MIRI LRS Recommended Strategies
Recommendations for planning MIRI LRS science observations, based on pre-launch knowledge of the instrument, are provided in this article.
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The MIRI low resolution spectrometer (LRS) offers slit and slitless spectroscopy from 5 to 14 μm. This page gives recommendations that, together with the MIRI Cross-Mode Recommended Strategies article, should help observers plan MIRI LRS observations.
For LRS slitless observations, please refer to the dedicated MIRI TSO Recommended Strategies page.
Detector readout mode
See also: Understanding Exposure Times, MIRI Cross-Mode Recommended Strategies
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See also: MIRI LRS Dithering
In slit mode, LRS dithering is required for science observations (see MIRI Low Resolution Spectroscopy Template in APT). Two options are available:
- The ALONG SLIT NOD represents a 2-point dither, which is recommended for point sources. This mode allows for both redundancy and background subtraction. The user should verify that there are no sources close to the target that could occupy the other dither position, which would defeat the purpose of the dither strategy—such unfavorable roll angles should be avoided.
- The MAPPING MODE allows the user to define a certain number of spectral and spatial steps and offsets, and it has been designed to allow for extended source mapping.
When defining a target in APT, users should specify, in the Extended parameter field, if the target is spatially extended; the options are YES, NO, and Unknown. The selected dither pattern should be consistent with the source definition. Deviations from these default options should be justified in the proposal.
Dithering is not allowed for the slitless mode as this option has been specifically designed to carry out time-series observations. Selecting the SLITLESSPRISM subarray in the APT LRS template will automatically select the Time Series Observation and No Parallel special requirements, which disables dithering.
Dwell time limit
See also: JWST Communications Subsystem
The dwell time is the length of time the target is maintained at a particular location, before a dither is applied. As a general rule, more groups in an integration improve our ability to calibrate the data. The minimum recommended number of groups for good absolute calibration is 5. For faint targets, it is best to maximise the duration of integrations to build signal to noise, and avoid having to perform multiple exposures. However, to mitigate the build up of cosmic rays on the detector, it is recommended to limit the duration of an exposure to approximately 1,000 s (360 groups in FASTR1/FULL mode, 42 in SLOWR1 mode, in FULL array configuration). A pre-launch study assuming typical cosmic ray arrival rates and properties showed that after a 1,000 s integration, most of the detector pixels are expected to be affected by cosmic rays. This not only includes pixels that are directly impacted by a cosmic ray, but also its adjacent pixels. An exposure can be broken up into multiple integrations to continue building signal, but the total duration of a single exposure should not exceed 10,000 s. This permits the regular repointing of the high-gain antenna for observatory communications.
For slitless LRS observations, dithering is disabled and the 10,000 s exposure time limit is waived to allow the observation to follow the time-variable phenomenon continuously. In slitless mode we can continue collecting photons through the HGA repointing events.
Please refer to the MIRI Cross-Mode Recommended Strategies for further guidance on this on MIRI-specific guidelines for the length of integrations within an exposure.
Target acquisition considerations
See also: MIRI Cross-Mode Recommended Strategies, MIRI LRS Slit Target Acquisition, MIRI LRS Slitless Target Acquisition, JWST Pointing Performance
For slitless TSO observations, target acquisition (TA) is required. Accurate target placement is especially important if multi-epoch transit observations will be combined. The TA procedure will ensure that the target is placed at the same location for each exposure, with <10 mas accuracy (corresponding to <0.1 px).
For slit observations, TA is highly recommended for point or compact sources given the size of the slit (length 4.7", 3.18 mm, 42.7 pix; width 0.51", 0.33 mm, 4.6 pix) and the sensitivity of the calibration to the location of the source in the slit. The no-TA option is intended to be used mostly for extended sources, and for dedicated background exposures, if required. When observations are carried out without a TA, the target placement accuracy is determined by the JWST pointing performance and the accuracy of the target coordinates (including proper motion).
Users should obtain a TA verification image, which will be obtained after the science target has been moved into the slit. This image will verify the placement of the target in the slit and will improve the observer's ability to correct for slit transmission. For further information, see LRS Slit Target Acquisition - Verification Image.
Some observing situations will require the use of an offset TA target. Observers should exercise care in these situations, because any error in the position of either the TA target or the spectroscopic target will result in the misplacement of the target in the slit. See LRS Slit Target Acquisition - Self TA vs. offset TA for additional information.
See also: JWST Background Model, Background-Limited Observations, MIRI Cross-Mode Recommended Strategies, APT Targets, MIRI TSO Recommended Strategies
For observations of point sources in the LRS slit, an ALONG SLIT NOD should be specified. In this configuration, the nods will be subtracted from each other pair-wise for background subtraction purposes - before being recombined into a single spectral image in the calibration pipeline.
For extended sources in the LRS slit, observers are strongly encouraged to obtain background observations by defining a separate background target. The coordinates should point to a suitable region nearby, preferably within 20", and the observation should otherwise be identical to that for the science target.
The science and background observations should be linked as a non-interruptible sequence in APT and then, for the science target, the background target should be specified as such (see Specifying APT Background Targets). This method will obtain 2 images of the science target, one in each nod position, and 2 corresponding background images. When a user assigns a background to a science target, that creates a formal association between them. By doing this, the pipeline will automatically subtract the background exposure from the target exposure. The background exposures are co-added into a single background image, which is subtracted from each nod separately. This method is preferred, because it mitigates for bad pixels on the array (whether permanent or due to a recent cosmic ray hit).
A MAPPING dither pattern with either 2 spatial positions or 2 spectral positions could also be used, but this method is strongly discouraged, because the pipeline will have no means of identifying the background position and will not subtract it. In addition, this approach would not mitigate for bad pixels. If the observer wishes to pursue this option, the extended science target will be observed in the center of one of the 2 slit positions. The background position will rotate with the position angle of the slit (at roughly 1° per day) and can only be constrained by constraining the observing time.
For slitless TSO observations, the background can be measured from the portions of the subarray not covered by the science spectrum. However due to the extensive detector-level scattering for bright targets, it may be difficult to get a "clean" measurement of the background, and the background subtraction step may end up over-subtracting the short wavelengths flux of the science target. To avoid this, it is advisable to take a dedicated background exposure immediately following the science exposure. This should have Ngroups matched to the science exposure, with a small number of integrations (e.g., 10), and the observation should be linked as a non-interruptible sequence. Further details are provided on the MIRI TSO Recommended Strategies page.
Kendrew et al. 2015, PASP, 127, 623K
The Mid-Infrared Instrument for the James Webb Space Telescope, IV: The Low-Resolution Spectrometer
Glasse et al. 2015, PASP, 127, 686G
The Mid-Infrared Instrument for the James Webb Space Telescope, IX: Predicted Sensitivity