NIRCam subpixel dithers are small pointing offsets performed between exposures in imaging or wide field slitless spectroscopy observations. They include subpixel offsets to improve the spatial resolution of the final combined (drizzled) image mosaic of all exposures.
This is especially important below 2 µm in the short wavelength channel (0.6–2.3 µm) and below 4 µm in the long wavelength channel (2.4–5.0 µm). Below these Nyquist wavelengths, NIRCam's detectors undersample the JWST PSF (the FWHM is less than 2 pixels). This undersampling can be most severe in the short wavelength channel, up to a factor of ~2 with F070W. (Wavefront errors may broaden the PSF below 2 µm, but PSF features will still scale as λ/D.) Therefore, subpixel dither patterns were designed to provide optimal benefits for the ~0.031" pixels in the short wavelength channel (Anderson 2009). Detailed background information on principles of dithered observations with JWST are also described in Koekemoer & Lindsay (2005), Anderson (2011), Anderson (2014), and Coe (2017).
For the STANDARD1 subpixel dithers described here, each dither step includes shifts along both detector axes (x and y) to mitigate bad pixels. Each shift is an integer plus a fractional pixel (subpixel) step (e.g., 2.5 pixels). The integer component mitigates bad detector pixels and flat field uncertainties, while the fractional subpixel step improves PSF sampling and achievable spatial resolution. These patterns are detailed below. Small patterns (0.4" across) are designed for up to 9 dither positions, and a single larger pattern (1.7" across) is used when requesting 10–64 dither positions.
A more compact subpixel SMALL-GRID-DITHER pattern is also available. It is performed more quickly (lower overheads) using the fine steering mirror instead of slewing the telescope. It also includes fewer integer pixel steps, reducing its ability to mitigate bad pixels or flat field uncertainties. The fractional pixel steps are exactly preserved for up to 9 dither positions.
NIRCam subpixel dithers are smaller than the primary dithers used to cover detector gaps and mitigate flat field uncertainties on larger scales. Primary dithers do offer some sub-optimal subpixel sampling.
Eight different STANDARD subpixel dither patterns were designed with 2–9 positions (Figure 1). The 4-point pattern samples the pixel-phase space with twice the resolution of the NIRCam pixels. The 9-point pattern samples 3 times the pixel resolution, which would be most useful for F070W observations.
All 8 patterns are kept small enough (within a 13 × 13 pixel box, 0.41" on a side) to ensure coherent subpixel shifts for all pixels on the detector, given geometric distortion. Allowing for 2% deviations in pixel scale at the detector edges, a 13-pixel shift at the detector center might differ by 0.26 pixel at the edges. This subpixel deviation maintains coherent subpixel shifts across the detector.
Each row shows a dither pattern defined for use with a different number of Subpixel Positions: N = 2–9. The two central columns show the full dither patterns in units of short wavelength pixels (0.031") and long wavelength pixels (0.063"), respectively. The outer two columns show the phase spacing, or fractional pixel offset, of each dither position. The open circles show the phase spacing pattern repeated outside the central pixel. Reproduced from Anderson 2009, Figures 17 and 18.
The 9-point pattern provides the finest spatial sampling required. Finer details would be blurred by the PSF, even in F070W imaging. Additional subpixel dithers cannot improve image resolution, but they can be used in a larger pattern to further mitigate bad pixels and flat-field uncertainties.
A larger 64-point pattern (Figure 2), designed within a 55 × 55 pixel box (1.74" on a side), consists of 16 sets of 4-point dither patterns, each fitting within a 10 × 10 pixel box (0.32" on a side) and doubling the native pixel sampling. The full 16-set pattern spirals outward from the center to cover a larger area with each set of 4 dither points. This strategy is similar to that used when imaging the Hubble Ultra Deep Field (HUDF).
The larger 64-point dither pattern designed for use with 10 or more subpixel positions. Top: pixel phase shifts. Bottom: full shifts. Left: short-wavelength channel in 0.031" pixel units. Right: long-wavelength channel in 0.063" pixel units. Reproduced from Anderson 2009, Figure 20.