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 is provided in this article.

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 or public compilations of observations from the community (e.g., TrExoLiSTS for transiting exoplanet observers; Nikolov et al., 2022), 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) and readout mode. 
   NIRCam Sensitivity
   NIRCam Bright Source Limits
   NIRCam Detector Subarrays
   NIRCam Detector Readout Patterns
   MIRI Sensitivity
   MIRI Bright Source Limits
   MIRI Detector Subarrays
   MIRI Detector Readout Patterns
   
   
3. Choose the appropriate pupil optic and/or filter configuration
   NIRCam Pupil and Filter Wheels, NIRCam 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.
   TSO Signal to Noise and Saturation
   JWST ETC Pandeia Engine 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 Acquisition, JWST 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. For periodically varying targets, such as transiting exoplanets, 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. You should consult the TSO Known Issues page to avoid some common APT errors for TSOs. 
   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 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 Series, NIRCam Sensitivity, NIRCam Bright Source Limits
   NIRISS Single Object Slitless Spectroscopy (SOSS), NIRISS Sensitivity, NIRISS Bright Limits
   NIRSpec Bright Object Time-Series Spectroscopy (BOTS), NIRSpec Sensitivity, NIRSpec Bright Source Limits
   MIRI Low Resolution Spectroscopy (LRS), MIRI Medium Resolution Spectroscopy (MRS), MIRI Sensitivity, MIRI Bright Source Limits

   
   Table 2. Overview of near-infrared TSO spectroscopy modes

   Mode	Disperser type	R	Subarray choices?	Readout pattern choices?	Target acquisition	Other
   NIRCam grism time series	Grism	1,600	4	yes	yes	Spectroscopy in the long wavelength channel, which can be accompanied by weak lens imaging or grism spectroscopy in short wavelength channel
   NIRCam short-wavelength time series	Grism	300-600	4 (multistripe)	yes	yes	Spectroscopy in the short wavelength channel, which can be accompanied by grism spectroscopy in long wavelength channel. Subarray uses the new NIRcam multistripe subarray mode.
   NIRISS SOSS	Grism	700	3	yes	yes	SOSS modes provides scientifically useful spectra in 2 orders
   NIRSpec BOTS	Prism or grating	100, 1,000, 2,700	5	yes	yes (WATA)	Gratings are combined with filters to determine the instantaneous spectral coverage

   Table 3. Overview of mid-infrared TSO spectroscopy modes

   Mode	Disperser type	R	Subarray choices?	Readout pattern choices?	Target acquisition	Other
   MIRI LRS	Double Prism	~100	no	no	yes	TSOs are only available in the slitless mode, which uses the SLITLESSPRISM subarray
   MIRI MRS	Grating	~1,500-3,500	no	yes	yes	A 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 TSOs, MIRI 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 (FASTR1) and, for LRS, subarray (SLITLESSPRISM) are fixed.
   NIRCam Detector Readout Patterns, NIRCam Detector Subarrays
   NIRISS Detector Readout Patterns, NIRISS Detector Subarrays 
   NIRSpec Detector Readout Modes and Patterns, NIRSpec Detector Subarrays, 
   MIRI Detector Readout Patterns, MIRI 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.
   TSO Signal to Noise and Saturation
   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 spectroscopy 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 Acquisition, JWST ETC NIRCam Target Acquisition
   NIRISS Target Acquisition, JWST ETC NIRISS Target Acquisition
   NIRSpec Target Acquisition Recommended Strategies,  JWST ETC NIRSpec Target Acquisition
   MIRI Target Acquisition, JWST 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. For periodically varying targets, such as transiting exoplanets, 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. You should consult the TSO Known Issues page to avoid some common APT errors for TSOs. 
   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 with Planning JWST Observations to complete the steps for proposal submission.

Example science programs

Example science programs that use this roadmap:

NIRCam Grism Time-Series Observations of GJ 436b

NIRISS SOSS Time-Series Observations of a Transiting Exoplanet

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

Nikolov, N. K, et al. 2022, RNAAS, 6, 272
TrExoLiSTS: Transiting Exoplanets List of Space Telescope Spectroscopy