The following patterns use the new syntax but have been constructed to duplicate the earlier pattern forms (if applicable). Their nomenclature is meant to be obvious; for example, “
STIS-CCD-BOX” means the former "box" pattern for the STIS CCD. After each description we show what the information would look like.
A specific entry for a parameter (such as 4 for Number_of_Points with
STIS-CCD-BOX) means that value may not be changed and doing so will cause an error. An indicated range (such as 0.0275-2.75 for
Point_Spacing in
STIS-MAMA-BOX) means you may select from within that range, and a “?” means any numeric value may be entered.
As previously noted, you may also nest patterns, but not all combinations make sense. If you use both a
Primary_Pattern and a
Sub-Pattern (or Secondary_Pattern in the Text Proposal File), they should be for the same instrument or at least one should be generic. Note that any convenience pattern can be added as a
Sub-Pattern when a generic pattern is selected as the
Primary_Pattern. If a pattern with large steps is to be combined with a pattern with small steps, the
Primary_Pattern should contain the large steps, to minimize the time spent on pointing maneuvers. In the following sections, we show which patterns may be selected as a
Sub-Pattern when the indicated convenience pattern is the
Primary_Pattern. The observer should consult the references given within the instrument-specific subsections for detailed advice on selecting patterns and modifying pattern parameters. Do not mix instrument-specific patterns.
Pattern_Type: STIS-PERP-TO-SLIT
Pattern_Type: STIS-ALONG-SLIT
This will produce a four-point parallelogram scan designed for dithering across the CCD pixels. With the default Point_Spacing of 0.567, the four points of the parallelogram will be obtained at the following POS TARG (X,Y) offsets relative to the default aperture position:
This default produces a parallelogram pattern (see diagram) with projected offsets from the starting point of 5n integer pixels in each coordinate, so that the above arcsec values correspond to pixel values of (0, 0), (10, 5), (15, 15), and (5, 10). With these integer-pixel offsets, the pattern is optimized to simultaneously compensate for hot pixels and small-scale detector non-uniformities. The pattern can be rescaled (e.g., by a factor of 0.5) to achieve shifts of N+half pixels for resolution enhancement by changing the Point_Spacing.
Pattern_Type: STIS-CCD-BOX
Point_Spacing: 0.0567-5.67 (default 0.567)
Line_Spacing: Equal to Point_Spacing (may not be changed)
Angle_Between_Sides: 143.1 (may not be changed)
Pattern_Orient: ? (default 26.6)
This will produce a four-point parallelogram scan designed for dithering across the MAMA pixels. With the default Point_Spacing of 0.275, the four points of the parallelogram will be obtained at the following POS TARG (X,Y) offsets relative to the default aperture position:
This default produces a parallelogram pattern (see diagram) with projected offsets from the starting point of 5n integer pixels in each coordinate, so that the above arcsec values correspond to pixel values of (0, 0), (10, 5), (15, 15), and (5, 10). With these integer-pixel offsets, the pattern is optimized to simultaneously compensate for hot pixels and small-scale detector non-uniformities. The pattern can be rescaled (e.g., by a factor of 0.5) to achieve shifts of N+half pixels for resolution enhancement by changing the Point_Spacing.
Pattern_Type: STIS-MAMA-BOX
Point_Spacing: 0.0275-2.75 (default 0.275)
Line_Spacing: Equal to Point_Spacing (may not be changed)
Angle_Between_Sides: 143.1 (may not be changed)
Pattern_Orient: ? (default 26.6)
This produces a spiral dither pattern, starting at the center and moving outward counterclockwise. Note that a STIS-SPIRAL-DITH with four points yields a square pattern, but the optimum pattern for detector dithering to enhance resolution is either STIS-CCD-BOX or STIS-MAMA-BOX.
Pattern_Type: STIS-SPIRAL-DITH
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:
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.) and parameters are updated using the latest distortion solution. Helpful diagrams and descriptions are provided along with Phase II pattern parameters.
