7.2 ACS Patterns

Most HST ACS observations are obtained with some motion of the telescope (e.g., dithering, mosaicking), and there are predefined patterns available in APT to support these motions.

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Format definitions

Boldface type indicates the name of an APT parameter or a value for a parameter.

(red star) Black text indicates an important note.

Magenta text indicates available but unsupported parameters (requires prior approval from STScI).

Red text indicates restricted parameters (for STScI use only).

Brown text indicates text file parameters.

Items in brackets - <value> - are required values.

Items in square brackets - [<value>] - are optional.

Introduction

There are 8 different predefined patterns that can be used with ACS, which are shown in Table 7.2.

Table 7.2: ACS Patterns


For general information on ACS pointing, and a library of carefully designed dither and mosaic pointing patterns (ready for use in a Phase II proposal), see the following web page:

http://www.stsci.edu/hst/instrumentation/acs/proposing/dither-strategies

In addition to the default patterns provided below, many non-default variations are included in the pattern library (e.g., for hot pixel rejection, sub-pixel dithering, etc.). As of Cycle 25, the ACS Team has incorporated the latest understanding of the Wide Field Channel (WFC) geometric distortion into a revised set of default parameters for the ACS-WFC-DITHER-LINE and ACS-WFC-DITHER-BOX convenience patterns discussed below. The matrix transformation between WFC pixel offsets and POSTARG offsets, provided on the webpage listed above, has also been updated.

Pattern_Type: ACS-WFC-DITHER-LINE

General WFC dither line pattern. The default for this pattern shifts the image by 5 pixels in x and 60 in y in order to span the gap between the two WFC detectors.

Pattern_Type: ACS-WFC-DITHER-LINE
Pattern_Purpose: DITHER
Number_Of_Points: 2-9 (default 2)
Point_Spacing: 0.01 - 10.0 (default 3.034)
Coordinate_Frame: POS-TARG
Pattern_Orient:? (default 85.29)
Center_Pattern:? (default NO)

Permitted  Sub_Pattern  (Secondary_Pattern) values: any other  ACS/WFC patterns, LINE, BOX, SPIRAL

(lightbulb) Note: All ACS/WFC users are encouraged to use some form of dithering to allow for correction of hot pixels during data processing. The standard CR-SPLIT approach does not eliminate hot pixels (see Optional Parameters). Users who would normally have taken that approach in the past are now encouraged to define and use instead the pattern:

Pattern_Type: ACS-WFC-DITHER-LINE 
Pattern_Purpose: DITHER
Number_Of_Points: 2
Point_Spacing: 0.146
Coordinate_Frame: POS-TARG
Pattern_Orient: 47.17
Center_Pattern: NO

This pattern will allow simultaneous elimination of hot pixels and cosmic ray hits in post-observation processing. The pattern parameters shift the image by 2 pixels in x and 2 in y along the direction that minimizes the effects of scale variation across the detector.

Pattern_Type: ACS-WFC-DITHER-BOX

This is the default WFC box pattern. It is a 4-point pattern with relative pixel coordinates (0, 0), (5.0, 1.5), (2.5, 4.5), (-2.5, 3.0) — a parallelogram pattern with a combination of integer and sub-pixel shifts, which is relatively compressed in one dimension compared to its STIS counterpart. This minimizes the effect of scale variation across the detector.

Pattern_Type: ACS-WFC-DITHER-BOX
Pattern_Purpose: DITHER
Number_Of_Points: 4

Point_Spacing: 0.01 - 10.0 (default 0.2637)
Line_Spacing: 0.01 - 10.0 (default 0.1856)
Angle_Between_Sides: 0 - 360 (default 69.02)
Coordinate_Frame: POS-TARG

Pattern_Orient: 0 - 360 (default 20.7)
Center_Pattern:? (default NO)

Permitted Sub_Pattern (Secondary_Pattern) values: any other ACS/WFC patterns, LINE, BOX, SPIRAL

Pattern_Type: ACS-WFC-MOSAIC-LINE

General WFC mosaic line pattern. The default shift is in the y direction by 47% of the detector dimension, resulting in a field-of-view that is about 200 × 300 arcsec. This is a compromise which allows the 2-point WFC mosaic to be performed with one set of guide stars and to cover the inter chip gap as well.

