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 slit observations (i.e., subarray set to FULL); for time-series spectroscopy with the slitless mode, dithering is disabled.

On this page

Dithering for low-resolution spectroscopy

Reference: MIRI LRS APT Template

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

Dithering can mitigate the effects of bad pixels and obtain subpixel sampling and background observations. It is mandatory for slit spectroscopy.

Two types of dither patterns are offered for MIRI LRS: ALONG SLIT NOD and MAPPING.


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.


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"). Mapping of an extended target can also be achieved with the mosaicking strategy in APT.

Although the maximum dither size is set by avoiding the need to acquire new guide stars, a useful guideline is that dithers larger than 20" will be much slower than ones smaller than this limit. A minimum of 2 mapping positions must be defined. If an even number of mapping positions are chosen in either direction, then the targeted coordinates will be observed at the center of one of those slit positions. The user does not control the order in which the exposures will be executed.

Figure 1. Layout of the LRS slit

LRS slit layout

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

Latest updates
    updated text to align with changes to APT

    updated figure
Originally published