JWST ETC NIRCam Target Acquisition

The JWST Exposure Time Calculator (ETC) has a NIRCam target acquisition (TA) mode which allows the user to estimate the exposure time necessary to achieve the signal to noise for the required centroid accuracy.

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The JWST Near Infrared Camera (NIRCam) instrument uses target acquisition (TA) for its coronagraphy, time-series and grism time-series observing templates. The JWST ETC includes a Target Acquisition calculation mode for NIRCam. This mode can be used to calculate the signal-to-noise ratio (SNR) for user specified exposure parameters, and covers all three of the relevant templates. The minimum recommended integrated (within the extraction aperture) SNR is ≥ 30 for NIRCam TA which is required to obtain centroid accuracy better than 0.1 pixels.

Creating a TA calculation

The steps involved in creating a TA calculation are: (1) define the TA scene with a source having appropriate spectral type and magnitude on the Scenes and Sources page, and (2) Specify the instrument and detector setup on the Calculations page

Defining the TA Scene and Source

The scene definition and source definition for the TA calculations are done along the same lines as for the other observing modes.

Creating a calculation

Target Acquisition is one of the modes available for all JWST instruments within the ETCTo initialize a TA calculation select Target Acquisition from the NIRCam instrument drop-down menu. Once selected, the calculation will utilize a default scene and source. This can be changed at the users discretion by accessing the Configuration pane where you can select the Scene tab to change the pre-defined TA source. 

Figure 1. Creating a NIRCam Target Acquisition calculation

Creating a target acquisition calculation by selecting the Target Acquisition mode from the NIRCam instrument drop-down menu.

What's supported 

The ETC supports TA for the following NIRCam observing templates: coronagraphytime series and grism time series. For each mode, there are only a few choices users can make regarding the Instrument Setup. The Detector Setup is much less restricted, allowing use of all exposure patterns, but values of Groups are limited to a few values ranging from 3 to 65 (the maximum is determined by limitations in the onboard data acquisition system). 

Instrument setup 

Time series and grism time series target acquisition

Target acquisition for both time-series and grism time-series observations utilizes subarrays on the LW detectors that are 32 pixels (2.16") on a side. The filters supported supported are F335M and F405N. The Instrument Setup for these target acquisitions is therefore very simple, involving only a single choice in the Acq Mode menu.

Coronagraphic target acquisition

For the coronagraphic imaging template, the instrument configuration includes choosing the desired occulting mask from the list of 5 available masks:

  • For MASK210R and MASKSWB (in the SW channel), the TA subarray is 128 pixels (4.0") square.
  • For MASK335R, MASK430R and MASKLWB (in the LW channel), the TA subarray is 64 pixels (4.0") square.

The next choice is whether the target that will be used for TA is in the "bright" or "faint" regime. For bright targets, the source will be placed behind a neutral density square for the TA exposure; for faint targets the source will be imaged through a transparent region. The division between bright and faint targets depends on the target spectrum, and on whether the SW or LW channel will be used. Roughly speaking, the division occurs at KVega = 6.3 (SW) and KVega = 4.8 (LW) for early type main sequence stars (these cutoffs are not specific to any given filter).

  • For  MASK210R and MASKSWB the filter choices are F210M (faint) or F210M + ND square (bright).
  • For MASK335R, MASK430R and MASKLWB the filter choices are F335M (faint) or F335M + ND square (bright).

Figure 2. NIRCam TA supported modes


(Top) The 5 NIRCam TA modes include one that applies for time-series or grism time-series observations, and 5 for coronagraphic imaging. (Bottom) Filter selection for coronagraphic TA includes the choice between "bright" and "faint" modes. 

Detector Setup

Target acquisition for coronagraphic imaging, time series, and grism time series uses specific subarrays as noted above under Instrument Setup. The subarray sizes can not be selected by users.

Users must choose the exposure pattern to use, and the value for Groups. For all TA modes the allowed selections are:

  • Readout pattern[RAPID, BRIGHT1, BRIGHT2, SHALLOW2, SHALLOW4, MEDIUM2, MEDIUM8, DEEP2, DEEP8] (i.e., all NIRCam patterns are available)
  • Groups = [3, 5, 9, 17, 33, 65] (additional options are available but not used for TA)

The maximum value of Groups is limited by onboard data acquisition limitations.

