MIRI MRS Recommended Strategies
This page gives recommendations that, together with the MIRI Generic Recommended Strategies, should help the observer to plan MIRI MRS observations. Note that these are pre-launch recommendations that will be updated with results from on-orbit commissioning.
The MIRI Medium-Resolution Spectrometer (MRS) offers integral field spectroscopy between 4.9 and 28.8 μm using four different nested Integral Field Units. The full spectral coverage is obtained by making three exposures using the three different spectral settings: SHORT (A), MEDIUM (B), and LONG (C).
MIRI Simultaneous Observations (MRS and Imaging)
See also: MRS Simultaneous Imaging
The default set-up for the MRS template in the Astronomer's Proposal Tool (APT) is to make use of the adjacent MIRI imaging detector for simultaneous imaging. Simultaneous imaging is a highly recommended procedure that will:
- Improve the MRS data astrometric accuracy and result in better-combined data cubes, not only within an observing program but across different ones.
- Provide scientific images of the field adjacent to the MRS, which are especially useful when observing extended objects.
There are cases in which the user should not consider this option, e.g., a very bright extended target that will saturate the imager field of view in all filters, even in subarray mode.
This observing mode makes use of all three MIRI detector arrays (two for the MRS, one for the imager), so it is important to consider MIRI Data Volume Limits.
Detector Readout Mode
The default readout mode for the MRS is SLOW. This is not mandatory and there are scenarios, such as observations of bright sources, where the user should select FAST mode. If the use of SLOW mode results in ramps shorter than 10 groups (about 239 seconds), observers are advised to switch to FAST mode to obtain longer ramps.
Users should determine the optimal readout mode for their specific observations using the ETC.
See also: MIRI MRS Dithering
MRS dithering is recommended in order to obtain better sampling of the point spread function (which is undersampled by the MRS optics).
For the MRS, however, a staring option is offered and may be desired in certain science cases or as a strategy to obtain background data. When making use of simultaneous imaging, the dither patterns used are those of the primary mode, which in this case is the MRS. In order to keep targets in the MRS field of view, the dithers are small; as a result, if the observer is using long wavelength filters, the system will issue a warning because the dither points are separated by less than one FWHM. However, at these longer wavelengths, the PSF is well-sampled and this dithered data will be suitable for combination.
When defining a target in APT, users should select whether the target is spatially extended; the options are YES, NO, and UNKNOWN. The dither pattern should be consistent with this choice: Sources defined as extended or unknown should use an extended source dither pattern and point sources should use a point source dither pattern. Deviations from these choices must be justified in the proposal.
Dwell Time Limit
Dwell time defines how long you can stay at a single dither position (i.e., your exposure time, not your integration time). All the ground-based detector testing data carried out so far indicates that, based on the presence of detector long term drifts, there are no restrictions on the length of an exposure per dither position for MRS.
Target Acquisition Considerations
Target Acquisition (TA) for the MRS is performed with the MIRI Imager (see Figure 1). TA for MRS observations is not mandatory, but proposers should take caution when deciding against using TA. Most importantly, proposers should consider the JWST pointing performance and the impact on their science case. The MRS field of view ranges from 3.3" × 3.7" (slice width 0.176") to 7.2" × 7.9" (0.645" slice width). Therefore, a large dither pattern coupled with a pointing error could be problematic. Even for a point source, the dither pattern results in overlapping fields of view in each channel that are smaller than the smallest field of view (3.3" × 3.7"), highlighting the importance in ensuring that the target is properly placed in the MRS aperture.
Maximum Separation Between MRS Science and TA Targets
TA cannot be performed on some science targets, either because they are extended and/or too faint. In this case, a nearby object (preferably a bright point source) can be used for TA. After TA is performed on this object, it is slewed into the MRS field of view. A second slew then places the science target in the MRS field of view.
Some restrictions apply to this TA procedure: Since the visit-splitting distance for fixed targets is 40" and the separation between the TA region on the MIRI imager and the MRS fiducial point is 24", the maximum separation between the TA target and the science target is therefore 16" (see Figure 1). The distance between the science and TA targets is computed by APT and if it is >40", APT will issue an error.
The need for dedicated background exposures is discussed on the MIRI MRS Dedicated Sky Observations page. Dedicated backgrounds (flagged as such in APT) will be automatically subtracted from science observations in the pipeline.
Wells et al. 2015, PASP, 127, 646
The Mid-Infrared Instrument for the James Webb Space Telescope, VI: The Medium Resolution Spectrometer
Glasse et al. 2015, PASP, 127, 686G
The Mid-Infrared Instrument for the James Webb Space Telescope, IX: Predicted Sensitivity