C.2 WFC3 Patterns
A number of different types of patterns are available to support dithered and mosaicked WFC3 observations. The pre-defined patterns that have been implemented in APT are described in the Phase II Proposal Instructions, which are updated when the selection of a new cycle of proposals is announced. The pre-defined WFC3 patterns in effect in APT at the time of publication of this handbook are summarized here. Patterns not predefined in APT can still be executed by specifying POS TARGs on exposures in the Phase II proposal. WFC3 ISR 2016-14 and WFC3 ISR 2020-07 tabulate the necessary POS TARGs for compact patterns with up to 9 steps for WFC3/IR and WFC3/UVIS, respectively. Those patterns are designed to preserve sub-pixel sampling as much as possible over the face of the detector, given the scale changes introduced by geometric distortion. Alternatively, observers can also develop custom Patterns within APT by starting with a Generic pattern and setting the required parameters as desired or by adjusting specific parameters within a pre-defined pattern.
Pre-defined WFC3 dither patterns designed to subsample pixels can 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. Due to geometric distortion (Appendix B), a large mosaic step shifts some objects by an integer number of rows (or columns), and others by an integer plus some fraction of a pixel. The PSF is thus not evenly sampled in the overlap region of the two exposures, so a PSF-sampling dither should be added if spatial resolution is important.
Sets of exposures with offsets executed using patterns or POS TARGs are associated and combined automatically during pipeline processing, as long as the same guide stars have been used for all exposures. Pointings must be contained within a diameter ~130 arcsec or less (depending on the availability of guide stars in the region) to use the same guide stars. Note that the rms pointing repeatability is significantly less accurate if the same guide stars are not used for all exposures (Appendix B of the DrizzlePac Handbook).
The names and purposes of the patterns in effect in APT at the time of publication are given in Table C.1. Note that the initially-adopted names of patterns have been preserved for continuity, although they do not always correspond to the distinction between dither steps and mosaic steps outlined above. The small BOX dither patterns are designed to optimally sample the PSF when 4 steps are used. Since time constraints do not always permit visits to be broken into multiples of 4 steps, LINE dither patterns that optimally sample the PSF in 2 or 3 steps are also given. The BOX and LINE dither patterns are illustrated in WFC3 ISR 2010-09. A full discussion and illustrations of patterns that optimally sample the PSF for different numbers of steps are available in Section C.2 of the DrizzlePac Handbook. Note that PSF sampling generally produces a more significant improvement for IR images than for UVIS images (Section 6.6.1 and Section 7.6.1). The remainder of the patterns in Table C.1 are special-purpose mosaic patterns that are expected to be commonly needed. There are no pre-defined patterns to deal with specific features in flats—notably, the circular dead spot on the IR detector (WFC3 ISR 2008-08) and the UVIS “droplets” (WFC3 ISR 2008-10); however, patterns that can be used to mitigate the effects of these artifacts are discussed in WFC3 ISR 2010-09.
Table C.1: Dithering and Mosaicking Patterns for WFC3.
WFC3 IR Patterns
Dithers over “blobs” (Section 7.9.6)
Provides optimal 4-step sampling of the PSF.
Produces an IR mosaic (despite the name) covering approximately the same area as the UVIS-CENTER aperture.
Provides optimal 2-step sampling of the PSF.
Provides optimal 3-step sampling of the PSF.
WFC3 UVIS Patterns
Provides optimal 4-step sampling of the PSF; produces spacings of >1 column for removal of hot columns.
Provides optimal 2-step sampling of the PSF; produces spacings of >1 column for removal of hot columns.
Provides optimal 3-step sampling of the PSF; produces spacings of >1 column for removal of hot columns.
Dithers over the interchip gap.
Produces a mosaic that can generally be executed with a single set of guide stars.
Combines a primary gap-stepping pattern with an optional dither at each primary position.
For full-frame UVIS with ACS/WFC in parallel; steps the gap on both detectors.
