MIRI TSO Recommended Strategies
Recommendations for planning MIRI time-series observations, based on pre-launch knowledge of the instrument.
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 are pre-launch recommendations that will be updated with results from on-orbit commissioning.
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 description represents our best pre-launch understanding of this issue, based on ground testing. We expect to update these recommendations after launch.
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 of additional groups in the integration outweighs the risks posed by saturation at the end of the ramp.The JWST pipeline is 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 pre-flight uncertainties in the overall throughput of the observatory.
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.
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". The pre-flight estimate for absolute blind pointing accuracy of the telescope, without target acquisition, is expected to be 0.1" (1-σ, per axis); this corresponds, approximately, to the size of a MIRI pixel (pixel scale: 0.11"/px). 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 will likely 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.
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 fields labelled 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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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
The Mid-Infrared Instrument for the James Webb Space Telescope, VII: The MIRI Detectors
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