pattern_Type. For patterns with a Pattern_Purpose of DITHER, POS-TARG is the only legal value.

Note: If the NICMOS OFFSET=FOM optional parameter is used, the pattern will be executed by moving the NICMOS Field Offset Mirror, rather than the telescope. In this case, Coordinate_Frame must be set to POS-TARG. All primary exposures in a pattern must use this parameter if any do, and all such exposures must specify the same set of values (if any) for the FOMXPOS and FOMYPOS Optional Parameters.

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.

Unless overridden via the Pattern_Type, the default value for Pattern_Orient is 0. This 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 Òdefault pointingÓ) 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 (Secondary_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 and NO. 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.