MIRI LRS Dithering
The MIRI low-resolution spectroscopy (LRS) 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.
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Dithering for low-resolution spectroscopy
Reference: MIRI LRS APT Template
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Two types of dither patterns are offered for MIRI LRS: ALONG SLIT NOD and MAPPING.
ALONG SLIT NOD
ALONG SLIT NOD is designed for compact sources and uses a 2-point "nod" where the point source is dithered between two positions that are located approximately 30% and 70% of the way along the length of the slit (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 measurements for their complement. When this dither pattern is selected, the calibration pipeline will carry out nod subtraction (i.e., A-B subtraction) to remove the background flux from both nod pointings.
MAPPING
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 be 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 the visit splitting distance, 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 cannot control the order in which the exposures will be executed.
Figure 1. Layout of the LRS slit
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 approximately 4.7" and 0.5", respectively. The bottom panel shows the locations of the default nod positions along the slit for the ALONG SLIT NOD dither pattern, with the same Airy circles overlaid.
References
Gordon , K. et al. 2015, PASP, 127, 953
The Mid-Infrared Instrument for the James Webb Space Telescope, X: Operations and Data Reduction