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), 4 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 4 output channels in 10.737 s.

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

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., FULL or SUB640 ), 4 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, the subarray pixels are reset pixel-by-pixel, then the remaining detector 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. When using the FULL and FULLP subarrays, all detector pixels are reset individually, thus no additional row resets are required.

The exposure times reported by APT and ETC include time for resets. Subarrays require both pre-exposure pixel resets (for all pixels in the subarray) and post-integration row resets (for additional numbers of rows specified below). The full detector requires neither reset for the first integration of an exposure; the pixels are reset between integrations.

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). Note that starting in Cycle 2, NIRCam coronagraphy is offered for both the SW and LW channel simultaneously. The coronagraphic subarrays shown in this figure refer to the channel that is naturally matched with each mask in use (e.g. the 210R mask is naturally matched to the SW channel and the 210R subarray is shown in blue). The second channel subarray is not shown for clarity, but the user should be reminded that a subarray of same pixel size will be used on the second channel. Depending on whether SW or LW is the natural channel for the coronagraphic mask in use (shown in blue/red, respectively)  the second subarray would respectively be twice/half the angular size, given the factor of 2 difference in pixel scale between SW and LW channels (see also Table 2 below).

Note that the short and long wavelength SUB64P subarrays and the short wavelength SUB160P subarrays have been moved slightly downwards on the detector. See NIRCam Subarray Primary Dithers.

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 5 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

Long wavelength FOV

Frame time

(s)

Noutputs

Reset rows

FULL

4 × 2048 × 2048 (SW)
 2048 × 2048 (LW)

4 × 64" × 64" + 4" to 5" gap128" × 128"10.736774
SUB640

4 ×  640 × 640 (SW)
640 × 640 (LW)

4 × 20" × 20" + 4" to 5" gap40" × 40"4.1858412048
SUB320

4 ×  320 × 320 (SW)
320 × 320 (LW)

4 × 10" ×  10" + 4" to 5" gap20" × 20"1.0690412048
SUB160

4 ×  160 × 160 (SW)
160 × 160 (LW)

4 × 5" × 5" + 4" to 5" gap10" × 10"0.278641512
FULLP2048 × 204864" × 64"128" × 128"10.736774
SUB400P400 × 40012.5" × 12.5"25" × 25"1.6562412048
SUB160P160 × 1605" × 5"10" × 10"0.278641512
SUB64P64 × 642" × 2"4" × 4"0.050161256

Subarrays ending in "P" are intended for point source imaging. They use only a single detector B1 in the short wavelength channel, in addition to the long wavelength detector B5. FULLP, added in APT 2021.2, exploits the full area of a single SW detector (B1) and the whole LW detector (B5), with the target placed in the upper right region where image quality and detector response are expected to be optimal.

The other non-"P" subarrays are intended for extended sources and use all 4 short wavelength detectors; the resulting images include 4"–5" gaps along the center of both axes.



Coronagraphic imaging

All coronagraphic imaging subarrays are on module A. The first 2 rows are for target acquisition, rows 3 and 4 are for target acquisition astrometric confirmation images, and the last 3 rows in the table are for occulted images at short and long wavelengths.

When observing with the FULL subarray, short wavelength coronagraphy returns data from all 4 detectors. Smaller subarrays return data in just 1 detector per channel.


Table 2. Subarrays for coronagraphy and target acquisition

Activity

MASK

Coronagraphy
subarray

Size in pixels
Nrows × Ncolumns

Short wavelength FOV

Long wavelength FOV

Frame time
(s)

Noutputs

TA SW

MASK210R, MASKSWB

SUB128

128 × 1284" × 4"-0.182001
TA LWMASK335R, MASK430R, MASKLWBSUB6464 × 64-4" × 4"0.050161

TA SW*

Astrom

MASK210R, MASKSWBFULL 4 × 2048 × 2048 (SW)

4 × 64" × 64" + 4" to 5" gap

-10.736774

TA LW*

Astrom

MASK335R, MASK430R, MASKLWBFULL2048 × 2048 (LW)

-

128" × 128"10.736774

Science

MASK210R, MASKSWB

 

SUB640

640 × 64020" × 20"40" × 40"4.185841

FULL

 1 × 2048 × 2048 (SW)
2048 × 2048 (LW)

1 × 64" × 64" + 4" to 5" gap

128" × 128"10.73677

4

4

Science

MASK335R, MASK430R

MASKLWB**

 

SUB320**


SUB400X256

320 × 320**

256 x 400

10" × 10"

7" × 12"

20" × 20"

24" × 15"

1.06904

1.06296

1

1

FULL

 1 × 2048 × 2048 (SW)
2048 × 2048 (LW)


1 × 64" × 64" + 4" to 5" gap

128" × 128"10.736774

For use with target acquisition only

* The astrometric confirmation images are taken only in the channel used for target acquisition

** Observations in subarray mode with the MASKLWB  taken before June 2023 used the SUB320 subarray. With the deployment of the simultaneous SW+LW coronagraphic imaging mode, this subarray was retired in favor of the SUB400X256 one, which allows covering the LW Bar occulter in the SW channel. The SUB400x256 subarray has been available in APT since version 2022.7 and above (Cycle 2 Call for Proposal and afterwards)



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

Long wavelength FOV

Frame
time (s)

Noutputs

Reset
rows
FULLP2048 × 204864"× 64"128" × 128"10.736774
SUB400P400 × 40012.5" × 12.5"25" × 25"1.6562412048
SUB160P160 × 1605" × 5"10" × 10"0.278641512
SUB64P64 × 642" × 2"4" × 4"0.050161256
SUB32TATS 32 × 322" × 2"0.014961256

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 4 detector outputs. Each observation obtains data on one long wavelength detector and 2 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

Reset
rows

FULL

2 × 2048 × 2048 (SW)
2048 × 2048  (LW)

2 × 64" × 64"  + 4" to 5" gap

128" × 128"

42.23000
10.73677

1
SUBGRISM256

2 × 256 × 2048 (SW)
256 × 2048  (LW)

2 × 8" × 64" + 4" to 5" gap16" × 128"

5.31480
1.34669

1
2048
SUBGRISM128

2 × 128 × 2048 (SW)
128 × 2048  (LW)

2 × 4" × 64" + 4" to 5" gap8" × 28"

2.67800
0.67597

1
512
SUBGRISM64

2 × 64 × 2048 (SW)
64 × 2048  (LW)

2 × 2" × 64" + 4" to 5" gap4" × 128"

1.35960
0.34061

1
512
SUB32TATSGRISM 32 × 322" × 2"0.014961256

For use with target acquisition only



Wide field slitless spectroscopy

Subarrays are not offered for grism wide field slitless spectroscopy (WFSS). Note grism spectroscopy is obtained in the long wavelength channel only, while imaging is obtained in the short wavelength channel. Additional supporting images are also obtained at long wavelengths.


Table 5. WFSS subarrays

WFSS
subarray

Size in pixels
Nrows × Ncolumns

Short wavelength FOV

Long wavelength FOV

Frame
time (s)

Noutputs

Reset
rows

FULL

4 × 2048 × 2048 (SW) 
2048 × 2048 (LW)

4 × 64" × 64" + 4" to 5" gap128" × 128"10.736774



References

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




Latest updates
  •  
    Added subarrays for simultaneous SW+LW coronagraphy, available starting in Cycle 2

  •   
    Added FULLP subarray and reset rows


  • 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
Originally published