NIRCam Subarray Primary Dithers

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The SUBARRAY_DITHER primary dither pattern is a legacy dither pattern. It was created before the start of Cycle 1 to increase the overlapping spatial coverage between the long and the short wavelength SUB64P subarrays, which are observed simultaneously. The SW SUB64P subarray is only 2" wide. The intent was to ensure that the target was observed in either channel for at least a subset of the 4 maximum allowed number of dithers. Prior to launch, the NIRCam team worked with the upper bound on the JWST required pointing performance of 1", which helped motivate the need for the SUBARRAY_DITHER primary dither pattern.

However, commissioning results demonstrated that the pointing performance greatly exceeds requirements, with a typical pointing accuracy of 0.1" (1-σ radial, Rigby et al. 2023). Moreover, during launch, the first fold mirror in the NIRCam short wavelength channel on module B underwent a very small motion (expected from ground tests) which serendipitously improved the overlap between the short and long wavelength channels. In addition, the NIRCam team redefined the spatial location of the SUB64P subarray (and the SUB160P as well) on the B1 and B5 detectors, to make them concentric.

For all the reasons above, the usefulness of the SUBARRAY_DITHER primary dither pattern is greatly reduced; this dither pattern is still offered at this time, but it may be discontinued in the future.

The NIRCam team instead recommends using the subpixel dither type STANDARD pattern, which subsamples the PSF while still mitigating detector defects.

See also: NIRCam Dithers and Mosaics

Figure 1. Left: the SUB64P subarray prior to launch. Right: the SUB64 subarray after commissioning

The overlap between the short wavelength channel (blue) subarray and the long wavelength channel (red) subarray was small before launch (about 2" × 1.1") as shown on the left, but improved with the SW module B first fold mirror motion experienced by NIRCam during launch, as shown on the right. The yellow star marks the target position of the source and the yellow circle marks the 2-σ pointing uncertainty assumed before launch (left) and measured during commissioning (right). 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 before launch (left) and after commissioning (right)

Screenshot from APT (v25.4.2 on the left, v2022.7 on the right) showing the Aladin view of SUB64P with 4 SUBARRAY_DITHERS. The background images are from Hubble WFC3/IR. These dithers were intended to increase the overlapping coverage of the long wavelength SUB64P subarray (large blue boxes) and the short wavelength SUB64P subarray (small blue boxes), which are observed simultaneously. The black box outlines the resulting coverage when all 4 dithers are used, approximately 2.6" × 1.9" before launch, and 2.6" × 2.6" after commissioning. The red crosshairs mark the position of the target.

References

Rigby, J. et al 2023, PASP, 135, 048001 
The Science Performance of JWST as Characterized in Commissioning



Notable updates

  • Updated to make it clearer that this is a legacy dither pattern, and that the STANDARD dither pattern is recommended instead.

  •  
    Updated for Cycle 2 using results from commissioning.
Originally published