NIRCam Detector Subarrays

The JWST NIRCam detector subarrays reduce data volumes and readout times, enabling rapid observations of bright objects without saturation.

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NIRCam users may either observe the full field of view (for a given observing mode) or read out smaller portions of the detectors, called subarrays.

Subarrays are read out more quickly than the full detector, allowing for shorter integration times. Shorter integration times can allow brighter objects to be observed without saturating the detector. See the bright source limits for more information.

Each pixel is read out in 10.00 µsec. For most subarrays, pixels are read out one at a time (Noutputs = 1). For subarrays spanning the full width of the detector (2,048 pixels), four parallel output channels can be utilized for faster read out times (Noutputs = 4).

Thus, the total time to read out each frame of Nrows × Ncolumns pixels through Noutputs output channels (including small overhead delays due to telemetry) is:

for Noutputs = 4, 

tframe = ((Ncolumns / Noutputs + 12) × (Nrows + 1) + 1) × 10.00 µsec


for Noutputs = 1,

tframe = (Ncolumns / Noutputs + 12) × (Nrows + 2) × 10.00 µsec 


The smallest subarray available for science (64 × 64 pixels) can be read out in tframe = 49.4 ms (the shortest integration time; though multiple groups of readouts are recommended: Ngroups > 1). The full 2048 × 2048 array is read out through four output channels in 10.737 s.

When observations are obtained simultaneously at short and long wavelengths, subarrays with identical numbers of pixels are used in both wavelength channels. Each long wavelength pixel covers 4× more area on the sky than each short wavelength pixel. So in some cases (e.g., SUB400P ), the long wavelength subarray covers 4× more area on the sky than the short wavelength subarray. In other cases (e.g., FULL1 or SUB640 ), four short wavelength detectors combine to roughly cover the same area observed by one long wavelength detector. Subarrays ending with a "P" are designed for point-source imaging (or for compact object imaging), and therefore have uneven coverage between the short wavelength and the long wavelength channel. 

Each detector has 2048 × 2048 pixels consisting of a central block of 2040 × 2040 pixels sensitive to light and a 4-pixel wide border of reference pixels along all edges used for calibration. All subarrays positioned along edges include reference pixels, subtracting slightly from their total area available for science.

After each subarray integration, additional rows outside the subarray region are quickly reset, one row at a time. While less effective at clearing out charge than standard individual pixel resets, this scheme should effectively mitigate latent images that might otherwise build up and leave persistence on portions of the detector outside the subarray region. The row resets contribute slightly to the overheads, generally less than a percent of the total integration time.

Figure 1 and the tables below summarize the supported subarray sizes, frame times, and fields of view for the NIRCam observing modes.  Subarrays are defined on one NIRCam module or the other (A or B), chosen for optimal performance for each observing mode.

Figure 1. NIRCam subarray locations (subject to change)

NIRCam subarray locations (subject to change)

Subarrays currently defined in the NIRCam field of view. Subarrays for wide field slitless spectroscopy are still under development. Blue and red correspond to the short and long wavelength channels, respectively. The 10 NIRCam detectors (A1–5 and B1–5) are labeled within each module. The coronagraphy field of view, located above the detectors on this plot, is projected onto the detectors when in use. ND and FS refer to target acquisition with attenuation by the neutral density squares and without, respectively (the latter used for faint sources that do not require attenuation).
Figure 2. Visualizations of imaging subarrays

Demonstrations of imaging subarrays

Demonstrations of the SUB640 and SUB400P subarrays for imaging extended sources and point sources, respectively. When using SUB640 in imaging mode, subarrays are read out from all five module B detectors. When using SUB400P in either imaging or time-series imaging mode, a subarray is read out from one detector in each wavelength channel. Center: Jupiter 5µm image obtained by VLT/VISIR (Credit: ESO/L. Fletcher) shown to scale with an angular diameter of 39". This diameter assumes (as in Norwood et al. 2016) that Jupiter is at a solar elongation of 90° and therefore in JWST's field of regard for observability. In the top right, the NIRCam point spread function in F200W, simulated by WebbPSF, is also shown to scale. Note the sizes in this figure may be outdated; refer to Table 1 for the most recent values.


Imaging

All imaging subarrays are on module B.

