Page tree

Versions Compared

Key

  • This line was added.
  • This line was removed.
  • Formatting was changed.

Content Block
overflowauto
meta-propertiestrue
nameSummary
id277319516
classpdf-full

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

Content Block
overflowauto
nameContent
id277805920
classpdf-full

Introduction

Parent page: NIRCam Instrumentation → NIRCam Detectors

NIRCam users may either observe the full field of view (for a given observing mode) or read outsmaller 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). 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. In some cases (e.g., FULL1 or SUB640 ), four short wavelength detectors combine to roughly cover the area observed by one long wavelength detector. In other cases (e.g., SUB400P ), the long wavelength subarray covers four times the area on the sky as the short wavelength subarray.

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 container
Figure title

Figure 1. NIRCam subarray locations (subject to change)

NIRCam subarray locations (subject to change)

Figure caption

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 coronography 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 (for faint sources), respectively.

Figure container
Figure title

Figure 2. Visualizations of imaging subarrays

Demonstrations of imaging subarrays

Figure caption

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 ortime-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.


Anchor
nircam_subarr_img
nircam_subarr_img
Imaging

All imaging subarrays are on module B.

Table 1. Imaging subarrays

Multiexcerpt
MultiExcerptNameNIRCam 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
HTML Wrap
margin10px 0 10px 0
idfootnote-font

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.

Warning

The overlapping area between the short wavelength SUB64P subarray and the long wavelength SUB64P subarray is smaller than JWST's 2-σ pointing accuracy. Use theSubarray Primary Dither Pattern 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
HTML Wrap
margin10px 0 10px 0
idfootnote-font

For use with target acquisition only.

Anchor
aptgui
aptgui
Multiexcerpt include
MultiExcerptNameaptgui
PageWithExcerptMR:APT GUI footnote

 

Restrict Content
groupsjdox-ins
usersboyer,dcoe
Panel
borderColorgreen
borderWidth2
borderStylesolid
Coronagraphy science exposure
subarray designation in SIAF1
APT Subarray
parameter value 2

APT Coronagraphic Mask
parameter value 2

Size in pixels
Nrows × Ncolumns
Short wavelength
FOV (each side) 
Long wavelength 
FOV (each side) 
Frame 
time (s)
Noutputs

SUB640A210R
SUB640ASWB

SUB640MASK210R
MASKSWB
640 × 64020.3"41.4"4.185841

SUB320A335R
SUB320A430R
SUB320ALWB 

SUB320MASK335R
MASK430R
MASKLWB 
320 × 32010.1"20.7"1.069041
Coronagraphy target acquisition
subarray designation in SIAF1
APT Acq Target Brightness 
parameter value 3
APT Coronagraphic Mask 
parameter value 2
Size in pixels
Nrows × Ncolumns 
Short wavelength
FOV (each side) 
Long wavelength 
FOV (each side) 
Frame 
time (s)
Noutputs

SUBNDA210R
SUBNDASWBL
SUBNDASWBS
SUBFSA210R
SUBFSASWB

BRIGHT
BRIGHT
BRIGHT
FAINT
FAINT 
MASK210R
MASKSWB
??
MASKSWB
??
MASK210R
MASKSWB
128 × 1284.1"8.3"0.182001
SUBNDA335R
SUBNDA430R
SUBNDALWBS
SUBNDALWBL
SUBFSALWB
SUBFSA335R
SUBFSA430R
BRIGHT
BRIGHT
BRIGHT
BRIGHT
FAINT
FAINT
FAINT
MASK335R
MASK430R
MASKLWB
??
MASKLWB
??
MASKLWB
MASK335R
MASK430R

64 × 642.0"4.1"0.050161

 

1 define SIAF here

2 APT Subarray and Coronagraphic Mask parameter values that correspond to the subarray's SIAF designation

3 When BRIGHT is chosen, acquisition target will be imaged through a neutral density (ND) square. (I'm not sure what FS means)

 

 


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
HTML Wrap
margin10px 0 10px 0
idfootnote-font

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
HTML Wrap
margin10px 0 10px 0
idfootnote-font

For use with target acquisition only.

 


Wide field slitless spectroscopy

Subarrays are not offered for grism wide field slitless spectroscopy.

 


 

Content Block
overflowauto
nameRelated links
id280628021
classpdf-hide

Related links

NIRCam Detector Subarrays
NIRCam Detector Readout Patterns
NIRCam Detector Readout
NIRCam Detector Performance
NIRCam Imaging
NIRCam Field of View
NIRCam Module
NIRCam Observing Modes
NIRCam Overview
NIRCam Time Series Imaging
NIRCam Wide Field Slitless Spectroscopy

Content Block
overflowauto
nameReferences
id281944791
classpdf-full

Anchor
ref
ref
References

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

Content Block
overflowauto
nameInstrument icon
id278719485
classpdf-hide

Multiexcerpt include
MultiExcerptNameNIRCam icon and name
PageWithExcerptMR:NIRCam icon and name

Content Block
nameUpdates
id1074221578
classpdf-hide
HTML Wrap
padding0
margin0
idupdatesbox
HTML Wrap
tagspan
classupdatesbox-heading

Last updated

Updated March 13, 2018

  • Corrected, updated, and clarified subarray sizes.


Updated November 18, 2017

  • Frame times updated for APT 25.4


Published December 28, 2016


Comment

The format for 'Last updated' is shown below. Enter updates above this internal"Comment" box. Only enter major updates (not typos or formatting changes).


Updated April 05, 2017

  • Lorem ipsum dolor sit amet, consectetur adipiscing elit. Aliquam fermentum vestibulum est. Cras rhoncus. 
  • Pellentesque habitant morbi tristique senectus et netus et malesuada fames ac turpis egestas. Sed quis tortor. 

Published March 02, 2017