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.
Permitted Sub_Pattern (
Secondary_Pattern) values: any other ACS/WFC patterns,
LINE,
BOX,
SPIRAL
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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 Section 11.2.4). 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.
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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.
Permitted Sub_Pattern (
Secondary_Pattern) values: any other ACS/WFC patterns,
LINE,
BOX,
SPIRAL
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 200x300 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.
Permitted Sub_Pattern (
Secondary_Pattern) values: any other ACS/WFC patterns,
LINE,
BOX,
SPIRAL
This is a 4-point box pattern for creating a WFC mosaic roughly 400x300 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).
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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
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Another useful WFC mosaic box pattern would maximize the field of view (~400x400 arcseconds). For this pattern, both the
x and
y shifts are ~95% of the detector dimensions, and would require multiple guide stars.
Permitted Sub_Pattern (
Secondary_Pattern) values: any other ACS/SBC patterns,
LINE,
BOX,
SPIRAL
Permitted Sub_Pattern (
Secondary_Pattern) values: any other ACS/SBC patterns,
LINE,
BOX,
SPIRAL
Permitted Sub_Pattern (
Secondary_Pattern) values: any other ACS/SBC patterns,
LINE,
BOX,
SPIRAL
This is a large 4-point box pattern for creating a mosaic roughly 4 times the SBC field-of-view, or ~64x64 arcsec. The shifts are 95% of the detector dimensions (~973 pixels or ~32 arcsec) along both the
x and
y axes of the detector.
Permitted Sub_Pattern (
Secondary_Pattern) values: any other ACS/SBC patterns,
LINE,
BOX,
SPIRAL
See the NICMOS Instrument Handbook for a full discussion of how and why to use patterns and for complete illustrations of their geometries.
Pattern_Type: NIC-XSTRIP-DITH
Permitted Sub-Pattern (
Secondary_Pattern) values:
NIC-YSTRIP-DITH,
NIC-SPIRAL-DITH,
NIC-SPIRAL-MAP,
NIC-SQUARE-WAVE-DITH,
LINE,
SPIRAL.
Pattern_Type: NIC-YSTRIP-DITH
Pattern_Orient: ? (default 90)
Permitted Sub-Pattern (
Secondary_Pattern) values:
NIC-XSTRIP-DITH,
NIC-SPIRAL-DITH,
NIC-SPIRAL-MAP,
NIC-SQUARE-WAVE-DITH,
LINE,
SPIRAL.
Pattern_Type: NIC-SPIRAL-DITH
Permitted Sub-Pattern (
Secondary_Pattern) values:
NIC-XSTRIP-DITH, NIC-YSTRIP-DITH, NIC-SPIRAL-MAP, NIC-SQUARE-WAVE-DITH, LINE, SPIRAL.
Pattern_Type: NIC-SPIRAL-MAP
Permitted Sub-Pattern (
Secondary_Pattern) values:
NIC-XSTRIP-DITH, NIC-YSTRIP-DITH, NIC-SPIRAL-DITH, NIC-SQUARE-WAVE-DITH, LINE, SPIRAL.
This specifies a square-wave pattern with the “amplitude” of the square wave along the POS TARG +Y axis and the main direction of motion along the POS TARG +X axis.
Pattern_Type: NIC-SQUARE-WAVE-DITH
Line_Spacing: Equal to Point_Spacing (may not be changed)
Angle_Between_Sides: 270 (may not be changed)
Pattern_Orient: ? (default 90)
Permitted Sub-Pattern (
Secondary_Pattern) values:
NIC-XSTRIP-DITH, NIC-YSTRIP-DITH, NIC-SPIRAL-DITH, NIC-SPIRAL-MAP, LINE, SPIRAL.
