JWST ETC Time Series Modes
Some aspects of ETC calculations for time series observations are different, when compared to non-time series modes, including residual flat field noise and the number of rejected frames in each integration.
The JWST Exposure Time Calculator (ETC) is capable of performing calculations for both time series and non-time series modes. The MIRI, NIRCam, NIRISS, and NIRSpec instruments all present modes for time series observations (TSOs) of, e.g., exoplanet transits, transient phenomena, and stellar occultations. The time series modes available in the ETC are listed below, with names as they appear in the calculation dropdown menus:
- MIRI Imaging Time Series
- MIRI Low Resolution Spectroscopy (LRS) Slitless
- MIRI MRS Time Series
- NIRCam SW Time Series
- NIRCam LW Time Series
- NIRCam LW Grism Time Series
- NIRISS SOSS
- NIRSpec Bright Object Time Series
Features of time series modes in the ETC
The ETC does not explicitly model instrument dithers, but does account for a decrease in the pixel-by-pixel residual flat field noise as a result of dithering. For non-time series modes, this is accounted for by increasing the number of exposures in the Exposures per specification box on the Detector Setup tab. Because each TSO sample is made using the same pixels, and the samples are compared relative to each other, this is not an issue for TSOs. More information on this is provided in the JWST ETC Residual Flat Field Errors article.
To reflect the fact that TSOs are immune to residual flat field effects and the fact that dithers are not supported for TSOs, the residual flat field noise component is not included in the computation of the total noise for time series modes in the ETC. This slightly increases the SNR compared to when the residual flat field noise is included. This can be easily seen when comparing SNRs for otherwise identical MIRI Imaging and MIRI Imaging Time Series, MIRI MRS and MIRI MRS Time Series, NIRCam SW/LW Imaging and NIRCam SW/LW Time Series, and NIRSpec Fixed Slit and NIRSpec Bright Object Time Series modes.
An additional change is made to the number of rejected frames for MIRI time series modes compared to the non-time series modes. The first and last frames are rejected by the pipeline and the ETC for non-time series MIRI integrations greater than 5 groups (with special handling for integration ramps with fewer than 5 groups) due to the first and last frame effects. In other words, an integration with 12 groups will be reduced to 10 groups, with the SNR computed for 10 groups, ignoring the first and last groups. No frames are rejected for any MIRI time-series modes, regardless of integration ramp length.
Time series mode calculations
All time series modes present the text shown in Figure 1 (below) on the Detector Setup tab, followed by a "context sensitive help link" (the blue question mark icon) that leads to a relevant JDOX page specific to the instrument and mode.
Users interested in evaluating time series observations in the ETC should heed the advice given by the orange text in Figure 1 (below) and any recommendations provided at the instrument-specific link. In general, users will want to calculate the SNR for a single sample (i.e., a single integration) rather than for the entirety of their observation (multiple integrations). Evaluating the SNR for the entire observation is incorrect for TSOs because the ETC coadds the results from each integration ramp, increasing the SNR by a factor of the square root of the number of integration ramps, resulting in a significant overestimation of the SNR.