In time-series observations (TSOs), JWST continuously observes a time-varying phenomenon. This covers periodic phenomena such as eclipsing binary stars or transiting exoplanets, or other types of variable sources such as flares from accreting black holes. The goal of such observations is typically to obtain relative (spectro-)photometry to characterize the variations with high precision.

The main difference between TSOs and other types of observations is that TSOs are typically executed in single, long duration exposures consisting of multiple integrations. Whereas regular observations may include multiple integrations in an exposure to optimize signal-to-noise ratio over a wide bandwidth (e.g., to increase SNR on faint features without saturating the bright regions), each integration in a TSO is treated as a separate time sample. The duration of an integration therefore sets the time cadence with which the time series is sampled. TSOs can include exposures that are much longer than the 10,000 s to which other exposures are limited. 

TSOs are marked in APT with the time-series observation special requirement, which allows the observations to be executed under conditions optimal for this type of performance, and processed with a dedicated set of algorithms and pipeline parameter reference files. As a result, the JWST Science Calibration Pipeline output products differ in format from "regular" observations. Steps and changes specific to TSO data are outlined below. 

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TSO Modes

The following instrument modes are capable of obtaining TSO-mode observations covered by this article:

• NIRCam time-series imaging
• NIRCam grism time-series spectroscopy 
• NIRCam short wavelength grism time-series spectroscopy
• NIRSpec bright object time series (BOTS)
• NIRISS single object slitless spectroscopy (SOSS)
• MIRI low-resolution spectroscopy (LRS) in slitless mode
• MIRI imaging in TSO mode

• MIRI medium-resolution spectroscopy (MRS) in TSO mode

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File segmentation

Because of the long single exposure execution strategy, TSO exposures can often have very large file sizes. This makes the initial pipeline stages, in particular the calwebb_detector1 stage, which performs detector-level calibrations and converts ramps into slopes, very computationally demanding. To manage this, TSO exposures are segmented by the data management system into segments with a maximum size of 2 GB. This segmentation is performed after the exposure—the exposure itself was recorded continuously by the observatory. 

This means a single TSO exposure may consist of multiple files. The segments can be recognized by "segNNN" in the filename, where NNN is a 3-digit number (e.g., "seg001" indicates the first segment of an exposure). Only segmented files contain this "segNNN" field in their exposure filenames. Header keywords in the primary FITS header record first and last integration packaged in the particular segment file, as follows:

• EXSEGNUM: The segment number of the current product
• EXSEGTOT: The total number of segments
• INTSTART: The starting integration number of the data in this segment
• INTEND: The ending integration number of the data in this segment

Each segment is processed independently in stages 1 and 2 of the pipeline. In stage 3, the segments are combined to produce a single time-series output product from the exposure. 

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Imaging

Imaging pipeline stages and steps

Imaging TSOs follow the pipeline in 3 stages:

• calwebb_detector1
• calwebb_image2
• calwebb_tso3

The stages listed above are linked to articles that provide further information on the steps in each of them, with additional links to dedicated software documentation. Stages 1 and 2 are generic to imaging observations, with the TSO-specific flow set by parameter reference files. These can be viewed and downloaded from the Calibration Reference Data System (CRDS). 

Stage 3 of the pipeline, calwebb_tso3, performs TSO-specific processing steps for both spectroscopy and imaging. For the case of imaging, 2 steps are executed in stage 3 of the pipeline:

1. Residual outliers are flagged.
2. Aperture photometry is performed and the resulting fluxes are written to a flux-calibrated photometric light curve output file.

If the original exposure file was segmented, the data will, at this stage, be merged to produce a single light curve.  

As in all stage 3 modules, the input to the calwebb_tso3 pipeline is an association file, which brings together the multiple segments.

