NIRCam Small Grid Dithers

JWST NIRCam small grid dithers are quick sub-arcsecond pointing offsets used primarily to improve coronagraphic PSF subtraction.

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Small grid dithers (SGD) are very small, fast, and precise pointing offsets of a target image. They are implemented in NIRCam coronagraphic imaging to obtain multiple images of a reference star to optimize the PSF subtraction (Lajoie et al. 2016).  As of APT 25.4, new SGD patterns designed for NIRCam imaging are available (Coe 2017).

Unlike other JWST dithers, which are executed by slewing the telescope, small grid dithers are executed using the fine steering mirror while remaining under control of the JWST Fine Guidance Sensor (FGS). The range of motion is restricted to ~1 FGS pixel (~69 mas on a side), but the pointings are performed very quickly (in a few seconds) and accurately (~2 mas uncertainty per axis).

By comparison, JWST slews of 25" or less (small angle maneuvers, or SAMS) take over 48 s to execute (~20 s + overheads) and have expected accuracies of ~5 mas per axis. Such slews are used in coronagraphic imaging to position science targets and PSF reference stars behind the occulters.  

Ideally the science and PSF targets would be positioned at exactly the same place behind the occulter. In practice, the subpixel pointing differences of ~5 mas per axis significantly alter the scattered light speckle pattern. Small grid dithers can be used to place one of those targets (usually the PSF reference) at multiple positions behind the occulter (see Tables 1 and 2 and Figure 1), and that grid of PSF images can be used to improve the PSF subtraction from the science image (Lajoie et al. 2016).

The limited range of motion for small grid dithers (~69 mas per axis) spans at most a few NIRCam pixels (32 and 65 mas across in the short and long wavelength channels, respectively). This restricts the utility of small grid dithers for improving flat fielding and bad pixel replacement in standard imaging and wide field slitless spectroscopy; these observing modes rely on somewhat larger subpixel dithers or SGD dithers in combination with larger primary dithers.

Coronagraphic imaging

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Figure 1. shows the five offered SGD patterns for NIRCam coronagraphic imaging. Tables 1 and 2 lists all their coordinates in milliarcseconds (mas). Taken a PSF reference star with more SGD positions ensures a better PSF subtraction (improved diversity) but increases the overall observing time for a given program. It is a trade-off. Certainly the largest number of positions is recommended for the most challenging programs (e.g. when the object to be imaged is in the close vicinity of the mask, typically from within the inner working angle to one arcsecond).
Figure 1. Small grid dither patterns for NIRCam coronagraphic imaging

Small-grid dither patterns

Illustration of the small grid dither patterns for round masks (left) and bar masks (right). Offsets are also listed in Tables 1 and 2.

Table 1. Small grid dither offsets for round masks

PositionOffsets (mas)
PositionOffsets (mas)




Table 2. Small grid dither offsets for bar masks

PositionOffsets (mas)
PositionOffsets (mas)




NIRCam imaging small grid dithers can be used in lieu of subpixel dithers. They perform the same optimal pixel subsampling as subpixel dithers, but with smaller patterns. That is, the fractional pixel steps are preserved, but fewer whole pixel steps are taken. Small grid dithers can still achieve some mitigation of bad pixels, especially in the short wavelength channel. The overheads are reduced and pointings are more precise compared to standard subpixel dithers. The nine available patterns are shown below, and the complete list of pointing offsets in arcseconds are given in the dither ASCII table:


Figure 2. Small grid dither patterns for NIRCam imaging

In these illustrations of the SMALL-GRID-DITHER patterns for NIRCam imaging, short and long wavelength pixels are shown in blue and red, respectively.  Dither points are black and numbered. The dither path is colored yellow – brown – black.  Light gray points mark the subpixel phasing covered by each pattern.


Coe, D. 2017, JWST-STScI-005798
More Efficient NIRCam Dither Patterns

Lajoie, C-P et al., 2016, SPIE, 99045K
Small-grid dithers for the JWST coronagraphs

Lajoie, C-P,  Soummer, R., Pueyo, L., and The JWST Coronagraphs Working Group, 2016, STScI Newsletter, Vol 32, Issue 02
Improving Webb Coronagraphic Performance with Small-Grid Dithers

Latest updates
    Added new pattern available for imaging in APT 25.4

    Added Figure 1

    Added Tables 1 and 2
Originally published