NIRCam Grism Time-Series APT Template

Instructions for designing JWST NIRCam grism time-series observations using APT, the Astronomer's Proposal Tool.

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See also: NIRCam Grism Time SeriesJWST Time-Series Observations RoadmapNIRCam Time-Series Observation Recommended StrategiesNIRCam Grism Time-Series Observations of GJ 436b

Grism time series is one of the 5 NIRCam observing modes and one of 2 NIRCam time-series observing modes. Each mode has a corresponding template in APT for users to design their observing programs.

This mode uses the NIRCam grisms for long wavelength observations (2.4–5.0 µm) and weak lenses for short wavelength observations (0.6–2.3 µm) obtained simultaneously.

No telescope moves (dithering nor mosaics) are allowed during NIRCam time series observations.

Lists of allowed values for each input parameter are documented and maintained in the NIRCam Grism Time-Series Imaging Template Parameters article.

Step-by-step APT instructions


Words in bold italics are buttons 
or parameters in GUI tools. Bold 
style represents GUI menus/
panels & data software packages.

The following parameters are generic to all templates, and are not discussed in this article: observation Number, observation Labelobservations CommentsTarget name, ETC Wkbk. Calc ID (in the Filters dialog box), Mosaic Properties, and Special Requirements.

Note: mosaics are not available for NIRCam grism time-series.

Target acquisition parameters

Target ACQ

See also: NIRCam Grism Time-Series Target Acquisition, JWST ETC NIRCam Target Acquisition

Users can opt to do target acquisition on any nearby object, including the science target. Target acquisition occurs with a 32 × 32 pixel subarray on the long wavelength channel of Module A located near the grism field positions. Target acquisition objects other than the science target can be defined in Targets. Choose SAME TARGET AS OBSERVATION to use the science target. The available filters for target acquisition are F335M and F405N. The F405N filter allows to perform target acquisition on bright science targets that would saturate if observed with the F335M filter. 

Figure 1. Target acquisition location

Target acquisition is performed with a 32 × 32 pixel subarray (yellow square) near the bottom of the long-wavelength detector A5. The grism subarrays are shown in black for long-wavelength channel. The corresponding short-wavelength subarrays are not shown; they span the short-wavelength detectors horizontally and are centered vertically within the long-wavelength subarrays. The target acquisition pointing is centered on the TA subarray. If any of the three grism subarrays is chosen, a slew places the target on one of the lower two yellow stars, depending on the science filter. If FULL array exposures are selected, the target is positioned at one of the upper two yellow stars, depending on the selected filter.

Acq exposure time

Please consult the Exposure Time Calculator (ETC). We recommend a signal-to-noise ratio of 30 or higher to obtain a centroid accuracy of 0.1 pixel for the TA source. We also recommend not saturating any pixels.

ACQ READOUT PATTERN: The NIRCam detectors are read out continuously using readout patterns.  Patterns with longer exposure times typically average more frames to reduce data volume (less of a concern for subarrays).

ACQ GROUPS/INT: The number of groups to include during an integration. Each group results in a saved image, which may be averaged from multiple frames (reads), depending on the readout pattern. 

Only one integration per exposure is permitted.

Thus the exposure time and integration time are equal.

Time-series parameters

See also: NIRCam Modules

Note: Module A is used for this observing mode.


See also: NIRCam Detector Subarrays 

Users may opt to either read out 3 detectors completely or more quickly read out 3 smaller detector subarrays. Smaller subarrays allow for brighter saturation limits in each integration.

The grism time-series subarrays span all detector columns and either some or all rows. The same numbers of pixels are read out from all subarrays, so each short wavelength subarray covers a quarter the area on the sky as the long wavelength subarray. When a subset of rows are selected, subarrays are defined in 2 short wavelength detectors to overlap and be centered vertically within the long wavelength subarray's footprint on the sky.

Note the weak lenses WLP4 and WLP8 produce defocused images roughly 66 and 132 pixels across, respectively. Therefore, WLP8 images would be significantly truncated by the smallest subarray SUBGRISM64.

Size in pixels
Nrows × Ncolumns 
time (s)
No. of output channels
FULL2048 × 2048

64" × (2 x 64" with 4"–5" gaps)

129" × 129"42.23000
SUBGRISM256256 × 20488.1" × (2 x 64" with 4"–5" gaps)16.6" × 129"


SUBGRISM128128 × 20484.1" × (2 x 64" with 4"–5" gaps)8.1" × 129"2.67800
SUBGRISM6464 × 20482.0" × (2 x 64" with 4"–5" gaps)4.0" × 129"1.35960

Number of output channels

See also: NIRCam Detector ReadoutJWST Data Volume and Data Excess

The detectors may be read out through a single output channel or more quickly through 4 output channels simultaneously. The latter produces roughly 4 times as much data for a given exposure time. (Put another way, for a given set of exposure parameters which produces a given data volume, the exposure time is roughly 4 times higher when using a single output channel instead of four.) Data rates and data volumes are limited somewhat by APT; tighter limitations may be required to facilitate scheduling (see Data Volume Limitations).

Number of exposures

Multiple exposures may be performed in sequence to increase the total exposure time. Each exposure is executed as defined in the remaining sections below.

In APT, this section is named "Exposures/Dith" (Exposures per Dither) for consistency with other observing modes, even though no dithering is allowed in this mode.

Short pupil + filter

Two weak lenses are available in this mode for short wavelength observations, and one or the other must be used:

  • WLP8 (8 waves of defocus at 2.12 µm)
  • WLP4 + F212N2 (4 waves of defocus at 2.12 µm; coupled to a 2.12 µm narrowband filter with a 2.3% bandpass)

The weak lenses defocus incoming light, mitigating uncertainties (jitter and flat fields) and allowing for observations of brighter objects before saturation in a given integration time.

WLP8 is located in the pupil wheel and must be used in combination with a medium or narrowband filter. WLP4 + F212N2 is located in the filter wheel and is used in combination with the CLEAR filter.

Short wavelength pupil wheelShort wavelength filter wheel
WLP8F070W, F0140, F182M, F187N, F210M, or F212N

WLP4 + F212N2

Long pupil + filter

The row dispersion grism GRISMR (in the pupil wheel) is used in combination with a wide long wavelength filter (in the filter wheel): F277W, F322W2, F356W, or F444W. See NIRCam Grism Time Series for more details.

Exposure time

See also: Understanding JWST Exposure Times

Each exposure is defined as a readout pattern, number of groups, and number of integrations. The resulting Total Exposure Time is reported. This readout configuration applies to both wavelength channels (short and long); the observations are obtained simultaneously using a dichroic.

Users should consult the Exposure Time Calculator (ETC) to achieve sufficient signal-to-noise for their science without saturating during each integration. Approximate saturation limits may be found at NIRCam Grism Time Series.

Each group yields saved data. Each integration accumulates charge for its duration, preceded and followed by detector resets. Shorter  integrations may prevent saturation. Saturated sources may be recovered (unsaturated) in earlier groups during the integration.

Each exposure is performed without moving the telescope nor any mechanisms, with one exception. Exposures of more than 10,000 s are permitted in this observing mode, but users are warned that the High Gain Antenna may need to move during a longer exposure. That movement may introduce jitter and affect data quality.


Latest updates
    Corrected frame times for Grism time series Noutputs = 1

  • Updated the TA figure to include the FULL array field points.
Originally published