JWST ETC Time-Series Modes

Some aspects of ETC calculations for time-series observations are different, when compared to modes that don't use time series, including residual flat field noise and the number of rejected frames in each integration. 

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See also: JWST Time-Series ObservationsJWST Time-Series Observations Roadmap, JWST Example Science Programs

The JWST Exposure Time Calculator (ETC) is capable of performing calculations for 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:



Features of time-series modes in the ETC

General

  

Words in bold italics are
parameters or buttons in 
tools
like APT & ETC. Similarly, a bold 
style represents menus & panels.

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 these otherwise identical modes: MIRI imaging vs. MIRI imaging time series, MIRI MRS vs. MIRI MRS time series, NIRCam SW/LW imaging vs. NIRCam SW/LW time series, and NIRSpec fixed slit vs. NIRSpec bright object time series.

MIRI-specific

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. (Keep in mind that the behavior is very different in APT: as of APT 2021.2 users will receive an error if they specify 1 integration for TSOs.)

Figure 1. Standard time-series text on the Detector Setup tab




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