MIRI MRS APT Template
See also: MIRI Medium Resolution Spectroscopy, JWST Integral Field Spectroscopy Roadmap, MIRI MRS Recommended Strategies, MIRI MRS Spectroscopy of a Late M Star, MIRI MRS and NIRSpec IFU Observations of Cassiopeia A, MIRI MRS and NIRSpec IFU Observations of SN1987A, Proposal Planning Video Tutorials
Medium-resolution spectroscopy is one of 4 observing modes available with the the Mid-Infrared Instrument (MIRI). The MIRI medium resolution spectrometer (MRS) will be used to obtain simultaneous spatial and spectral information between 4.9 to 27.9 μm over a contiguous field of view up to 6.6'' × 7.7'' in size.
The MRS is the only JWST observing mode offering spectroscopy (R = 1,500–3,500) longward of ~ 14 μm. MRS observations are carried out using a set of 4 integral field units (IFUs), each of which covers a different portion of the MIRI wavelength range. The MRS IFUs use slicers to split the field of view into spatial slices. Each slice produces a separate dispersed "long slit" spectrum. Post-processing produces a 3-dimensional (2 spatial and one spectral dimension) data cube.
MRS operations have been designed to allow for efficient observations of point sources, compact sources, and extended sources. The observer will have control over 3 primary variables for MRS spectroscopy:
- wavelength coverage
- dithering pattern
- detector readout mode and exposure time (via the number of frames and integrations)
Step-by-step APT instructions are provided below.
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
MIRI Medium Resolution Spectroscopy tab
Target Acquisition Parameters
The target acquisition parameters Acq Target, Acq Filter, and Acq ExposureTime are found under the MIRI Medium Resolution Spectroscopy tab, in the Target Acquisition Parameters panel.
See also: MIRI MRS Target Acquisition
For MRS spectroscopy, choose Acq Target from the pull down menu. Filters available are F560W, F1000W, F1500W, and FND. As of APT 25.4.2, NONE may also be selected for target acquisition, which will disable target acquisition for the observation.
Acq Exposure Time
The Acq Exposure Time fields must be completed by selecting a MULTIACCUM exposure configuration. Each exposure is specified by setting the readout pattern and characteristics parameters: Acq Readout Pattern and Acq Groups/Int.
Acq Readout Pattern
See also: MIRI Detector Readout Overview
Choices for the Acq Readout Pattern are
Number of groups and integrations
See also: Understanding JWST Exposure Times
Groups/Int = 2 requires permission from STScI, which can be obtained with a request through the Help Desk.
The MIRI readout timing pattern for the TA exposure is defined by only one of the MULTIACCUM parameters: Acq Groups/Int, the number of groups per integration, where a group is the product of cycling through all the pixels.
Acq Integrations/Exp, Acq Total Integrations and Acq Total Exposure Time cannot be changed by the user.
The Primary Channel parameter, as well as Dithers, Simultaneous Imaging, and Exposure Parameter dialog boxes are found under the MIRI Medium Resolution Spectroscopy tab, in the MRS Parameters panel.
See also: MRS Channels
The Primary Channel defines the pointing origin (i.e., where the science target would be centered in the absence of dithering), available dither patterns, and field boundaries (for mosaic overlap calculations). Options are:
- ALL MRS*
The IMAGER option should only be used to place the science target in the simultaneous imaging field while the MRS is observing a dedicated background.
* The default choice should be ALL MRS, unless the science case wishes to prioritize data quality in a particular wavelength channel at the expense of other wavelengths (e.g., maximizing long wavelength source separation at the possible expense of the target not being in the short wavelength field of view).
See also: MIRI MRS PSF and Dithering
The Dithers dialog box handles the creation of dither patterns, which must be created before defining your exposure sequence. Each DITHER is specified by setting several parameters: Dither Type, Optimized For, and Direction.
- 2-Point dithering allows for basic image separation in order to use one exposure as the "background image" for another, and to detect the source with redundant detector pixels. It does not achieve optimal half-integer spatial sampling throughout much of the field of view.
