MIRI TSO Recommended Strategies
Recommendations for planning MIRI time-series observations, based on commissioning and Cycle 1 experience with the MIRI TSO modes, are provided in this article.
This page, together with the MIRI Cross-Mode Recommended Strategies and JWST Time-Series Observations articles, should help the observer plan MIRI time-series observations (TSOs). Note that these recommendations are based on findings from commissioning and initial Cycle 1 science observations.
Time-series observations (TSO) are carried out to monitor astrophysical sources that are time variable. A time-series observation is typically a long staring observation, optimized to detect and characterize faint temporal modulations in the source flux. Common examples are observations of stellar variability, eclipsing variables, brown dwarf variability, and exoplanet transits.
TSOs can be performed with MIRI in the following modes:
- MIRI imaging
- MIRI low resolution spectroscopy (slitless; LRS)
- MIRI medium resolution spectroscopy (MRS)
For dedicated instructions on preparing TSOs and comparing TSO modes across the instruments, see the JWST Time-Series Observations Roadmap.
For time-series observations, the challenge is to achieve maximum detector stability and measurement precision over timescales of several to many hours. This section provides guidance for achieving the best possible results for TSOs using MIRI observing modes.
TSOs are best executed in a single staring exposure for a particular transit or eclipse event, to achieve optimal detector and telescope pointing stability.
Contrary to "regular" science observations, the total length of the exposure is not chosen based on the required S/N ratio, but on the duration of the transit event. Knowledge of the transit timing and duration is therefore essential for planning such observations (see Scheduling TSOs). In the data calibration and processing of TSOs, each integration ("ramp") is treated as an individual exposure.
The number of groups in each integration should be calculated using the JWST Exposure Time Calculator (ETC), either optimizing for the required S/N ratio and/or the desired time resolution (i.e., the cadence at which the light curve should be sampled). In the case of very bright targets, the user has to balance the number of groups (where, generally, more is better) against potential saturation issues. In Figure 1 we provide some guidance for these choices. In general, the minimum recommended number of groups in an integration for accurate calibration is 5; values between 2 and 5 are also permitted, though the calibration accuracy may be affected for such observations. The maximum recommended length of an integration is approximately ~1,000 s, based on our current best understanding of cosmic ray arrival statistics and cosmic ray impact on the MIRI detectors.
The number of integrations in the exposure should be calculated based on the required duration of the total exposure. This should include sufficient time out-of-transit compared to the transit duration, and some additional time for detector settling (see section below). The maximum number of integrations permitted in a single exposure is 65,535.
Detector readout mode
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Detector readout patterns
For the spectroscopic modes, the detector readout patterns are fixed. When used in slitless mode, the low resolution spectrometer (LRS) uses the SLITLESSPRISM subarray, which provides a higher dynamic range than is available in the FULL array configuration (see MIRI Bright Source Limits). For time-series imaging, all imaging subarrays may be selected for TSOs.
If a MIRI TSO imaging target saturates in <5 groups, a smaller subarray should be chosen to allow for a higher number of groups for a given integration time.
Detector settling time
We recommend that users start their exposure approximately 30 minutes prior to the time period they actually need to cover in order to allow for detector settling.
MIRI's mid-infrared detectors display a number of non-ideal behaviours such as nonlinearity, latents, reset anomalies and drifts (see Rieke et al. 2015, Ressler et al. 2015). There are several mechanisms behind these observed response variations, some arising in the detector substrate itself, others caused by the cryogenic readout electronics. TSO observations use multi-integration exposures which may be affected by drifts, i.e., where the response measured between integrations gradually changes at the beginning of the observation, settling to a more stable response over time. While these data are still considered "science quality" (i.e., accurate ramps can be constructed), adding a settling time at the start of the observation is thought to help improve the relative photometry and/or enable a more accurate subtraction of the drift function.
To achieve the highest possible measurement precision in the time series, users should therefore add additional time to their exposure to account for this initial settling. A complicating factor is that the strength of the effects and the required settling time is strongly dependent on the flux levels received in the preceding integration prior to the reset, so there is no single number for the detector settling time that will cover all cases. In addition, for TSOs where the exposure covers a wide wavelength range with a strongly varying SED (e.g., a stellar-type SED observed with the MIRI LRS), the detector settling time may even vary across the pixels in a given exposure. The value of 30 minutes is considered a conservative estimate on the settling time that should cover most observations.
This recommendation is supported by findings during commissioning, where we observed a response drift of ~0.3% in the first 20 minutes of the time series.
