Coordinated Parallel Dither Tables
Custom dither tables for JWST coordinated parallel observations are available to help users select appropriate dithers for their observations.
See also: JWST Dithering Overview, APT Coordinated Parallel Observations, JWST Instrument Ideal Coordinate System
ASCII files containing spatial offsets for dither patterns for coordinated parallel observations are available to users who want to study the characteristics of dither patterns in more detail, prior to selecting the appropriate pattern for their needs.
Spatial offsets are given in units of arcsec, as well as in pixels of the prime and parallel instruments (see file headers). All offsets are relative to the reference position of the selected aperture of the prime instrument, in that aperture's ideal coordinate system.
Tables are provided in two formats, as described below. In both cases, abbreviations for the instruments are as follows: MIR = MIRI, NIS = NIRISS, NRC = NIRCam, and NRS = NIRSpec.
File names have the following format:
- APT_<Prime Instrument>_<Parallel Instrument>.txt tables list all dither patterns for the prime-parallel instrument combinations in the file name. The spatial offsets for each dither pattern are preceded by a line stating the name of the dither pattern in question.
- <Prime Instrument>_<Parallel Instrument>_dithers.zip are zip files containing tables for all individual dither patterns in ASCII format for those prime-parallel instrument combinations.
For prime-parallel combinations involving NIRCam and NIRISS (or vice versa), the names of dither patterns for small, medium, and large spatial offsets are preceded by a character line ending in S, M, and L, respectively. The small dither offsets are typically <0.4", the medium ones are ~0.6", and the large ones are ~1.0". Note: dithers with small offsets are not available for all custom patterns.
For prime-parallel combinations involving MIRI, names of dither patterns are listed at the MIRI filter level, with syntax MIR followed by the filter's central wavelength in tenths of microns. Only wide MIRI filters are considered in this context. Example: dither patterns calculated for MIRI's F1280W filter have names with the characters MIR128. Note: not all custom dither patterns are available for all MIRI filters. If the pattern you are interested in is not available for a given MIRI filter, using the pattern for a filter at the next longer central wavelength is recommended (e.g., use F770W instead of F560W, F1280W instead of F1130W, etc.).
For the prime-parallel combination NIRSpec MOS + NIRCam Imaging, the dither patterns come in 3 offset sizes (size1, size2, and size3). These constitute "secondary" dither patterns which are executed at each NIRSpec MOS "nod" (the latter are moves from one shutter in the Multi-Shutter Array to an adjacent one "above" or "below" it). The zip file "NRS_NRC_dithers.zip" contains tables that list the resulting dither offsets for the cases of 2 and 3 NIRSpec "nods". The names of the latter tables are "<dither pattern name>.2nods.txt" and "<dither pattern name>.3nods.txt" for 2 and 3 NIRSpec nods, respectively. The relative NIRCam pixel phases populated by these dither patterns are illustrated in Figure 1 below.
Tables with all dither patterns for a given prime/parallel instrument combination:
Zip files containing tables of all individual dither patterns for a given prime/parallel instrument combination:
Click on the figure for a larger view.
Each panel shows the relative NIRCam SW pixel phases (i.e., fractional pixels in 2 dimensions) populated by the dither pattern in question. For reference, dither #1 is always plotted at pixel phase (0.5, 0.5). Black circles indicate positions for the 2-pt dither patterns (which have 4 dithers total for 2 NIRSpec MOS nods and 6 dithers total for 3 NIRSpec MOS nods), while the red circles indicate the "extra" positions populated by the third dither point (for each NIRSpec MOS nod) of the 3-pt dither patterns. The dither labels indicate the order in which the dithers are executed (in black for the 2-pt patterns and in red for the 3-pt patterns). The open circles represent the positions of the nominal NIRSpec nods (i.e., the ones that end up exactly in the center of the NIRSpec MOS shutters). Note: the middle row of panels shows the dithers for the case of 3 nods and 3-shutter NIRSpec MOS slitlets, while the top row of panels shows the dithers for the case of 3 nods and 5-shutter NIRSpec MOS slitlets. Observations that use 5 nods and 5-shutter MOS slitlets will sample the NIRCam SW pixel phases shown in the middle and top rows. The yellow rectangles on the right-hand side of the middle row of panels represent 3-shutter slitlets. The numbers in the shutters indicate the dither locations for the case of 3 nods (not to scale - just for illustrative purposes).
Finally, we remind the reader that these dither patterns are made available through the APT templates for each prime instrument in the combination. Details are shown in the article APT Coordinated Parallel Observations.