Pattern_Type: NIC-ONE-CHOP
Pattern_Type: NIC-ONE-CHOP
Other permitted Sub-Pattern (
Secondary_Pattern) values: none.
This requires two pattern definitions. It specifies a repeated “chop” along the POS TARG +X axis, similar to NIC-ONE-CHOP, but using four points instead of two.
Pattern_Type: NIC-TWO-CHOP
Pattern_Type: NIC-TWO-CHOP
Line_Spacing: Equal to Point_Spacing (may not be changed)
Angle_Between_Sides: 0 (may not be changed)
Other permitted Sub-Pattern (
Secondary_Pattern) values: none.
Pattern_Type: NIC-XSTRIP-DITH-CHOP
Center_Pattern: ? (default NO)
Pattern_Type: NIC-XSTRIP-DITH-CHOP
Pattern_Orient: ? (default 90)
Other permitted Sub-Pattern (
Secondary_Pattern) values: none.
Pattern_Type: NIC-YSTRIP-DITH-CHOP
Pattern_Orient: ? (default 90)
Center_Pattern: ? (default NO)
Pattern_Type: NIC-YSTRIP-DITH-CHOP
Other permitted Sub-Pattern (
Secondary_Pattern) values: none.
Pattern_Type: NIC-SPIRAL-DITH-CHOP
Pattern_Type: NIC-SPIRAL-DITH-CHOP
Other permitted Sub-Pattern (
Secondary_Pattern) values: none.
Other permitted Sub-Pattern (
Secondary_Pattern) values: none.
Pattern_Type: NIC-SKY-ONE-CHOP
Pattern_Type: NIC-SKY-ONE-CHOP
Other permitted Sub-Pattern (
Secondary_Pattern) values: none.
Pattern_Type: NIC-SKY-TWO-CHOP
Pattern_Type: NIC-SKY-TWO-CHOP
Line_Spacing: Equal to Point_Spacing (may not be changed)
Angle_Between_Sides 0 (may not be changed)
Other permitted Sub-Pattern (
Secondary_Pattern) values: none.
(Secondary_Pattern) Pattern_Type: NIC-SKY-SPIRAL-DITH-CHOP
This requires two pattern definitions. It specifies a combination of a spiral pattern and a two-point line (the “chop”), similar to NIC-SPIRAL-DITH-CHOP except that the pattern is oriented on the sky.
Pattern_Type: NIC-SKY-SPIRAL-DITH-CHOP
Center_Pattern: ? (default NO)
Pattern_Type: NIC-SKY-SPIRAL-DITH-CHOP
Other permitted Sub-Pattern (
Secondary_Pattern) values: none.
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 Sections 2.4 and 2.5 of the
Multidrizzle Handbook. 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.).
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.
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.
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.
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).
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.
Permitted Sub-Pattern (
Secondary_Pattern) values:
WFC3-UVIS-DITHER-BOX, WFC3-UVIS-DITHER-LINE, WFC3-UVIS-DITHER-LINE-3PT
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.).
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.
Permitted Sub-Pattern (
Secondary_Pattern) values:
WFC3-UVIS-DITHER-BOX, WFC3-UVIS-DITHER-LINE, WFC3-UVIS-DITHER-LINE-3PT
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.
Permitted Sub-Pattern (
Secondary_Pattern) values:
WFC3-UVIS-DITHER-BOX, WFC3-UVIS-DITHER-LINE, WFC3-UVIS-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 (3.5, 3.5). The default values are optimized for a 2-step pattern.
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.
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).
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.
Permitted Sub-Pattern (
Secondary_Pattern) values:
WFC3-IR-DITHER-BOX-MIN, WFC3-IR-DITHER-LINE, WFC3-IR-DITHER-LINE-3PT
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.
Permitted Sub-Pattern (
Secondary_Pattern) values:
WFC3-IR-DITHER-BOX-MIN, WFC3-IR-DITHER-LINE, WFC3-IR-DITHER-LINE-3PT
