JWST ETC Time Definitions
The JWST Exposure Time Calculator uses 3 different definitions of time to compute the full range of quantities presented in the Reports pane: 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 ETC. For consistency, the equations used by the ETC are identical to those used by the JWST Astronomers Proposal Tool, but APT does not make use all the times described here. Users interested in the full equations should read the JWST technical document by Holler et al. (2021).
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The different times can be divided into 2 different 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 ETC, described in the following sections. Readout pattern terms throughout this article are shown graphically in Figure 1.
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 read 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 rejected groups at the beginning or end of an integration (Figure 2), but does not include any observatory overheads or instrument overheads. The ETC presents this time in the Reports pane as the "Single Exposure Time." In APT, this is referred to as the "Exposing Duration."
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. 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 ETC also reports the "Time Between First and Last Measurement, per exposure," which is a pixel-based time between the first and last reads of any individual pixel in an integration, multiplied by 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. The "Time Between First and Last Measurement, per exposure" is directly proportional to the the number of incident photons and the ETC uses "Time Between First and Last Measurement, per exposure" to calculate the signal and the photon noise in the determination of the SNR of an observation. In APT, this is referred to as the "Photon Collect Time."
The "Time Between First Reset and Last measurement, per integration" is another pixel-based time that is similar to the "Time Between First and Last Measurement, per exposure," except that it includes the time for reset frames. Specifically, it is the time between the reset of a pixel and the final read of that pixel in an integration (Figure 4). 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. The ETC uses the "Time Between First Reset and Last measurement, per integration," along with target brightness information and detector full well depth, to determine whether or not saturation has occurred.
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.
The "Fraction of Time Spent Collection Flux" (commonly referred to as the "duty cycle") is currently improperly calculated. To match the duty cycle calculated from FITS header quantities produced by the JWST calibration pipeline, the calculation should be the "Time Between First Reset and Last measurement, per integration" multiplied by the number of integrations in an exposure, divided by the the "Single Exposure Time."
Holler, B., et al. 2021, JWST-STScI-006013, SM-12, Rev. A (PDF)
Consistent Times and Repetition Factors in ETC and APT