As charge accumulates during a NIRCam integration, the detectorsare read out multiple times, non-destructively, sampling the data while conserving the charge in each pixel. This MULTIACCUM technique enables “up-the-ramp” fitting of count rates to multiple data samples obtained over time. Up-the-ramp fitting facilitates cosmic ray rejection, reduces the effective readout noise (approximately by the square root of the number of samples), and increases the dynamic range of the final image (sampling count rates of bright sources before they saturate).
Each 16-bit (2 byte) pixel is read out, in turn, in 10 µs. This data rate could reach 540 GB/day (accounting for overheads) when using all 10 detectors with four simultaneous outputs each. This would overfill the onboard solid state recorder, which can store about 57 GB of data for science, downloaded twice daily.
To reduce the data rates and enable longer exposures, readout patterns are defined (see Figure 1). Each readout pattern is a group of up to 20 detector reads (Nsamples). Each group yields a single saved image obtained by averaging as many as eight of the reads (Nframes). Any remaining reads are discarded (Nskip).
Choosing to average more frames yields a more precise average but also allows more time for a potential cosmic ray impact, in which case the entire group must be discarded from the integration ramp. The choice involves a tradeoff, but initial estimates suggest that averaging more frames generally yields slightly higher signal to noise for a given group length and integration time (Robberto 2009, 2010).
Multiple groups of non-destructive reads are generally taken consecutively, resulting in an integration ramp for each pixel. Integrations are terminated by a reset, which clears accumulated charge from the pixels. Multiple integrations can be executed without interruption in a single exposure, yielding a photon collection duration at each dither position.
NIRCam has nine available readout patterns (see Table 1 and Figure 2). Their names encode their group size (Nsamples) followed by the number of averaged samples (Nframes). The five available group sizes (Nsamples = 1, 2, 5, 10, and 20) are named according to their potential applications (RAPID, "BRIGHT," "SHALLOW," "MEDIUM," and "DEEP," respectively). So for example, a DEEP8 group contains 20 samples (Nsamples = 20), eight of which are averaged (Nframes = 8) and 12 of which are skipped (Nskip = 12).
Each integration may contain up to 10 groups (Ngroups = 10) or more for the DEEP2 and DEEP8 patterns in most observing modes. And in most cases, 10 integrations are allowed per exposure, so long as the total exposure time remains within allowed limits. Tighter restrictions are placed on the RAPID and BRIGHT2 patterns, which save all frames: when all 10 full detectors are being read, exposures are limited to a single integration of Ngroups = 4 or less.
Each detector readout takes 10.737 s for the full frame (2048 × 2048 pixels using four outputs) or as little as 49.4 ms for the smallest supported science subarray (64 × 64 pixels). Each pixel is read out in turn; therefore, the integration start time varies slightly from one pixel to the next. The total integration time is identical for all pixels.
The tables and diagrams below illustrate the nine readout patterns available for NIRCam observations. Tables 2 and 3 give total integration times achievable with multiple groups.
Table 1. Available NIRCam MULTIACCUM readout patterns
Each readout pattern name describes its potential application based on the number of samples (Nsamples). For example, a RAPID group consists of a single sample, and a "DEEP" group consists of 20 samples. Each name (except RAPID) ends in a number signifying the number of averaged frames in each group (Nframes). Any remaining frames are discarded (Nskip).
For all readout patterns that involve averaging frames into groups (i.e., all patterns except RAPID or BRIGHT1), the initial frame will always be saved and is termed “frame 0”. It is saved as a separate extension in the data file, and its purpose is to increase the dynamic range of the data. If the first averaged group of an integration is saturated, then the photon count rate cannot be determined; however, frame 0 may not have reached full well and could therefore be used to estimate the count rate (see Figure 3). Similarly, if an integration has been contaminated by a cosmic ray that hits within the first group (and after the first frame), frame 0 may still be trusted even though the group has to be discarded.
Because the count rate in such cases will be determined by calculating the slope using frame 0, it is important that the bias of the detectors is very well characterized. Any uncertainty on the signal level at the very beginning of an integration (the “reset” or “bias” level) due to, for example, global electronic offsets or pixel-dependent kTC (thermal) noise, will have a significant effect on slopes determined using only frame 0 (Rest 2018, in preparation). If the count rate measured in frame 0 is very high, the associated Poisson noise can dominate the uncertainty of the reset level, making the use of the single frame 0 data point entirely appropriate. To read more about the bias correction of the JWST science calibration pipeline, please refer to the first stage of calibrations: CALWEBB_DETECTOR1.
Tables 2 and 3 give integration times for groups of reads of the full frame detector (2048 × 2048 pixels) using four outputs.
Integration time = Tframe × (Ngroups × Nsamples – Nskip)
Nsamples = Nframes + Nskip
Tframe = 10.73677 s for the full detector that's read out through four outputs.
Note that skipped reads at the end of each integration are not executed.
For example, three groups of SHALLOW4 consist of two groups of five reads plus a final group of four reads. The 14 total reads of the full detector take 150.3 s. Each pixel collects photons for this amount of time. The times given below follow this definition. These are reported as Science time (at the Duration field) by APT.
However, the last pixel in each detector begins collecting photons 10.7 s after the first pixel. Thus the exposure and readout process takes 161 s to complete for the full detectors. This is the Exposure Time reported by the APT and ETC.
Table 2. Pixel integration times (s) for groups of short readout patterns for the full detector
The RAPID and BRIGHT2 patterns are limited to four groups per integration when reading out the full detectors in both modules. This limit increases to 10 groups (shown in gray) when using a single module (full detectors or subarrays). The limit is always 10 groups for BRIGHT1, SHALLOW2, SHALLOW4, MEDIUM2 and MEDIUM8 patterns listed in Table 3. Numbers of saved and skipped frames (Nframes and Nskip) are shown in green and orange, respectively, as in Figure 1.
Table 3. Pixel integration times (s) for groups of long readout patterns for the full detector
Robberto, M., 2009, JWST-STScI-001721, SM-12
Robberto, M., 2010, JWST-STScI-002100, SM-12