The WFI uses a set of pre-defined multi-accumulation tables to sample the detector within an Exposure. These tables are described below.

Changes to the technical details and other specifications presented here are expected as a part of the process of defining the Core Community Surveys. This includes modifications to the Multi Accumulation tables.

Multi-Accumulation Tables

The multi-accumulation (sometimes abbreviated multi-accum or MA) tables describe the sequence of individual reads (or samples) that comprise the up-the-ramp sampling during a single exposure of the Wide Field Instrument (WFI) detectors (see Pass Plans page to learn how to specify the MA Tables in APT). Onboard the spacecraft, reads are averaged into resultants (what JWST calls groups) using an unweighted arithmetic mean, and it is the resultants that are downlinked to the ground system as science data for calibration and archiving. Note that unlike other instruments that users may be familiar with, the WFI is capable of creating unevenly-spaced resultants, which are resultants in which the number of reads within a resultant and/or the number of skipped reads between resultants is not constant over the Exposure (see the Figure of Reads and Resultants over Time). Furthermore, each science MA table is executed from the beginning of the sequence, but may be truncated to any arbitrary resultant before the end of the table. For example, a user may select the High Latitude Wide Area Survey Imaging MA table and choose to end the exposure after resultant 6, rather than all 8 resultants in the full table.

Some MA table truncation points may result in undesirable effects in the data. The recommended minimum number of resultants for each MA table is indicated below. Users are also advised to choose a final resultant that contains a single read for optimal cosmic-ray rejection during data processing.

In addition to the science reads, the MA tables may include a reference read (what JWST calls a frame zero) and a number of reset reads. The reference read is an additional science read that is subtracted from all science reads before they are added to the accumulator and averaged into resultants. The purpose of the reference read is to improve the efficiency of the compression of science data before downlinking to the ground system. The reset read(s) flush the signal in the pixels before the integration begins. Typically, one reset read is sufficient, but additional reset reads could be included at the beginning of select MA tables in operations.

The presence of a reference read and the number of reset reads are not user configurable parameters, but rather are part of the MA table specification itself.

At present, the reference read and reset read(s) are not included in the downlink of science data from the spacecraft to the ground system; therefore, users will not have access to these reads.

This figure shows the MA table "High Latitude Wide Area Survey - Imaging." Star symbols represent the midpoint time of each resultant, though they are vertically offset to aid in visibility, while the ranges around each star symbol show the resultant duration. Note that resultant 8 (R8) contains fewer reads than all other resultants, which demonstrates the uneven sampling capability of the WFI. The y-axis has been hidden for this example, though it depicts the signal level observed for a given count rate. The total duration is the time between the start of the first reset read and the end of the final read in the MA table. The effective exposure time is the difference between the midpoint time of the first and last resultants. The total duration and effective exposure time shown correspond to an execution of the full MA table; however, these values would be different if the table had been truncated.
Changes to the technical details and other specifications presented here are expected as a part of the process of defining the Core Community Surveys.

MA Table Specifications

Definition of Terms

In the following tables, we include general information for each MA table as well as information for each resultant in the MA table. We use the following definitions:

Number of skips: These are the number of skipped reads that occurred before the resultant. For example, if the first resultant had two skips, that would mean two reads were skipped between the reference read and the first read of the first resultant. Alternatively, if the second read contained two skips, that would mean there were two skipped reads between the last read of the first resultant and the first read of the second resultant. At present, skips do not occur within a resultant.

Number of science reads: This represents the number of science reads that will be averaged and downlinked as a resultant.

Resultant duration: The duration between the start time of the first read in a resultant and the end time of the final read of the same resultant. See the Figure of Reads and Resultants over Time for an example.

Total integration duration: The time between the start of the integration (including any reset reads and the reference read) and the end of the final science read. See the Figure of Reads and Resultants over Time for an example.

Total accumulated exposure time: The time between the start of the first science read or skip in the MA table and the last science read in the final resultant.

Effective exposure time: The difference in the midpoint time of the first resultant and the midpoint time of the last resultant. See the Figure of Reads and Resultants over Time for an example.

In the following resultant tables, orange highlighted rows indicate resultants that are below the recommended minimum number of resultants.