TSO Roadmap

A roadmap to guide users, step-by-step, through the process of designing an observing program using one of the dedicated time-series modes on board JWST.

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For time-series observations (TSOs), we assume that the total length of the observation is driven by the duration of the transit, rotation, or variability period rather than the signal-to-noise requirement, and that these quantities are known from previous observations or from the literature. A useful reference source for this information, aside from published literature on your target, is the ExoMAST database.

The first step is to select the TSO mode that is most appropriate for your observation. The first choices to make here are

(a) imaging or spectroscopy
(b) near- or mid-infrared?


Table 1. A decision matrix for time-series imaging and spectroscopy observations



TSOs roadmap: imaging

  1. Determine the required wavelength coverage: near-infrared or mid-infrared.
    The links in Table 1 provide more information about the wavelengths covered and filters provided by the imaging TSO modes.

  2. By studying the sensitivity, saturation limit, and dynamic range of the relevant detectors, determine the required array configuration (full array or subarray)
    NIRCam SensitivityNIRCam Bright Source LimitsNIRCam Detector Subarrays
    MIRI SensitivityMIRI Bright Source LimitsMIRI Detector Subarrays

  3. Choose the appropriate pupil optic and/or filter configuration
    NIRCam Pupil and Filter WheelsNIRCam Filters
    MIRI Filters

  4. Calculate the required exposure configuration using the JWST Exposure Time Calculator. Note that for TSOs each integration is treated as a separate image by the calibration pipeline; the purpose is to monitor the target over a period of time, not to co-add the integrations to increase the SNR. We therefore recommend that the ETC be used to model the SNR for a single integration. The number of integrations is then determined by the required length of the observation (e.g., based on the known transit duration). The ETC can also be run from its code engine, Pandeia.
    NIRCam Detector Readout Patterns
    MIRI Detector Readout Patterns
    Pandeia Tutorial

  5. For NIRCam time-series imaging, use the Exposure Time Calculator (ETC) to determine the appropriate target acquisition strategy. For multi-epoch observations, target acquisition ensures that the target is placed in the same subpixel location for each epoch, which may be important to control for systematics such as intra-pixel responsivity variations. Note: target acquisition is currently not available for MIRI imaging.
    NIRCam Time-Series Imaging Target AcquisitionJWST ETC NIRCam Target Acquisition
    JWST Pointing Performance

  6. Using the outcomes of steps 1 to 6, complete the Astronomer's Proposal Tool template using the appropriate NIRCam or MIRI template. Remember to check that the Time Series Observation* and No Parallel special requirements are selected in the Special Requirements tab of the observation form in the proposal. The Time Series Observation special requirement disables the use of dithering and mosaicking, and allows a single exposure to run longer than 10,000 s. You will also need to specify the phase constraints and transit timing specifications in the Special Requirements section; without this information the observation cannot be successfully scheduled. 
    NIRCam Time-Series APT Template
    MIRI Imaging APT Template
    APT Special Requirements


* Bold italics style indicates words that are also parameters or buttons in software tools (like the APT and ETC). Similarly, a bold style represents menu items and panels.



TSOs Roadmap: Spectroscopy

  1. Determine the required wavelength coverage: near-infrared or mid-infrared.
    The links in Table 1 provide more information, in broad terms, about the wavelength coverage of the available spectroscopic TSO modes (near- vs. mid-infrared). For more details on the (instantaneous) wavelength coverage of each mode, check the instrument mode articles (listed below). Additional parameters to consider for selection between the near-IR modes are (1) sensitivity and saturation limits and (2) spectral resolving power. Table 2 gives a summary overview of the NIR modes; for spectroscopy beyond 5 µm, the MIRI low resolution spectrometer (LRS) and medium resolution spectrometer (MRS) are both available. Note that MIRI LRS with the slit cannot be used for TSOs —these observations always use the LRS slitless mode in the SLITLESSPRISM subarray.
    NIRCam Grism Time SeriesNIRCam SensitivityNIRCam Bright Source Limits
    NIRISS Single Object Slitless Spectroscopy (SOSS), NIRISS SensitivityNIRISS Bright Limits
    NIRSpec Bright Object Time-Series Spectroscopy (BOTS), NIRSpec SensitivityNIRSpec Bright Source Limits
    MIRI Low Resolution Spectroscopy (LRS), MIRI Medium Resolution Spectroscopy (MRS), MIRI SensitivityMIRI Bright Source Limits