Dealing with saturation
The issue of saturation for MIRI and TSOs is described in more detail in the reference articles above. Observing very bright targets with MIRI can cause detector artifacts, and these issues become worse as the pixels approach saturation. In general, users should be cautious about integrating up to saturation. In the case of MIRI, there are situations where the benefits to the detector stability in additional groups in the integration outweighs the risks posed by saturation at the end of the ramp. For Ngroups > 5, the JWST calibration pipeline does not use the final group of a ramp in the ramp fitting (due to the last frame effect), so saturation in the final group in those cases will not impact the ramp fit. The JWST pipeline is also able to flag saturated groups in an integration, and only use the non-saturated groups to construct the ramp; saturation in the final groups of an integration therefore does not cause the data to be "lost."
Figure 1 shows a decision flow diagram to help the user choose the appropriate number of groups for their time-series observation in the case of very bright targets. These recommendations apply to all 3 MIRI TSO modes.
The user should always rely on the JWST Exposure Time Calculator (ETC) to estimate when saturation will occur for their target. The count level at which the ETC reports saturation is a conservative estimate that accounts for uncertainties in the overall throughput of the instrument.
Dithering is not permitted for TSOs with MIRI. If the user has assigned a dither pattern to an observation when the Time Series Observation special requirement is selected in APT, an error will be returned.
Time Series Observations with the LRS use the SLITLESSPRISM subarray, which measures 416 rows by 72 px in size. The typically bright nature of TSO targets, especially at the shortest wavelengths, creates a significant amount of scattered light (the cruciform artifact, seen in all MIRI modes). This artifact is extensive enough to fill the entire width of the subarray, creating an additive component to the background. When performing background subtraction using off-target regions of the subarray, this causes an over-subtraction of the background flux. For a better background subtraction result, adding a dedicated background pointing to the observation after the main exposure is recommended, so that it does not impact the critical start time of the TSO.
Target acquisition is mandatory for LRS TSOs, so the off-target background will also have to use a TA sequence in APT. A good way to achieve this is to set the target for the background observation to the science target, and add a spatial offset in the special requirements. Any spatial offset can be used (e.g. 15–20" in X) within the limit of the visit splitting distance, taking care to avoid placing bright targets in the subarray. The background exposure should be matched in number of groups per integration with the science data, but only a small number of integrations is required (e.g., 10). Science and background exposures should be linked as a non-interruptible sequence, to ensure that the background is measured immediately following the science exposure.
The background observation does not require additional timing constraints.
Target acquisition considerations
Because of the high stability and precision required in a time-series observation, target placement at the subpixel level is very important. This is particularly the case where multi-epoch observations will be carried out and combined to reach the required measurement precision. Target acquisition is therefore highly recommended for such observations.
For slitless LRS and MRS, TA is mandatory when the Time Series Observation special requirement is selected. Please refer to the articles above for detailed descriptions of the procedures and choices for TA in these modes.
Target acquisition is not currently supported for MIRI imaging TSOs. For these observations, the target placement will be limited to the JWST pointing accuracy and stability. The smallest subarray for MIRI imaging measures 64 × 64 pixels (SUB64), or about 7" × 7". Provided the coordinates of the target and its proper motion are well known, the telescope will be reliably able to acquire a target into the MIRI imaging array, even when using the SUB64 subarray.
Performing multi-epoch imaging TSOs without target acquisition may see the target placed onto a different set of pixels with each epoch. This may introduce additional systematics when the multiple epochs are combined, as different pixels will be subject to different noise residuals from flat fielding, intra-pixel gain variations, and other calibrations. This is likely to result in a somewhat degraded performance.
When the target is dispersed, it is challenging to reconstruct precisely where the target was positioned on the detector, either from blind pointing or from target acquisition. Verification images have proven very useful as a pointing verification check, and to diagnose potential calibration issues. This capability has been given extra flexibility as of APT2023.1.1, to make this feature maximally useful. For slitless observations, the image will be taken by default. A verification image can be taken with any of the MIRI imager filters, and with custom groups/integration settings.
Scheduling observations of periodic phenomena requires the user to give input on the timing of the event (e.g., exoplanet transit) and when the observation should start. The user should have this information on hand for their target, from the literature or community databases such as Exo.MAST. This information should be provided using the timing special requirements in APT; the Timing Special Requirements article provides further information on this feature in APT. In the Phase special requirement, under the Timing special requirement in APT, the user should provide the Zero phase timing (in heliocentric Julian Date (HJD)), and the Period duration. The field labeled Phase range should specify the range of phases within which the observation should be started. It's recommended to provide a window of at least 1 hour; if a smaller window is provided, the observation will incur a 1-hour additional overhead for schedulability.
An additional ~30 minutes of padding, prior to the scientifically relevant observation period, is recommended to account for detector settling.
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