Table 1. Imaging subarrays

Imaging
subarray
Size in pixels
Nrows × Ncolumns
Short wavelength
FOV (each side)
Long wavelength
FOV (each side) 
Frame
time (s)
Noutputs
FULL2048 × 20482 × 64" + 4–5" gap129"10.736774
SUB640
640 × 6402 × 19.9" + 4–5" gap40.4"4.185841
SUB320320 × 3202 × 9.9" + 4–5" gap20.2"1.069041
SUB160160 × 1602 × 5.0" + 4–5" gap10.1"0.278641
SUB400P400 × 40012.4"25.0"1.656241
SUB160P160 × 1605.0"10.0"0.278641
SUB64P64 × 642.0"4.0"0.050161

Subarrays ending in "P" only use a single detector in the short wavelength channel. The other subarrays use all four short wavelength detectors; the resulting images include 4–5" gaps along the center of both axes.

The overlapping area between the short wavelength SUB64P subarray and the long wavelength SUB64P subarray is smaller than JWST's 2-σ pointing accuracy. Set the Primary Dither Type parameter to SUBARRAY DITHER to increase the spatial coverage and ensure the target is observed in both channels.



Coronagraphic imaging

All coronagraphic imaging subarrays are on module A. The first three rows in the table are for occulted images at short and long wavelengths. The last two rows are for target acquisition. Each observation obtains data on a single detector in a single wavelength channel (short or long).


Table 2. Subarrays for coronagraphy and target acquisition

Coronagraphy
subarray

Size in pixels Nrows × Ncolumns

Short wavelength
FOV (each side) 

Long wavelength 
FOV (each side) 

Frame 
time (s)

Noutputs

FULL2048 × 204863"129"10.736774

SUB640

640 × 64020"
4.185841

SUB320

320 × 320
20"1.069041

SUB128

128 × 1284.0"
0.182001
SUB64 64 × 64
4.0"0.050161

For use with target acquisition only

Bold italics style indicates words that are also parameters or buttons in software tools like the APT and ETC. Similarly, a bold style represents menu items and panels.



Time-series imaging

Time-series imaging subarrays are on module B. Each observation obtains data on one short wavelength detector and one long wavelength detector.


Table 3. Time-series imaging subarrays

Time-series
subarray

Size in pixels
Nrows × Ncolumns

Short wavelength
FOV (each side) 

Long wavelength
FOV (each side) 

Frame
time (s)

Noutputs

FULL2048 × 20482 × 64" + 4–5" gap129"10.736774
SUB400P400 × 40012.4"25.0"1.656241
SUB160P160 × 1605.0"10.0"0.278641
SUB64P64 × 642.0"4.0"0.050161
SUB32TATS 32 × 322.0"0.014961

For use with target acquisition only




Grism time series 

Grism time-series subarrays are on module A. In this mode, the user is offered a choice between one and four detector outputs. Each observation obtains data on one long wavelength detector and two short wavelength detectors.


Table 4. Subarrays for grism time series

Grism
subarray

Size in pixels
Nrows × Ncolumns

Short wavelength
FOV

Long wavelength
FOV

Frame
time (s)

Noutputs

FULL2048 × 2048

64" × (2 × 64" + 4"–5" gaps)

129" × 129"

42.23000
10.73677

1
SUBGRISM256256 × 20488.1" × (2 × 64" + 4"–5" gaps)16.6" × 129"

5.31480
1.34669

1
SUBGRISM128128 × 20484.1" × (2 × 64" + 4"–5" gaps)8.1" × 129"

2.67800
0.67597

1
SUBGRISM6464 × 20482.0" × (2 × 64" + 4"–5" gaps)4.1" × 129"

1.35960
0.34061

1
SUB32TATSGRISM 32 × 322.0" × 2.0"0.014961

For use with target acquisition only



Wide field slitless spectroscopy

Subarrays are not offered for grism wide field slitless spectroscopy.



References

Norwood, J., Hammel, H., Milam, S. et al. 2016, PASP, 128, 025004
Solar System Observations with the James Webb Space Telescope




Published

 

Latest updates

  • Added some clarification text to the subarray description
  •  
    Corrected frame times for Grism time series Noutputs = 1

  • Updated imaging section to match current APT terminology

  •  
    Missing spaces before hyperlinks added


  • Corrected, updated, and clarified subarray sizes

  •  
    Frame times updated for APT 25