The duration of a JWST observation can be described using 3 characteristic time definitions, each with their own purpose: exposure time, measurement time, and saturation time.

The complex readout patterns of the JWST instrument detectors lead to multiple ways to measure the duration of an observation. Different times are used for different purposes in the Astronomers Proposal Tool (APT) and the Exposure Time Calculator (ETC), and the JWST Science Calibration Pipeline reports a variety of different time-related keywords. Users interested in the full equations should read the JWST technical document by Holler et al. (2021). See also Understanding Exposure Times for a basic introduction to JWST up-the-ramp readout.

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Time definitions

The different times can be divided into 2 broad categories:

• Pixel-based time: Time with respect to a single pixel.
• Array-based time: Time with respect to a subarray. Also known as "first pixel to last pixel" time.

The distinctions between these 2 categories are best understood in the context of the times used by the calibration pipeline, APT, and ETC as described in the following sections. Readout pattern terms throughout this article are shown graphically in Figure 1.

For convenience, short descriptions of all quantities presented in the JWST ETC Reports pane, including the times described in this article, can be found in JWST ETC Reports.

Exposure time

The term exposure time is a generic phrase in astronomy but has a specific meaning in the context of the JWST instrument detectors. Exposure time is an array-based time, the time from the first reset of the first pixel to the final read of the last pixel in the array, and is defined as the time the detector is operating during a single expose command (i.e., set of integrations). This includes the time for reset frames and dropped frames at the beginning or end of an integration (Figure 2), but does not otherwise include any observatory overheads or instrument overheads. This time corresponds to the “Single Exposure Time” quantity in the ETC Reports and the TELAPSE keyword in FITS headers. APT does not report the individual exposure time, only the total exposure time (see below).

Two additional quantities are reported in the ETC: "Total Exposure Time" and "Total Time Required for Strategy." The "Total Exposure Time" is the "Single Exposure Time" multiplied by the Total Dithers quantity and corresponds to the "Total Exposure Time" in APT templates. The "Total Time Required for Strategy" is the "Total Exposure Time" multiplied by the number of "pointings." These 2 quantities are different only when using the NIRSpec IFU, MIRI MRS, or MIRI MRS time-series modes and the IFU On-Target and Off-Target Pointing strategy, in which case the "Total Time Required for Strategy" is double the "Total Exposure Time," representing the 2 different "pointings." The pipeline does not report either of these quantities.

Measurement time

The measurement time is a pixel-based time between the first and last reads of any individual pixel in an integration, multiplied by the number of integrations per exposure (Figure 3). This time does not account for resets between integrations or groups that are rejected at the beginning or end of an integration. This corresponds to the "Time Between First and Last Measurement, per exposure" in the ETC and the TMEASURE keyword in FITS headers. This quantity is not reported by APT.

The ETC uses the measurement time to compute the signal-to-noise ratio (SNR) for a given calculation and the pipeline implicitly uses this time for slope calculations.

Saturation time

The saturation time is the time between the reset of a pixel and the final read of that pixel in an integration (Figure 4), which is relevant for determining how close a given pixel may get to saturation. It is close to, but not quite, the full amount of time between resets when a pixel is collecting photons; the time after the final read of a pixel is not included because the photons incident on the pixel after that time are never read. This corresponds to the "Time Between First Reset and Last measurement, per integration" in the ETC and the EFFINTTM keyword in FITS headers. This time is not explicitly reported by APT but can be calculated by dividing the "PhotonCollect" quantity in the "times" file by "Integrations/Exp."

The ETC uses the saturation time, along with target brightness information and detector full well depth, to determine whether or not saturation has occurred.

Other

The ETC also reports the "Fraction of Time Spent Collecting Flux," which is the fractional time spent collecting photons during an exposure. It is defined as the "Time Between First and Last Measurement, per exposure" divided by the "Single Exposure Time." This quantity is a measure of the efficiency of an observation.

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Time-related header keywords

The data products created by the JWST science calibration pipeline include a variety of time-related FITS header keyword, each of which may be useful for different purposes.  A summary of these keywords is given below, see the JWST Keyword Dictionary for the detailed equations.

• MJD-BEG, MJD-MID, and MJD-END
  • FITS-standard keywords, located in the "SCI" FITS extension header. These give the start, midpoint, and ending time for a given exposure in Modified Julian Days (MJD)
  • Identical to the EXPSTART, EXPMID, and EXPEND keywords in the primary FITS header (non-standard keywords included for backwards compatibility)
    
    
• TDB-BEG, TDB-MID, and TDB-END
  • Located in the "SCI" FITS extension header. These give the start, midpoint, and ending Barycentric Dynamical Time for a given exposure.
    
    
•  TELAPSE
  • FITS-standard keyword, located in the "SCI" FITS extension header. This gives the total exposure time as defined above; the difference between MJD-BEG and MJD-END.
  • Identical to the DURATION keyword in the primary FITS header (non-standard keyword included for backwards compatibility)
  • APT equivalent: "Total Exposure Time" divided by "Total Dithers"
  • ETC equivalent: "Single Exposure Time"
    
    
• EFFINTTM
  • Located in the primary FITS header. This gives the saturation time as defined above.
  • APT equivalent: "Photon Collect Time" divided by "Integrations/Exp"
  • ETC equivalent: "Time Between First Reset and Last Measurement, per integration"
    
    
• TMEASURE
  • Located in the "SCI" FITS extension header. This gives the measurement time as defined above, which is typically most relevant for SNR purposes. Note that this is a new keyword in jwst pipeline version 1.14.0 (build 10.2) and later.
  • APT equivalent: None reported
  • ETC equivalent: "Time Between First and Last Measurement, per exposure"
    
    
• XPOSURE
  • FITS-standard keyword, located in the "SCI" FITS extension header. This is similar to TELAPSE as defined above, but does not include time for any resets.
  • Identical to the EFFEXPTM keyword in the primary FITS header (non-standard keyword included for backwards compatibility)

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References

Holler, B., et al. 2021, JWST-STScI-006013, SM-12, Rev. A (PDF)
Consistent Times and Repetition Factors in ETC and APT

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