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

Changes to the technical details and other specifications presented here are expected as a part of pre-launch development. 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 (called groups in JWST) using an unweighted arithmetic mean. The resultants are down-linked to the ground system as science data. Note that the WFI supports unevenly-spaced resultants, characterized by a variable number of reads within a resultant and/or a variable number of skipped reads between resultants 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 at any arbitrary resultant before reaching the end of the table. For example, a user might select the High Latitude Wide Area Survey Imaging MA table and choose to end the exposure after resultant 6 rather than executing all 8 resultants in the table.

MA table truncation 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 is referred to as frame zero in JWST) 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 down-linking 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 selected MA tables during 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 down-link of science data from the spacecraft to the ground system; therefore, users will not have access to these reads.

This figure shows MA Table 4. The dashed vertical lines represent the midpoint time of each resultant, though they are vertically offset to aid in visibility, while the colored ranges around each dashed vertical line show the resultant duration. Note that resultants three and four (R3, R4, and R9) contains fewer reads than all other resultants, which demonstrates the uneven sampling capability of the WFI. The integration 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 integration duration and effective exposure time shown correspond to the execution of the full MA table.

MA Table Specifications

Definition of Terms

The following tables provide general information about each MA table, as well as details for each resultant within the MA table. The definitions used are as follows:

Number of skips: The number of skipped reads occurring before the resultant. For example, if the first resultant had two skips, this would mean that two reads were skipped between the reference read and the first read of the first resultant. Similarly, if the second read contained two skips, it would mean that two reads were skipped 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: The number of science reads that are averaged and down-linked as a resultant.

Resultant duration: The time span 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.

Integration duration: The time span 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.

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.

The MA tables described below are documented in RST-WFI-DESC-0110 Revision F, which is available on TDMS for authorized users.

In the following resultant tables, rows highlighted in orange indicate resultants that are below the recommended minimum number of resultants. Note that the MA Table ID is the stringified MA Table Number, padded to four digits.

1: DEFOCUS_MOD Restricted Mode

This mode is intended to be used for engineering purposes and is not anticipated to be used for science observations.