- 4-Point adds an additional offset compared to 2-Point dithering and achieves optimal half-integer spatial sampling throughout the entire field of view of all 4 channels.
The default choice should be 4-Point. Dithering may be turned off in order to create zero-offset exposures in the Exposure Parameters table.
- POINT SOURCE
- EXTENDED SOURCE
The POINT SOURCE dithering schemes provide the maximum possible separation between the dither locations. The EXTENDED SOURCE dithering scheme provides the minimum possible separation that still achieves ideal spatial sampling and use of redundant detector locations. When observing point sources or otherwise small objects, the POINT SOURCE dithering scheme should generally be selected. When observing extended sources that fill much of the field of view, or when mosaicing together multiple pointings, some science cases may wish to use the EXTENDED SOURCE scheme. Note that programs using the EXTENDED SOURCE pattern should also include a separate dedicated background exposure at an off-source location since the image separation achieved in this dither scheme will be insufficient to achieve proper background subtraction. The BACKGROUND dithering scheme provides large offsets suitable for optimizing MIRI simultaneous imaging data while the MRS is observing an empty dedicated background field.
This choice defines the orientation of the dither offsets in the JWST focal plane; they are mirror symmetric with respect to the IFU slice orientation and achieve identical sampling. This option is provided to allow for flexibility in dithering direction if the telescope orientation is to be fixed for a given observation. The default is NEGATIVE, and is unimportant for the majority of science cases.
See also: MRS Simultaneous Imaging
MIRI offers the ability to obtain simultaneous with the MIRI imager while obtaining MRS spectroscopy. Choose:
Grating Wheel Direction
For engineering purposes only, the direction of the grating wheels can be controlled with this limited access parameter.
See also: MIRI Detector Subarrays
If you choose simultaneous imaging, MIRI imaging supports the use of a pre-defined set of Imager Subarrays for observing targets bright enough to saturate the image in full frame readout. Each subarray is associated with a brightness limit, depending on the filter used, above which a point source will saturate in the shortest 2-group integration. A MIRI simultaneous imaging observation can only support a single subarray. If the target must be imaged using another subarray, it is necessary to create another MIRI simultaneous imaging observation.
Usable size (arcsec)
1024 × 1032
74" × 113"
512 × 512
56.3" × 56.3"
256 × 256
28.2" × 28.2"
128 × 136
14.1" × 14.1"
64 × 72
7" × 7"
An MRS sequence is specified by first setting the Wavelength Range desired for each exposure. There are 3 available settings: SHORT (A), MEDIUM (B), and LONG (C). Each setting covers one-third of the wavelength range of each channel; a full observation from 4.9 to 27.9 μm would thus require an exposure at each of these 3 settings.
Each sequence must then be completed by selecting a MULTIACCUM exposure configuration for each detector being used (in the Exposure Parameters dialog box). Each exposure is configured by setting the readout pattern and characteristics parameters: Readout Pattern, Groups/Int, and Integrations/Exp. If you choose simultaneous imaging, you must select an imaging Filter for each MRS sequence. Each MRS sequence can support only a single imaging Filter. Finally select a Dither pattern from the list of Dithers already defined for the observation (or None for no dithering). The exposures will be repeated at each dither position following an offset of the telescope to the new position. The number of exposures at each dither position is set by Exposures/Dith, but this input must be the same for all detectors.
See also: MIRI Detector Readout Overview
MIRI offers 2 readout modes:
Number of groups and Integrations
See also: Understanding JWST Exposure Times
The MIRI timing pattern per exposure is defined by only 2 MULTIACCUM parameters:
- Groups/Int: the number of groups during an integration, where a group is the product of cycling through all the pixels
- Integrations/Exp: the number of integrations during an exposure, where integration is defined as the time between resets.
The MIRI MRS may be used to obtain data for a region larger than their size by creating a MIRI MRS mosaic pattern under the Mosaic Properties tab.
A variety of observatory level Special Requirements may be chosen under the Special Requirements tab.
The Comments field (under the Comments tab) should be used for observing notes.