Pattern_Type: ACS-WFC-MOSAIC-LINE
Pattern_Purpose: MOSAIC
Number_Of_Points: 2-9 (default 2)
Point_Spacing: 10.0 - 130.0 (default 96.816)
Coordinate_Frame: POS-TARG
Pattern_Orient:? (default 90.0)
Center_Pattern:? (default YES)

Permitted  Sub_Pattern  (Secondary_Pattern)  values: any other  ACS/WFC patterns, LINE, BOX, SPIRAL

Pattern_Type: ACS-WFC-MOSAIC-BOX

This is a 4-point box pattern for creating a WFC mosaic roughly 400 × 300 arc seconds. This pattern strikes a compromise with a y-shift that covers the inter chip gap (with one set of guide stars) and the maximum x-shift to expand the field of view. So another set of guide stars would be needed only for the x shifts, which are 95% of the detector x dimension (~193 arcsec).

(lightbulb) Note: This pattern is not supported in the STScI ground system because it would require two sets of guide stars, so no pattern parameters are given. The pattern can be set up by hand using the following POS TARG offsets:

Point 1: -96.497, -55.801
Point 2: -96.497, 41.015
Point 3: 96.497, 55.801
Point 4: 96.497, -41.015

Another useful WFC mosaic box pattern would maximize the field of view (~400 × 400 arcseconds). For this pattern, both the x and y shifts are ~95% of the detector dimensions, and would require multiple guide stars.

This pattern is also not supported in the ground system, but may be set up by hand using the following POS TARG offsets:

Point 1: -96.497, -104.084
Point 2: -96.497, 89.298
Point 3: 96.497, 104.084
Point 4: 96.497, -89.298

Pattern_Type: ACS-SBC-DITHER-LINE

General SBC dither line pattern. This pattern shifts the image on the diagonal (10 pixels in x and y).

Pattern_Type: ACS-SBC-DITHER-LINE
Pattern_Purpose: DITHER
Number_Of_Points: 2-9 (default 2)
Point_Spacing: 0.01 - 10.0 (default 0.472)
Coordinate_Frame: POS-TARG
Pattern_Orient:? (default 44.40)
Center_Pattern:? (default NO)

Permitted  Sub_Pattern  (Secondary_Pattern)  values: any  other  ACS/SBCpatterns, LINE, BOX, SPIRAL

Pattern_Type: ACS-SBC-DITHER-BOX

Same as ACS-WFC-DITHER-BOX, but for the SBC detector.

Pattern_Type: ACS-SBC-DITHER-BOX
Pattern_Purpose: DITHER
Number_Of_Points: 4
Point_Spacing: 0.01 - 10.0 (default 0.179)
Line_Spacing: 0.01 - 10.0 (default 0.116)
Angle_Between_Sides: 0 - 360 (default 63.65)
Coordinate_Frame: POS-TARG
Pattern_Orient: 0 - 360 (default 20.02)
Center_Pattern:? (default NO)

Permitted  Sub_Pattern  (Secondary_Pattern)  values: any other  ACS/SBC patterns, LINE, BOX, SPIRAL

Pattern_Type: ACS-SBC-MOSAIC-LINE

General SBC mosaic line pattern. This pattern shifts the image by 95% in y to roughly double the FOV.

Pattern_Type: ACS-SBC-MOSAIC-LINE
Pattern_Purpose: MOSAIC
Number_Of_Points: 2-9 (default 2)
Point_Spacing: 10.0 - 130.0 (default 28.801)
Coordinate_Frame: POS-TARG
Pattern_Orient:? (default 90.0)
Center_Pattern:? (default YES)

Permitted  Sub_Pattern  (Secondary_Pattern)  values:  any  other  ACS/SBC patterns, LINE, BOX, SPIRAL

Pattern_Type: ACS-SBC-MOSAIC-BOX

This is a large 4-point box pattern for creating a mosaic roughly 4 times the SBC field-of-view, or ~64 × 64 arcsec. The shifts are 95% of the detector dimensions (~973 pixels or ~32 arcsec) along both the x and y axes of the detector.

Pattern_Type: ACS-SBC-MOSAIC-BOX
Pattern_Purpose: MOSAIC
Number_Of_Points: 4
Point_Spacing: 10.0 - 130.0 (default 28.801)
Line_Spacing: 10.0 - 130.0 (default 32.957)
Angle_Between_Sides: 0 - 360 (default 264.24)
Coordinate_Frame: POS-TARG
Pattern_Orient: 0 - 360 (default 90.0)
Center_Pattern:? (default YES)

Permitted  Sub_Pattern  (Secondary_Pattern)  values:  any  other  ACS/SBC patterns, LINE, BOX, SPIRAL

Related Links

7.1 STIS Patterns
7.3 WFC3 Patterns



Table of Contents


Change Log

Version Cycle 30 August 2022   

         PROPINST-91387 (Update to ACS WFC Dither Box Pattern)