The MIRI wide field slitless spectroscopy (WFSS) mode combines the large field offered by the MIRI imager over an unobstructed 74" × 113" field of view with the double prism P750L to obtain 5–14 µm, low-resolution (R ~ 100) spectra of multiple targets in a single observation. The mode thus provides an efficient way to obtain spectra of multiple sources in a single observation.

See also: MIRI Low Resolution Spectroscopy, Parallel Observations

As of Cycle 5, the MIRI WFSS mode is available for prime use only; it is not yet supported for coordinated or pure parallel operations. However, in prime mode, MIRI WFSS permits pure parallel observations. Future cycles may allow MIRI WFSS as pure or coordinated parallels.

The MIRI WFSS mode has only a single disperser, the P750L double prism mounted in the filter wheel. Adjacent sources in the imager FOV may have overlapping spectra in the dispersed image. Multiple observations at different position angles may be required to disentangle overlapping spectra. This is described in more detail in the WFSS Recommended Strategies page. 

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MIRI WFSS observational sequence

See also: MIRI Imaging

An observation in the MIRI WFSS mode proceeds as follows:

1. A direct image filter is selected in the imager filter wheel.
2. A single (i.e., non-dithered) image of the scene is taken, called the "pre-image."
3. The filter wheel is set to the double prism, P750L.
4. Dithered prism images are taken.
5. At the last dither position, a further image is taken of the scene with the same filter as the pre-image; this is called the "post-image." This post-image can optionally be executed with 2 extra dithers, if desired.
6. Optional: select a different filter and repeat steps 1–5.
   
7. Optional: duplicate the observation with a PA Offset link to assist in disentangling overlapping spectra.

 

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WFSS imaging filters

See also: MIRI Filters and Dispersers

A typical WFSS observation will obtain direct images of the field both prior to and after the dispersed exposure sequence, the pre- and post-images. The goal of these images is:

• To enable proper identification of object(s) seen in the dispersed image(s), and 
• To determine the origin of the absolute wavelength scale for the prism spectra for each object in the field.  

The direct images can be taken with any of MIRI's imaging filters that provide a (near-)overlap with the throughput range of the prism:

• F560W
• F770W
• F1000W
• F1280W
• F1500W

The default direct pre- and post-WFSS imaging must use the same filter. 

If images of the field are required in multiple imaging filters, the WFSS sequence can be repeated multiple times to gain the additional filter coverage. If images in other MIRI filters are required (i.e., those at wavelengths beyond the F1500W coverage), these should be added as regular MIRI imaging observations. 

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Subarrays

See also: MIRI Detector Subarrays

All exposures will be taken in FULL array configuration. Subarray configurations are not available for MIRI WFSS observations and associated direct images. 

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Readout pattern

See also: MIRI Detector Readout Overview

MIRI WFSS observations can use both FASTR1 and SLOWR1 readout modes. The FASTR1 mode should be considered the default choice; SLOWR1 is only recommended for data volume considerations. Switching between readout patterns within an observation (e.g., using a different readout pattern for the direct image vs. the dispersed exposures) is not permitted, as the settling effects associated with a readout pattern change may cause undesirable detector behaviors. 

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Dither patterns

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-WFSS image can be used with a 3-position dither pattern to image any out-of-field sources that may be partially dispersed into the science field. 

The dispersed exposures must use a dither pattern. STScI offers the default imager CYCLING pattern for maximum flexibility in the number of dither points. A minimum of 4 dither points are recommended to provide enough redundancy. It offers 3 different pattern sizes, with CYCLING-LARGE considered the default. 

The CYCLING pattern consists of 311 points. This pattern is a random Gaussian pattern designed to be flexible. Observers will be able to choose (1) the starting position in the dither table and (2) the number of dither positions to maximize observational flexibility. For observers who request more than 311 dither positions, the CYCLING pattern will wrap so that the 312^th dither position is the same as the 1^st position. Each set of 4 consecutive dithers provides complete ½ pixel sampling.

The table of dither positions is available on the MIRI Imaging Dithering page. 

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Mosaics

See also: MIRI Imaging Mosaics

Mosaics are allowed for WFSS, and follow the same guidelines as for the imager.

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Target acquisition

See also: MIRI Imaging Target Acquisition

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Variations along the field of view

Calibration data to measure trace, flux, and wavelength variations across the field of view will be obtained and analyzed in Cycle 4. However, the STScI MIRI team estimates that differences in trace curvature and spectral resolution should not exceed differences between the currently available LRS slit and slitless observations. The sensitivity of the MIRI WFSS data will be most similar to the currently available slitless mode.

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References

Kendrew et al. 2015, PASP, 127 623.

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