Simultaneous use of the JWST MIRI imager and the Medium Resolution Spectrometer (MRS) is offered for all observations where the MRS is the primary observing mode. Astrometry of stars in the imager field will result in more accurate data cube construction.
The MIRI imager can be used for mid-infrared imaging simultaneously with the MIRI medium resolution spectrometer. This is referred to as "simultaneous imaging" and is not a parallel imaging mode. This is the default operational mode for MRS observations, and it is highly recommended that simultaneous imaging be specified for all MRS observations as there are no data volume issues when using the and the MRS simultaneously. The only instance where simultaneous imaging should be turned off is if saturation of the imager would occur due to a long MRS observation or a bright target is in the imager field of view.
Since the MIRI MRS has an extremely small field of view, its absolute astrometric solution cannot always be tied to an external reference frame using MRS data alone. If data are taken without a prior target acquisition observation (for instance, when mapping an extended diffuse source) the absolute astrometry of a given visit may be in error by 0.5" or more. This poses a challenge to combining exposures across multiple different visits, and also for comparing source locations against extant multi-wavelength data. The simultaneous imaging mode helps to alleviate this problem by obtaining data across the much larger MIRI imaging field of view that can be used offline (in combination with the well-known relation between the imaging and MRS fields) to improve the astrometric solution of individual MRS exposures by cross-matching multiple sources in the imaging field against known catalog positions.
Simultaneous imaging can also be used to obtain additional science observations of an extended object. For example, MRS observations of a small portion of a nebula can be complemented by simultaneous imaging of adjacent regions of the nebula (depending on the roll of the observatory, which is significantly constrained for JWST). Figure 1 shows the spacing and orientation of the imager and MRS FOVs. Specific examples of simultaneous imaging science applications can be found in the descriptions of the GTO programs (JWST GTO Observation Specifications).
Selecting simultaneous imaging in APT
The following figures walk through how to select simultaneous imaging when MIRI Medium Resolution Spectroscopy1 is selected as the observing template. Note: MRS parameters cannot be specified when MIRI Imaging is selected as the observing template.
Selecting multiple simultaneous imaging filters
A different imaging filter can be selected for each exposure in a given observations. For instance, if an observation contained 3 exposures (one each for the MRS wavelength ranges SHORT, MEDIUM, and LONG, thereby covering the entire MRS wavelength range), then simultaneous imaging could correspondingly be obtained in 3 different filters.
Long wavelength filter warning
Selecting the F1280W, F1500W, F1800W, F2100W, or F2550W filters for simultaneous imaging will result in the following warning message: "Warning (Form): Imager Filter overlap." This is due to the large size of the PSF FWHM in those filters compared to the smallest of the MRS dither patterns; executing a small dither for the MRS will result in the PSFs of point sources in the imager overlapping between subsequent exposures, which can be problematic when performing image subtraction.
Simultaneous imaging and target acquisition
MRS target acquisition (TA) uses the FULL subarray of the MIRI imager. If TA is selected, the FULL imager subarray must also be selected for simultaneous imaging, otherwise APT will generate a warning that the imager data quality may be adversely affected. If no MRS TA is selected (option available starting in APT 25.4.2), there is no restriction on the subarray mode available for simultaneous imaging.
Simultaneous imaging overheads
If simultaneous imaging is selected, there will be an additional small overhead for each dither position. For example, if a 4-point dither is selected, and observations are set up with all 3 Wavelength Ranges, this results in 12 dithers and each of those dithers incurs its own small overhead.
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