The default specifications of the patterns are summarized in Table C.2. The equivalent POS TARG moves are summarized in Table C.3, along with the approximate number of pixels corresponding to these moves. The number of pixels was computed using only the linear distortion terms with coefficients measured at the center of each detector. This is an excellent approximation for small moves and for objects that remain in the central region of the detector (Figure B.1 and B.3 in Appendix B).
Note that you can easily scale up the patterns in APT to make them larger; e.g., multiply the Point Spacing and Line Spacing of patterns with half-pixel sampling (WFC3-IR-DITHER-BOX-MIN, WFC3-IR-DITHER-LINE, WFC3-UVIS-DITHER-BOX, WFC3-UVIS-DITHER-LINE) by an odd number to preserve the half-pixel sampling (this is equivalent to multiplying the POS TARGs and steps in pixels by that number). You may want to do this in the IR, for example, to achieve the recommended spacing of at least 10 pixels between positions for photometric repeatability (WFC3 ISR 2019-07) or to move a saturated persistence-generating core of a target by a greater distance than the minimal default distance.
As an example, for the WFC3-IR-DITHER-LINE, one would scale the 0.636" spacing x3 to preserve half-pixel sampling, making this ~15 pixels along the diagonal instead of the standard ~5 pixels. This will help minimize the effect of self-persistence from bright stars taken in prior exposures from the same visit.
Be aware, however, that larger steps result in larger variations in sub-pixel sampling of the PSF over the face of the detector due to non-linear geometric distortion (WFC3 ISR 2016-14).
Table C.2: Default values of the parameters that define the WFC3 convenience patterns
No. of Points
Point Spacing (arcsec)
Line Spacing (arcsec)
Pattern Orient (degrees)
Angle between Sides (degrees)
Table C.3: Steps in arcsec in the POS TARG frame and in detector pixels for the WFC3 convenience patterns.
POS TARG X (arcsec)
POS TARG Y (arcsec)
For the IR detector, the linear relation between POS TARGs and pixels is simply
POS TARG X = a11 * x
POS TARG Y = b10 * y
where a11 ~ 0.1355 arcsec/pixel and b10 ~ 0.1211 arcsec/pixel near the center of the detector. For the UVIS detector, there is a cross-term that takes into account the fact that the projected axes are not perpendicular:
POS TARG X = a11 * x
POS TARG Y = b11 * x + b10 * y
where a11 ~ 0.0396 arcsec/pixel, b11 ~ 0.0027 arcsec/pixel, and b10 ~ 0.0395 arcsec/ pixel near the center of the detector. This relationship is illustrated in Figure C.1.
The values of these coefficients were derived using optical models and apply to the centers of the detectors. On-orbit geometric distortion solutions give marginally different coefficients (WFC3 ISR 2010-09). The corresponding changes in pixel steps in small dithers are insignificant. The corresponding changes in pixel steps in large dithers or mosaic steps are inconsequential, since non-linear distortion makes the step size in pixels variable over the detector.