    Table 2. Overview of near-infrared TSO spectroscopy modes

    ModeDisperser typeRSubarray choices?Readout pattern choices?Target acquisitionOther
    NIRCam grism time seriesGrism1,6004yesyesSpectroscopy in long wavelength channel only, accompanied by weak lens imaging in short wavelength channel
    NIRISS SOSSGrism7003yesyesSOSS modes provides scientifically useful spectra in 2 orders
    NIRSpec BOTSPrism or grating100, 1,000, 2,7005yesyes (WATA)Gratings are combined with filters to determine the instantaneous spectral coverage

    Table 3. Overview of mid-infrared TSO spectroscopy modes

    ModeDisperser typeRSubarray choices?Readout pattern choices?Target acquisitionOther
    MIRI LRSDouble Prism~100nonoyesTSOs are only available in the slitless mode, which uses the SLITLESSPRISM subarray
    MIRI MRSGrating~1,500-3,500noyesyesA single exposure covers 1/3rd of the full wavelength range; full coverage of 4.9-28.8 µm would require 3 separate exposures. The MRS simultaneous imaging capability for MRS is disabled for TSOs.
  2. Select an instrument observing mode. The recommended strategies articles are particularly valuable to help you choose between the near-IR modes.
    NIRCam Time-Series Observation Recommended Strategies
    NIRISS SOSS Recommended Strategies
    NIRSpec BOTS Operations
    MIRI LRS TSOsMIRI MRS TSOs

  3. Based on the target properties, determine the detector readout pattern and subarray configuration for the instrument mode. For MIRI TSOs, the readout pattern (FAST) and, for LRS, subarray (SLITLESSPRISM) are fixed.
    NIRCam Detector Readout PatternsNIRCam Detector Subarrays
    NIRISS Detector Readout PatternsNIRISS Detector Subarrays 
    NIRSpec Detector Readout Modes and PatternsNIRSpec Detector Subarrays
    MIRI Detector Readout PatternsMIRI Detector Subarrays

  4. Calculate the required exposure configuration using the JWST Exposure Time Calculator. Note that in TSOs each integration is treated as a separate image; the purpose is to monitor the target over a period of time, not to co-add the integrations to increase the SNR. We therefore recommend that the ETC is used to model the SNR for a single integration. The number of integrations is then determined by the required length of the observation (e.g. based on the known transit duration). Note that the ETC can also be run from its code engine, Pandeia.
    JWST ETC Pandeia Engine Tutorial
    JWST ETC SOSS Spectral Extraction Strategy

  5. Users are encouraged to use PandExo for more detailed modelling of spectroscopic exoplanet transit observations (Batalha et al.). PandExo is an exoplanet modelling tool built on Pandeia, the code engine for the ETC. While Pandeia can only model a stellar spectral energy distribution (SED), PandExo accepts as input a stellar SED, a planet spectrum, and the transit duration and provides, as output, the error obtained on your spectrum. You can also optimize the number of groups per integration. PandExo has good front-end plotting functions, as well as parallelized bash scripts for running multiple noise models at once. It provides improved noise computation for TSOs compared with the ETC. 

  6. Determine whether target acquisition is required, and use the ETC to determine the appropriate strategy. Target acquisition is highly recommended (for some modes, mandatory) for spectroscopic TSOs given the importance of target placement on the detector, particularly when data will be combined from different transit epochs.
    NIRCam Target AcquisitionJWST ETC NIRCam Target Acquisition
    NIRISS Target AcquisitionJWST ETC NIRISS Target Acquisition
    NIRSpec Target Acquisition Recommended Strategies,  JWST ETC NIRSpec Target Acquisition
    MIRI Target AcquisitionJWST ETC MIRI Target Acquisition

  7. Using the outcomes of steps 1 to 6, complete the Astronomer's Proposal Tool template using the appropriate template. Remember to check that the Time Series Observation and No Parallel special requirements are selected in the Special Requirements tab of the observation form in the proposal. The Time Series Observation special requirement disables the ability to use dithering and mosaicking, and allows a single exposure to run longer than 10,000 s. You will also need to specify the phase constraints and transit timing specifications in the Special Requirements section; without this information the observation cannot be successfully scheduled. 
    NIRCam Grism Time-Series APT Template
    NIRISS Single-Object Slitless Spectroscopy APT Template
    NIRSpec Bright Object Time-Series APT Template
    MIRI LRS APT Template
    MIRI MRS APT Template
    APT Special Requirements

Go to the Getting Started Guide to complete the steps for proposal submission.



Example Science Programs

Example science programs that use this roadmap:

NIRCam Time-Series Imaging of HAT-P-18 b

NIRCam Grism Time-Series Observations of GJ 436b

NIRISS SOSS Time-Series Observations of HAT-P-1

NIRSpec BOTS Observations of WASP-79b



References

PandExo homepage

Batalha, N. E. et al. 2017, PASP, 129, 064501
PandExo: A Community Tool for Transiting Exoplanet Science with JWST & HST




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