Imaging data products

The pipeline generates a number of additional products for TSOs compared with standard imaging observations. The calwebb_detector1 pipeline produces a "rate.fits" file, but specifically also a "rateints.fits" file. The "rateints" file contains slope images for the exposure, or exposure segment, for each individual integration. In the "rateints.fits" file, the science, DQ, error, and variance extensions are 3-D, instead of 2-D, products, with the 3^rd dimension matching the number of integrations in the exposure or exposure segment. 

An additional extension, "INT_TIMES", contains a table listing the beginning, middle, and end time stamps for each integration in the exposure, or exposure segment.

The calwebb_tso3 pipeline stage produces 2 output products. The first, with file extension "crfints.fits", is produced from the outlier detection step, and contains updated outlier flags in the DQ array. No changes are made to the science or error extensions. Finally, the photometric time series is captured in the "phot.ecsv" file, which is an ecsv file. This file consists of a source catalog containing photometry results from all of the "crfints" products, organized as a function of integration time stamps. 

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Spectroscopy

Spectroscopic pipeline stages and steps

Spectroscopic TSOs follow the pipeline in 3 stages:

• calwebb_detector1
• calwebb_spec2
• calwebb_tso3

The stages listed above are linked to articles that provide further information on the steps in each of them, with additional links to dedicated software documentation. Stages 1 and 2 are generic to spectroscopic observations, with the TSO-specific flow set by parameter reference files. These can be viewed and downloaded from the Calibration Reference Data System (CRDS). 

Stage 3 of the pipeline, calwebb_tso3, performs TSO-specific processing steps for both spectroscopy and imaging. For the case of spectroscopy, 3 steps are executed in stage 3 of the pipeline:

1. Residual outliers are flagged.
2. The segments are merged and spectra are re-extracted and packaged into a single file, using the same parameters as in the calwebb_spec2 pipeline.
3. The spectrum is summed in wavelength space to produce a white light curve.

If the original exposure file was segmented, the segments will at this stage be merged to produce a single extracted spectrum per integration and a single white light curve for the entire exposure. The white light curve is produced by summing the flux between pre-defined wavelength boundaries in the spectrum in each extracted spectrum, yielding a single flux data point per integration. 

As in all stage 3 modules, the input to the calwebb_tso3 pipeline is an association file which brings together the multiple segments.

Spectroscopic data products

The pipeline generates a number of additional products for TSOs compared with standard spectroscopic observations. The calwebb_detector1 pipeline produces a "rate.fits" file, but specifically also a "rateints.fits" file. The "rateints" file contains slope images for the exposure, or exposure segment, for each individual integration. In the "rateints.fits" file, the science, DQ, error and variance extensions are 3-D, instead of 2-D, products, with the 3^rd dimension matching the number of integrations in the exposure or exposure segment. 

An additional extension, "INT_TIMES", contains a table listing the beginning, middle, and end time stamps for each integration in the exposure, or exposure segment.

Similar to calwebb_detector1, calwebb_spec2 returns 2-D spectro-photometrically calibrated images as "calints.fits" files, and 1-D extracted spectra are returned as "x1dints.fits" files. At this stage, segmented files are processed separately and each exposure segment will produce its own "x1dints.fits" file. 

The calwebb_tso3 pipeline stage produces 3 spectroscopic output products for each additional step as described above, merging the exposure segments for segmented observations. The first, with file extension "crfints.fits", is produced from the outlier detection step, and contains updated outlier flags in the DQ array. No changes are made to the science or error extensions in this step. Second, the exposure segments will be merged, and the spectra are re-extracted as a single product (with the segments merged together), producing a new "x1dints.fits" file. The white light curve is captured in an ecsv file with suffix "wtlt.ecsv". This file lists the integrated white light flux as a function of time, based on the integration time stamps.

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Pipeline parameter reference files

TSO-specific steps and step configurations are enabled using special parameter reference files available through CRDS (see JWST Pipeline Overview) that identify TSO flags in the file header metadata. Examples of TSO-specific data processing are captured in the TSO JWebbinar materials, available online.

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