NIRCam Wide Field Slitless Spectroscopy Dithers

JWST NIRCam wide field slitless spectroscopy dithers are telescope pointing maneuvers performed between multiple exposures to improve sky coverage, image quality, and spectral resolution.

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Both primary and subpixel dithers are recommended for NIRCam wide field slitless spectroscopy (WFSS) observations. Subpixel dithers are small offsets that improve the spatial resolution, and therefore the spectral resolution in grism mode, by increasing the pixel sampling of the PSF. Primary dithers are larger offsets designed to cover the gaps between detectors in the field of view.

Subpixel dithers 

With NIRCam, WFSS observations are available only in the long wavelength (LW) channel (2.4–5.0 µm), which is Nyquist sampled at 4 µm. The resolving power (R = λ/Δλ) is 1,525 at 4 µm, but decreases to R = 1,200 at 2.5 µm. This can be improved with subpixel dithering. The subpixel dither patterns for WFSS mode are optimized both for LW grism data and short wavelength (SW) imaging, which are obtained simultaneously. The SW imaging data will serve to precisely measure the dither offsets, which are affected by JWST’s slew uncertainty (~5 mas, 1-sigma/axis).  Knowledge of the dither offsets is crucial for determining the wavelength calibration.

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There are 3 available subpixel dither patterns: 2-point, 4-point, and 9-point. Another option is NONE. (NONE and 2-point were added in APT 2020.2.) The offsets are shown in Table 1 and Figure 1.

Table 1. Subpixel dither patterns for NIRCam WFSS observations 

PositionOffsets (SW pixels approx.)Offsets (arcsec)
#X pixY pixV2V3
2-point dither pattern
4-point dither pattern
9-point dither pattern

Table note: NIRCam's coordinates are very close to the JWST instrument ideal coordinate system (V2,V3).

Figure 1. WFSS subpixel dither patterns

Subpixel dither patterns for NIRCam WFSS observations (values shown in Table 1).

Primary dithers

INTRAMODULE primary dithers are also enabled for WFSS observations. These serve to cover the gaps between SW detectors for the simultaneous SW imaging (there are no gaps in the WFSS spatial coverage itself because that mode is available exclusively in the LW channel). At least 3 INTRAMODULE dithers are necessary to cover all SW gaps. 

Dithers for out-of-field source identification

Direct imaging with NIRCam is required for WFSS observations at the end of each dither sequence to identify sources in the WFSS data. However, the grisms deflect the light from the target in a direction parallel to the dispersion direction, so some sources that are outside of the imaging FOV will nonetheless produce spectra dispersed onto the detector when the grism is in use. To image the affected areas, a third type of dither is automatically implemented that images the area to each side of the detector along the dispersion direction. These are called out-of-field dithers.

When observing with the grism that disperses along the V2 axis (GRISMR), the out-of-field dither offsets are ±12" along the V2 axis. When observing with the grism that disperses along the V3 axis (GRISMC), the offsets are larger because the pickoff mirrors are larger in the V3 direction to allow for the coronographic substrates. In that case, the offsets are -12" and +35" along V3. Depending on the Galactic latitude of the target, the GRISMC offsets may be larger than the visit splitting distance, which requires multiple visits (and increased overheads). (See APT Visit Splitting for further information on visit splitting.)

Latest updates
    2-point subpixel dither option added in APT 2020.2
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