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.

complete set of primary and secondary subpixel dither patterns (specified in arcseconds) is available as ASCII tables in a .zip file:  

NIRCamDitherPatterns.zip

This .zip file contains a collection of separate files for all the NIRCam-related dither patterns. Specifically relevant to the dither patterns described in this article, it contains these dither patterns (mapping their names to the APT pattern names):

FilenameAPT dither pattern type and pattern name
NircamWfssSubpixel.txtSubpixel Dither Type: 2-Point, 4-Point, 9-Point
NircamImagingIntramodule.txtPrimary Dither Type: INTRAMODULE
NircamImagingIntramoduleBox.txtPrimary Dither Type: INTRAMODULEBOX
NircamImagingIntramoduleX.txtPrimary Dither Type: INTRAMODULEX

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 (arcsec)

Offsets (LW pixels approx.)

Offsets (SW pixels approx.)

#

X ideal

ideal

X pix

Y pix

X pix

Y pix

2-point dither pattern

1

0

0

0

0

0

0

2

0.598

0.598

9.5

9.5

19.3

19.3

4-point dither pattern

1

0

0

0

0

0

0

2

0.598

-0.126

9.5

-2.0

19.3

-4.1

3

-0.126

0.598

-2.0

9.5

-4.1

19.3

4

-0.598

-0.598

-9.5

-9.5

-19.3

-19.3

9-point dither pattern

1

0

0

0

0

0

0

2

-0.483

-0.126

-7.7

-2.0

-15.5

-4.1

3

0.483

-0.252

7.7

-4.0

15.5

-8.1

4

-0.126

-0.483

-2.0

-7.7

-4.1

-15.5

5

-0.546

-0.546

-8.7

-8.7

-17.6

-17.6

6

0.546

-0.359

8.7

-5.7

17.6

-11.5

7

-0.252

0.483

-4.0

7.7

-8.1

15.5

8

-0.357

0.546

-5.7

8.7

-11.5

17.6

9

0.357

0.357

5.7

5.7

11.5

11.5

Table note: The NIRCam Ideal coordinate system represents an undistorted reference frame. Ideal frame coordinates can be treated as a location in a tangent plane projection. The tangent point (i.e., X,Y ideal origin) is defined at the given aperture’s fiducial/reference position. For WFSS the reference positions are the NRCALL or the NRCAALL ones (see NIRCam Apertures). For NIRCam, given the orientation of the detectors in the JWST focal plane, the X and Y axis of the Ideal system are very well aligned with the (V2,V3) telescope reference frame.

Figure 1. WFSS subpixel dither patterns (arcsec)

Subpixel dither patterns for NIRCam WFSS observations (values shown in Table 1).
Figure 2. WFSS subpixel dither patterns (LW pixel phase)

LW pixel phase plot demonstrating how the source will land on the pixel for each dither pattern if dither 0 is perfectly centered on the pixel center. 
Figure 3. WFSS subpixel dither patterns (SW pixel phase)

SW pixel phase plot demonstrating how the source will land on the pixel for each dither pattern if dither 0 is perfectly centered on the pixel center.


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.)

During commissioning the NIRCam team measured small deflections of the spectra with respect to the location of the direct image, in the direction perpendicular to the dispersion (cross-dispersion). These deflections are of the order of 1” and they imply that sources slightly beyond the edges of the detectors in direct images may still produce spectra that land on the detector, if not accounted for correctly.

For this reason, the NIRCam team has slightly modified the out-of-filed dithers, to account for the cross-dispersion deflections.

Note that the amount of deflection is different between module A and module B. For this reason an average shift is used when Module = ALL. This implies that on either module there could still be sources at the very edges (order of 10 pixels) that may not have a counterpart in the direct images. Users are advised to account for these edge effects, which basically make the extreme edges of the detectors sub-optimal for WFSS science.

For observations in module A only, the out-of-field dithers are optimized to account for the module A appropriate cross-dispersion deflection, and therefore the out-of-field images allow a fuller recovery of the sources responsible for spectra landing at the edges of the module A LW detector




Latest updates
  •   
    Added updated dither tables and figures

  •  
    2-point subpixel dither option added in APT 2020.2
Originally published