Introduction

There are 12 different predefined patterns that can be used with WFC3, which are shown in Table 7.3: WFC3 Patterns.

Table 7.3: WFC3 Patterns

Please see ISR WFC3 2010-09 and the WFC3 Instrument Handbook (Sections 6.10, 7.10 and Appendix C) for information on dither strategies and patterns. For general advice on dither strategies, see the DrizzlePac website. The step sizes in pixels specified below apply to the central regions of the detectors. For smaller steps, the step sizes are accurate over a larger region of the detectors. (The step size in pixels depends on the geometric distortion; see Appendix B of the WFC3 Instrument Handbook.) 

Additional UVIS dither patterns that optimize PSF sampling are being introduced for cycle 28, including patterns with more than 4 steps.  The steps are prescribed in pixels and POS-TARGs in ISR WFC3 2020-07. We will be releasing a new dither pattern (defined via postargs) later this cycle, which will optimize pixel phase sampling when the two primary imagers (WFC3 & ACS) are used simultaneously.

WFC3 dither patterns designed to subsample pixels can now optionally be selected as secondary patterns when WFC3 patterns with larger steps are selected as primary patterns. WFC3 patterns can also be added as secondary patterns to any of the generic pattern types (BOX, LINE, SPIRAL). When combining patterns, the smaller dither pattern should be the secondary pattern to minimize the time spent moving the telescope. We remind the user that IR dither patterns should be at least 10 pixels to avoid self-persistence for sources that are larger than 5 pixels in size. The standard box pattern is significantly smaller than this. For more information, see ISR WFC3 2016-14.

Pattern_Type: WFC3-UVIS-DITHER-LINE

This pattern dithers the UVIS aperture by (2.5, 2.5) pixels to sample the point spread function with fractional pixel steps. The default values are optimized for a 2-step pattern.

Pattern_Type: WFC3-UVIS-DITHER-LINE 
Pattern_Purpose: DITHER 
Number_Of_Points: 2 - 9 (default 2)
Point_Spacing:? (default 0.145) 
Coordinate_Frame: POS-TARG 
Pattern_Orient:? (default 46.840) 
Center_Pattern:? (default NO)

Pattern_Type: WFC3-UVIS-DITHER-LINE-3PT

This pattern dithers the UVIS aperture by (2.33, 2.33) pixels to sample the point spread function with fractional pixel steps. The default values are optimized for a 3-step pattern.

Pattern_Type: WFC3-UVIS-DITHER-LINE-3PT
Pattern_Purpose: DITHER
Number_Of_Points: 3 - 9 (default 3)
Point_Spacing:? (default 0.135)
Coordinate_Frame: POS-TARG
Pattern_Orient:? (default 46.840)
Center_Pattern:? (default NO)

Pattern_Type: WFC3-UVIS-DITHER-BOX

This pattern for WFC3/UVIS samples the point spread function with fractional pixel steps and produces spacings of more than one column to move hot columns. The relative steps in pixels are (0, 0), (4.0, 1.5), (2.5, 4.0), and (-1.5, 2.5).

Pattern_Type: WFC3-UVIS-DITHER-BOX
Pattern_Purpose: DITHER
Number_Of_Points: 4
Point_Spacing: ? (default 0.173)
Line_Spacing: ? (default 0.112)
Angle_Between_Sides: ? (default 81.785)
Coordinate_Frame: POS-TARG
Pattern_Orient: ? (default 23.884)
Center_Pattern: NO

Pattern_Type: WFC3-UVIS-GAP-LINE

This pattern dithers over the gap between the two chips of the UVIS detector with relative steps of (-2.25,-30.25) and (2.25,30.25) pixels. Additional subpixel dithering can be achieved by adding one of the WFC3-UVIS dither patterns as a Secondary Pattern.

Primary_Pattern:

Pattern_Type: WFC3-UVIS-GAP-LINE
Pattern_Purpose: MOSAIC
Number_of_Points:? (default 2)
Point_Spacing:? (default 2.414)
Coordinate_Frame: POS-TARG
Pattern_Orient:? (default 85.759)
Center_Pattern: YES

Permitted Sub-Pattern (Secondary_Pattern) values:

WFC3-UVIS-DITHER-BOX,
WFC3-UVIS-DITHER-LINE,
WFC3-UVIS-DITHER-LINE-3PT

Pattern_Type: WFC3-UVIS-MOS-DITH-LINE

The primary pattern dithers over the gap between the two chips of the UVIS detector with relative steps of (-4.5,-60.25), (0, 0), and (4.5, 60.25) pixels. The secondary pattern adds a dither of (2.5, 1.5) pixels to the primary pattern (Use WFC3-UVIS-GAP-LINE if you need a greater selection of permitted Secondary Patterns.).

Primary_Pattern:

Pattern_Type: WFC3-UVIS-MOS-DITH-LINE
Pattern_Purpose: MOSAIC Number_of_Points:? (default 3)
Point_Spacing:? (default 2.400)
Coordinate_Frame: POS-TARG Pattern_Orient:? (default 85.754)
Center_Pattern: YES

Secondary_Pattern:

Pattern_Type: WFC3-UVIS-MOS-DITH-LINE
Number_of_Points:? (default 2)
Pattern_Purpose: DITHER Point_Spacing:? (default 0.119)
Coordinate_Frame: POS-TARG
Pattern_Orient:? (default 33.606)
Center_Pattern: NO

Pattern_Type: WFC3-UVIS-MOS-BOX-LRG

This pattern produces a UVIS mosaic that can be executed with a single set of guide stars. It dithers the gap between the chips so that no region lies in the gap more than once. The relative steps in pixels are approximately (-1000, -997), (1000, -1001), (1000, 997), and (-1000,1001). Subpixel dithering can be achieved by adding one of the WFC3-UVIS dither patterns as a Secondary Pattern.

