Time Series Observations Roadmap

This page provides 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.

For example programs that use this roadmap, please see this page. We assume that the total length of the observation of TSOs is driven by the duration of the transit, rotation or variability period rather than the signal to noise requirement, and that this quantity is known.

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; and (b) near-or mid-infrared? We show a decision matrix for these choices below. We provide below separate roadmaps for imaging and spectroscopy.

Jump to:

TSO Imaging Roadmap
TSO Spectroscopy Roadmap


PhotometrySpectroscopy
λ ≤ 5 µmNIRCam Time Series Imaging

NIRCam Time Series Grism Spectroscopy

NIRISS Single-Object Slitless Spectroscopy (SOSS)

NIRSpec Bright Object Time Series Spectroscopy (BOTS)


λ ≥ 5 µmMIRI ImagingMIRI Low Resolution Slitless Spectroscopy

TSOs Roadmap: Imaging

  1. Determine the required wavelength coverage: near-infrared or mid-infrared. The links in the table above 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 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.
    NIRCam Detector Readout Patterns
    MIRI Detector Readout Patterns
    Pandeia Tutorial

  5. For NIRCam Time Series Imaging use the Exposure Time Calculator to determine the appropriate target acquisition strategy. For multi-epoch observations, target acquisition ensures that the target is placed in the same sub-pixel location for each epoch, which may be important to control for systematics such as intra-pixel responsivity variations. Note: target acquisition is not currently supported 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 description of the proposal. The TSO Special Requirement disables the ability to use dithering and mosaicking, and allows a single exposure to run longer than 10,000 seconds.
    NIRCam Time-Series APT Template
    MIRI Imaging APT Template
    APT Special Requirements

TSOs Roadmap: Spectroscopy

  1. Determine the required wavelength coverage: near-infrared or mid-infrared. The links in the table above provide more information about the wavelength coverage of the available spectroscopic TSO modes, in broad terms (near- vs. mid-infrared). For more details of the (instantaneous) wavelength coverage of each mode, check the relevant pages for the instrument modes. Additional parameters to consider to select between the near-IR modes are (i) sensitivity and saturation limits; and (ii) spectral resolving power. The table below gives a summary overview of the NIR modes; for spectroscopy beyond 5 µm, the MIRI Low Resolution Spectrometer is the only option. Note that the MIRI LRS cannot be used for TSOs with the slit; TSOs will 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 SensitivityMIRI Bright Source Limits

    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
  2. Select an instrument observing mode. The recommended strategies are particularly valuable to help you choose between the near-IR modes.
    NIRCam Time-Series Observation Recommended StrategiesNIRCam-Specific Time-Series Observations
    NIRISS SOSS Recommended StrategiesNIRISS-Specific Time-Series Observations
    NIRSpec BOTS OperationsNIRSpec-specific Time-Series Observations
    MIRI-Specific Time-Series Observations 


  3. Based on the target properties, determine the detector readout pattern and subarray configuration for the instrument mode. For MIRI, the readout pattern and subarray for a TSO 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.
    Pandeia Tutorial
    JWST ETC SOSS Spectral Extraction Strategy

  5. Users are particularly 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 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. The user 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 the best noise computation for TSOs. 

  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 Acquisition OverviewJWST ETC NIRCam Target Acquisition
    NIRISS Target AcquisitionJWST ETC NIRISS Target Acquisition
    NIRSpec Target Acquisition Recommended Strategies,  JWST ETC NIRSpec Target Acquisition
    MIRI Target AcquisitionsJWST 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 description of the proposal. The TSO Special Requirement disables the ability to use dithering and mosaicing, and allows a single exposure to run longer than 10,000 seconds.
    NIRCam Grism Time-Series APT Template
    NIRISS Single-Object Slitless Spectroscopy APT Template
    NIRSpec Bright Object Time Series Template Parameters
    MIRI LRS APT Template
    APT Special Requirements

Go to the General Proposal Planning Workflow to complete the steps for proposal submission.


PandExo homepage


References

Batalha, Natasha E., Mandell, Avi, Pontoppidan, K., et al. 2017, PASP, 129, 064501


Published

 

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