NIRCam Subarray Primary Dithers

The SUBARRAY_DITHER primary dither pattern can be used to increase the overlapping spatial coverage between the long wavelength SUB64P subarray and the short wavelength SUB64P subarray, which are observed simultaneously to ensure that the target is observed in both channels.

Main Article: NIRCam Dithers and Mosaics

Several subarrays are available for imaging with NIRCam. Since the long wavelength (LW) and short wavelength (SW) detectors do not overlap perfectly, the SW and LW overlapping spatial coverage can be smaller than the nominal subarray size. For the smallest subarrays, the overlapping area is small enough that targets may be placed near the edge of the subarray, or may possibly be missed entirely.

The absolute fine pointing accuracy, without a science target acquisition, is expected to be 0.45" to 0.30" (1-σ radial error), depending on the distance between the guide star and the science instrument aperture.  This uncertainty is dominated by guide star catalog position errors and pointing errors due to roll control. 

The smallest subarray (SUB64P) is 64 × 64 pixels. In the SW channel, that corresponds to about 2" × 2".  However, the area that overlaps with the LW channel is only about 2" × 1.1" (Figure 1). A primary dither pattern (SUBARRAY_DITHER) is available for imaging with the subarray to provide larger spatial coverage. The step size is approximately 0.3" and users can choose 2–4 dithers (Table 2, Figure 2). When all 4 dithers are used, the area covered by both the SW and LW channel is approximately 2.6" × 1.9", which encompasses a 2-σ pointing uncertainty. Alternatively, users can use a mosaic.

The SUBARRAY_DITHER option can be used with any of the subarrays. The step size remains the same for all cases.

Figure 1. The SUB64P subarray

The overlap between the short wavelength channel (blue) subarray and the long wavelength channel (red) subarray is small (about 2" × 1.1"). The yellow star marks the target position of the source and the yellow circle marks the 2-σ pointing uncertainty. The location of the SUB64P subarray with respect to the full field of view can be found on the NIRCam Detector Subarrays article.

Table 1. SUBARRAY_DITHER positions

PositionOffsets (arcsec)
#XY
1-0.335-0.335
2+0.335+0.335
3-0.295+0.295
4+0.295-0.295

Figure 2. Subarray primary dithers

Screenshot from APT (v25.4.2) showing the Aladin view of SUB64P with 4 SUBARRAY_DITHERS. The background image is a Hubble WFC3/IR image at 1.27 µm. These dithers increase the overlapping coverage of the long wavelength SUB64P subarray (large boxes) and the short wavelength SUB64P subarray (small boxes), which are observed simultaneously. The black box outlines the resulting coverage when all 4 dithers are used, approximately 2.6" × 1.9". The red crosshairs mark the position of the target.



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