Pattern_Type: WFC3-UVIS-MOS-BOX-LRG
Pattern_Purpose: MOSAIC
Number_of_Points: 4
Point_Spacing: ? (default 79.40)
Line_Spacing: ? (default 77.50)
Angle_Between_Sides: ? (default 95.30) 
Coordinate_Frame: POS-TARG 
Pattern_Orient: ? (default 5.55) 
Center_Pattern: YES

Permitted Sub-Pattern (Secondary_Pattern) values:

WFC3-UVIS-DITHER-BOX, 
WFC3-UVIS-DITHER-LINE, 
WFC3-UVIS-DITHER-LINE-3PT

Pattern_Type: WFC3-UVIS-MOSAIC-LINE

This pattern is designed for observations using the full WFC3/UVIS detector for primary exposures and the full ACS/WFC detector for parallel exposures. It dithers over the inter-chip gap on both detectors. The relative steps on the WFC3/UVIS detector are (0, 0) and (36.5, 71.5) pixels. Subpixel dithering can be achieved by adding one of the WFC3-UVIS dither patterns as a Secondary Pattern.

Pattern_Type: WFC3-UVIS-MOSAIC-LINE
Pattern_Purpose: MOSAIC
Number_of_Points: 2-9 (default 2)
Point_Spacing: ? (default 3.264)
Coordinate_Frame: POS-TARG
Pattern_Orient: ? (default 63.697)
Center_Pattern: NO

Permitted Sub-Pattern (Secondary_Pattern) values:

WFC3-UVIS-DITHER-BOX, 
WFC3-UVIS-DITHER-LINE, 
WFC3-UVIS-DITHER-LINE-3PT

Pattern_Type: WFC3-IR-DITHER-LINE

This pattern for WFC3/IR takes steps large enough for photometric accuracy and samples the point spread function with fractional pixel steps. The relative steps in pixels are (0, 0) and (3.5, 3.5). The default values are optimized for a 2-step pattern.

Pattern_Type: WFC3-IR-DITHER-LINE
Pattern_Purpose: DITHER
Number_of_Points: 2 - 9 (default 2)
Point_Spacing:? (default 0.636)
Coordinate_Frame: POS-TARG
Pattern_Orient:? (default 41.788)
Center_Pattern: NO

Pattern_Type: WFC3-IR-DITHER-LINE-3PT

This pattern for WFC3/IR takes steps large enough for photometric accuracy and samples the point spread function with fractional pixel steps. The relative steps in pixels are (0, 0) and (3.33, 3.33). The default values are optimized for a 3-step pattern.

Pattern_Type: WFC3-IR-DITHER-LINE-3PT
Pattern_Purpose: DITHER
Number_of_Points: 3 - 9 (default 3)
Point_Spacing:? (default 0.605)
Coordinate_Frame: POS-TARG
Pattern_Orient:? (default 41.788)
Center_Pattern: NO

Pattern_Type: WFC3-IR-DITHER-BOX-MIN

This pattern for WFC3/IR takes steps just large enough for photometric accuracy and samples the point spread function with fractional pixel steps. The relative steps in pixels are (0, 0), (4.0, 1.5), (2.5, 4.0), and (-1.5, 2.5).

Pattern_Type: WFC3-IR-DITHER-BOX-MIN
Pattern_Purpose: DITHER
Number_of_Points: 4
Point_Spacing: ? (default 0.572)
Line_Spacing: ? (default 0.365)
Angle_Between_Sides: ? (default 74.653)
Coordinate_Frame: POS-TARG
Pattern_Orient: ? (default 18.528)
Center_Pattern: NO

Pattern_Type: WFC3-IR-DITHER-BOX-UVIS

This is a four-point box pattern that produces an IR mosaic covering approximately the same area as the UVIS detector. IR imaging with aperture IR-FIX is intended to be accompanied by a UVIS exposure (or small dither pattern) using the aperture UVIS-FIX or UVIS-CENTER. If UVIS-FIX is used, the UVIS exposures should use the Target Position Requirement POS TARG 0.0", -12" to achieve the same centering of the target in the UVIS exposures and the IR mosaic. Subpixel dithering can be achieved by adding one of the WFC3-IR dither patterns as a Secondary Pattern.

Pattern_Type: WFC3-IR-DITHER-BOX-UVIS
Pattern_Purpose: DITHER
Number_of_Points: 4
Point_Spacing: 23.020
Line_Spacing: 35.212
Angle_Between_Sides: 89.287
Coordinate_Frame: POS-TARG
Pattern_Orient: 0.713
Center_Pattern: YES

Permitted Sub-Pattern (Secondary_Pattern) values:

WFC3-IR-DITHER-BOX-MIN,
WFC3-IR-DITHER-LINE, 
WFC3-IR-DITHER-LINE-3PT

Pattern_Type: WFC3-IR-DITHER-BLOB

This pattern dithers over the IR "blobs,"described in ISR WFC3 2010-09, using relative steps of (-14.25,-14.25) and (14.25,14.25) pixels. Additional subpixel dithering can be achieved by adding one of the WFC3-IR dither patterns as a Secondary Pattern.

Pattern_Type: WFC3-IR-DITHER-BLOB
Pattern_Purpose: DITHER
Number_of_Points:? (default 2) 
Point_Spacing:? (default 5.183) 
Coordinate_Frame: POS-TARG
Pattern_Orient: 41.859 
Center_Pattern: YES

Permitted Sub-Pattern (Secondary_Pattern) values:

WFC3-IR-DITHER-BOX-MIN,
WFC3-IR-DITHER-LINE, 
WFC3-IR-DITHER-LINE-3PT

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