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.
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.
 Note: Many programs use both ACS/WFC and WFC3/UVIS or ACS/WFC and WFC3/IR together in parallel. One instrument will always be considered the prime instrument and one the parallel. In general, patterns are optimized for the prime instrument, which should have the observations with the higher scientific priority. However, if the data from BOTH cameras are of equal importance scientifically, there is a combined ISR: ACS ISR 2023-04 and WFC3 ISR 2023-05, which helps show how to optimize the dithering in both cameras. It is mainly about a small-scale dither pattern. If larger chip-gap-covering patterns are desired to be optimized as best as possible simultaneously in both cameras, it should be noted that with larger dithers, the integrity of the dither pattern is lost more quickly across the field of view as one moves away from the reference aperture whether in prime-only or parallel usage. Please contact the ACS Help Desk or WFC3 Help Desk for additional help and advice if desiring to try and simultaneously optimize such larger chip-gap dithers in both cameras. |
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
Pattern_Type: ACS-WFC-DITHER-LINE 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: 4Point_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-TARGPattern_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-TARGPattern_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).
Point 1: -96.497, -55.801 |
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: 4Point_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-TARGPattern_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-TARGPattern_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: 4Point_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-TARGPattern_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