See also: MIRI Dithering, MIRI Imaging Dithering

The default direct pre- and post-WFSS images are taken as a single exposure, not dithered, and must use the same filter. The post-image can be followed by 2 additional dithers, to visualize out-of-field sources. 

The dispersed exposures must use one of the imager cycling dither patterns. The CYCLING pattern is available in 3 different pattern sizes: CYCLING-SMALL, CYCLING-MEDIUM and CYCLING-LARGE.  These patterns are described in detail in the MIRI Imaging Dithering pages, with recommendations for usage of the 3 pattern sizes. A minimum of 4 dither points is recommended. As described in MIRI Cross-Mode Recommended Strategies, the recommended maximum exposure duration is ~300 s (~100 groups in  FULL array, FASTR1 readout mode), to avoid the detector becoming too affected by cosmic rays. Observations should therefore aim to dither every ~300 s, adding dither positions to make up the required exposure time. 

The CYCLING-LARGE pattern is the default dithering pattern. 

---

Overlapping sources

When observing crowded fields with the WFSS mode, spectra of different objects can overlap and contaminate each other, making it difficult to extract "clean" spectra. The prism disperses the spectra approximately vertically along the detector y-axis, with the 5–14 µm range covering around 400 pixels. The dispersion profile is asymmetric around the source position, with the blue end of the spectrum being dispersed around 100 pixels upwards on the detector, and the red end around 300 pixels downward. This is illustrated in Figure 1 with the green circle indicating the spatial position of the source on the detector. Any sources lying along the dispersion axis within these ranges will therefore produce overlapping spectra. 

The MIRI WFSS mode only uses one disperser, the P750L double prism, mounted in the imager filter wheel. The instrument does not have the ability to disperse in orthogonal directions, as is the case for NIRISS and NIRCam. To achieve different orientations of the field on the detector to de-blend overlapping sources, additional WFSS observing sequences are recommended with "Position Angle Offset" links in the Special Requirements section in APT. Even for sparse extragalactic fields, two PA observations are recommended; for more crowded fields, more may be required. There is no limit on the number of sequences that can be requested in a WFSS program, as long as they can be scientifically and technically justified. 

---

Both FASTR1 and SLOWR1 readout patterns are available for the MIRI WFSS mode; FASTR1 should be considered the default for the majority of use cases. The SLOWR1 mode should be used predominantly to avoid data volume issues. Users should review the MIRI Cross-Mode Recommended Strategies page.