MIRI LRS Dithering

The JWST MIRI low-resolution spectroscopy mode provides dither templates for both point and extended sources. Dithering is only available for LRS observations with SLIT (i.e. subarray set to FULL); for time series spectroscopy with the slitless mode, dithering is disabled.

Dithering for low-resolution spectroscopy

Reference: MIRI LRS APT Template

Dithers can mitigate the effects of bad pixels, and obtain subpixel sampling and background observations. It is recommended for slit spectroscopy.

Two types of dither patterns are offered for MIRI LRS:

  1. ALONG SLIT NOD is designed for compact sources, and uses a 2-point "nod" where a point source is dithered between positions that are located approximately 30% and 70% of the way along the slit length (see Fig. 1). This should be the default choice for LRS slit observations of compact sources. Using this strategy, the sky regions of the nod pointings are used as sky background measurement for their complement.

  2. MAPPING has a customizable number of pointings in the spatial (along-slit) and spectral (across-slit) directions. The user can choose the step size for each direction (to be specified in arcseconds). We recommend that the chosen step sizes are smaller than, or on the order of the slit size (approx. 4.7 x 0.5"). Note that mapping of an extended target can also be achieved with the mosaicing strategy in APT.

To observe an extended target in the center of the slit, use the MAPPING dither pattern and set the number of steps in both directions to 1.  A dedicated background observation will be needed.

LRS slit layout

Figure 1: Layout of the LRS slit, showing the size of a diffraction-limited Airy disk at 5, 7.5 and 12 µm with respect to the slit (to scale). The slit length and width are approx. 4.7" and 0.5", respectively. The bottom panel shows the locations of the default nod positions along the slit, with the same Airy circles overlaid. 


Gordon , K. et al. 2015, PASP, 127, 953
The Mid-Infrared Instrument for the James Webb Space Telescope, X: Operations and Data Reduction



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