WFC3 Instrument Handbook
- • Acknowledgments
- Chapter 1: Introduction to WFC3
- Chapter 2: WFC3 Instrument Description
- Chapter 3: Choosing the Optimum HST Instrument
- Chapter 4: Designing a Phase I WFC3 Proposal
- Chapter 5: WFC3 Detector Characteristics and Performance
Chapter 6: UVIS Imaging with WFC3
- • 6.1 WFC3 UVIS Imaging
- • 6.2 Specifying a UVIS Observation
- • 6.3 UVIS Channel Characteristics
- • 6.4 UVIS Field Geometry
- • 6.5 UVIS Spectral Elements
- • 6.6 UVIS Optical Performance
- • 6.7 UVIS Exposure and Readout
- • 6.8 UVIS Sensitivity
- • 6.9 Charge Transfer Efficiency
- • 6.10 Other Considerations for UVIS Imaging
- • 6.11 UVIS Observing Strategies
- Chapter 7: IR Imaging with WFC3
- Chapter 8: Slitless Spectroscopy with WFC3
Chapter 9: WFC3 Exposure-Time Calculation
- • 9.1 Overview
- • 9.2 The WFC3 Exposure Time Calculator - ETC
- • 9.3 Calculating Sensitivities from Tabulated Data
- • 9.4 Count Rates: Imaging
- • 9.5 Count Rates: Slitless Spectroscopy
- • 9.6 Estimating Exposure Times
- • 9.7 Sky Background
- • 9.8 Interstellar Extinction
- • 9.9 Exposure-Time Calculation Examples
- Chapter 10: Overheads and Orbit Time Determinations
Appendix A: WFC3 Filter Throughputs
- • A.1 Introduction
A.2 Throughputs and Signal-to-Noise Ratio Data
- • UVIS F200LP
- • UVIS F218W
- • UVIS F225W
- • UVIS F275W
- • UVIS F280N
- • UVIS F300X
- • UVIS F336W
- • UVIS F343N
- • UVIS F350LP
- • UVIS F373N
- • UVIS F390M
- • UVIS F390W
- • UVIS F395N
- • UVIS F410M
- • UVIS F438W
- • UVIS F467M
- • UVIS F469N
- • UVIS F475W
- • UVIS F475X
- • UVIS F487N
- • UVIS F502N
- • UVIS F547M
- • UVIS F555W
- • UVIS F600LP
- • UVIS F606W
- • UVIS F621M
- • UVIS F625W
- • UVIS F631N
- • UVIS F645N
- • UVIS F656N
- • UVIS F657N
- • UVIS F658N
- • UVIS F665N
- • UVIS F673N
- • UVIS F680N
- • UVIS F689M
- • UVIS F763M
- • UVIS F775W
- • UVIS F814W
- • UVIS F845M
- • UVIS F850LP
- • UVIS F953N
- • UVIS FQ232N
- • UVIS FQ243N
- • UVIS FQ378N
- • UVIS FQ387N
- • UVIS FQ422M
- • UVIS FQ436N
- • UVIS FQ437N
- • UVIS FQ492N
- • UVIS FQ508N
- • UVIS FQ575N
- • UVIS FQ619N
- • UVIS FQ634N
- • UVIS FQ672N
- • UVIS FQ674N
- • UVIS FQ727N
- • UVIS FQ750N
- • UVIS FQ889N
- • UVIS FQ906N
- • UVIS FQ924N
- • UVIS FQ937N
- • IR F098M
- • IR F105W
- • IR F110W
- • IR F125W
- • IR F126N
- • IR F127M
- • IR F128N
- • IR F130N
- • IR F132N
- • IR F139M
- • IR F140W
- • IR F153M
- • IR F160W
- • IR F164N
- • IR F167N
- Appendix B: Geometric Distortion
- Appendix C: Dithering and Mosaicking
- Appendix D: Bright-Object Constraints and Image Persistence
Appendix E: Reduction and Calibration of WFC3 Data
- • E.1 The STScI Reduction and Calibration Pipeline
- • E.2 The SMOV Calibration Plan
- • E.3 The Cycle 17 Calibration Plan
- • E.4 The Cycle 18 Calibration Plan
- • E.5 The Cycle 19 Calibration Plan
- • E.6 The Cycle 20 Calibration Plan
- • E.7 The Cycle 21 Calibration Plan
- • E.8 The Cycle 22 Calibration Plan
- • E.9 The Cycle 23 Calibration Plan
- • E.10 The Cycle 24 Calibration Plan
- • E.11 The Cycle 25 Calibration Plan
- • E.12 The Cycle 26 Calibration Plan
- • E.13 The Cycle 27 Calibration Plan
- • E.14 The Cycle 28 Calibration Plan
- • E.15 The Cycle 29 Calibration Plan
- • E.16 The Cycle 30 Calibration Plan
- • Glossary