Near-infrared non-destructive readout
The NIRISS detector is readout non-destructively ("sampling up the ramp") which provides lower average readout noise than if the detector were read out once at the end of an exposure. This mode also allows the identification and rejection of cosmic ray hits in the time domain. During an integration, the pixels are digitized at the sampling rate of 10.0 μsec/pixel. In the full array mode, the detector is read with 4 parallel outputs (one output for each 512 × 2048 pixels) resulting in a full frame readout time of 10.737 seconds. The NIRISS array uses pixel-by-pixel reset which also takes 10.737 seconds. This reset mode offers better latent-image mitigation than other reset modes and allows the total exposure time per pixel to be independent of the position on the detector.
The detector array is read using the MULTIACCUM readout mode, which is illustrated in Figure 1. An exposure can consist of multiple integrations (MULTIACCUM data ramps) . The detector is reset at the beginning of each integration, clearing the accumulated charge. Each integration consists of a number of groups (NGroups), which can consist of a number of read frames (NFrames). In general MULTIACCUM mode, some frames can be ignored in an integration (DropFrames), but for the NIRISS readout patterns, no frames are skipped.
Figure 1. Illustration of MULTIACCUM mode basics
Each pixel is first reset and readout non-destructively several times during an integration. Sampling occurs in groups within which some number of frames ("ReadFrames") may be averaged while some are ignored ("DropFrames"). The resets may be executed in idle mode or between consecutive integrations ("Resets1" and "Resets2") and can be individually configured. In the general MULTIACCUM mode, the number of frames to drop at the beginning ("DropFrames1"), within Groups ("DropFrames2"), and at the end of an integration ("DropFrames3") can all be tailored individually. For NIRISS there are no dropped frames and there is 1 reset at the beginning of each integration.
NIRISS detector readout patterns
NIRISS has two readout patterns, NISRAPID and NIS, summarized in Table 1, and illustrated in Figures 2 and 3, respectively.
In NISRAPID, all the frames are read out (NFrames = 1), and there is no averaging of frames. In full frame mode, four frames are averaged together (NFrames = 4). This mode should be used if the number of integrations per exposure is small and data volume is not an issue. When using a subarray for the AMI or SOSS observing modes, this readout pattern will be used for large numbers of integrations per exposure.
In the NIS mode, 4 frames are averaged together (NFrames = 4). This mode produces a lower data rate for longer exposures of faint targets.
The detector is reset at the beginning of each integration, so that the exposure time per integration is:
t = NGroups × NFrames x 10.737s + 10.737s
Table 1. Number of frames per group for each NIRISS readout pattern
|NIRISS Readout Pattern|
Number of Frames per group
Figure 2. Illustration of the NISRAPID readout patter
In the NISRAPID readout mode, each frame is saved. Here, NFrames = NGroups.
Figure 3. Illustration of NIS readout pattern
In the NIS readout mode, four frames are averaged per group.