Figure 3. NIRCam TA Detector Setup

UI screen shot of supported NIRCam TA Detector Setup tab. Note that values of Groups larger than 65 are not actually supported, and will issue a warning in the Reports pane.
Table 1. NIRCam target acquisition modes, subarrays, and filters supported by ETC

TA Acq ModeCoronagraphic maskSubarrayFilter
Time series or grism time series--SUB32 Time Series TAF335M, F405N


MASK210RSUB128 (128 × 128)


F210M (Faint)

Use for sources fainter than KVega=6.2 

F210M+ND square (Bright)

MASK335R SUB64 (64 × 64) 

F335M (Faint)

Use for Kmag fainter than KVega=4.7 

F335M+ND square (Bright)
MASK430RSUB64 (64 × 64) F335M (Faint)
F335M+ND square (Bright)
MASKSWBSUB128 (128 × 128)F210M (Faint)
F210M+ND square (Bright)
MASKLWBSUB64 (64 × 64) F335M (Faint)
F335M+ND square (Bright)

† Time series and grism time series have identical configurations in the ETC. 


Minimum recommended SNR

The exposure specification for the TA should be chosen to obtain at least the minimum recommended integrated (within the extraction aperture) SNR (~30). At lower SNR a warning will be issued, and centroid accuracy may not be adequate to fully support acquisition of the target and/or calibration of the data.

Saturation limits for target acquisition

Saturation is also a concern for TA exposures, and should be avoided if possible. Assuming an early type main sequence spectral type, the brightest targets that can be observed in the various TA modes are:

  • Short wavelength coronagraphic imaging: KVega = -1.1 
  • Long wavelength coronagraphic imaging: KVega = -2.0
  • Time series and grism time series: KVega = See the "Time-series saturation limits" section below

Time-series saturation limits

For imaging time-series and grism time-series templates, targets that cause saturation in some pixels can still be used for target acquisition, depending on the centroid accuracy required for a particular observation. Figures 3–5 in both the NIRCam Time-Series Imaging Target Acquisition and NIRCam Grism Time-Series Target Acquisition articles present the observed centroid accuracy versus KVega and number of saturated pixels. Observers are encouraged to check these plots in order to determine the limiting magnitude or number of saturated pixels that can be tolerated for their required accuracy and science case. For sources which are too bright and saturate too many pixels for the required centroid accuracy, users may use F405N or be required to perform TA on a nearby offset target with a more suitable brightness.

For the particular case of full saturation (in the 1st group) of one pixel, the ETC will report an SNR of 0, but TA will still work. Approximate magnitude ranges, when using F335M, corresponding to these cases are:

  • No saturation: KVega = 7.1
  • Partial saturation: 6.7 < KVega< 7.0
  • Full saturation: 6.3 < KVega< 6.6 (ETC reports SNR=0, but TA will still work)

Note that using F405N allows acquisition of targets approximately 3.0-3.5 magnitudes brighter than F335M, depending on the spectrum of the target

TA Failure

The ETC will issue a "TA may fail" message if the number of saturated pixels in the centroid box exceeds one pixel. TA will still work in this case, although observers are encouraged to check Figures 3-5 in the Grism Time-Series Target Acquisition or Time-Series Imaging Target Acquisition articles to see that their analysis can accept the number of ETC-reported saturated pixels. 

Figure 5. ETC Output 2D SNR plots for NIRCam TA

ETC output 2D SNR for NIRCam TA setup. Left: example using coronagraphic MASK210R TA on a bright KVega=5 target through a neutral density square. Note that the target is offset from the center of the scene in order to avoid falling behind the coronagraphic mask, which is centered at (0,0). Time-series TA setup (right) using a very faint KVega=15 target.

Latest updates
    Updated for ETC 3.0.

  • Updated for ETC 2.0.

  • Updated for ETC 1.5.

  • Updated for ETC 1.3.
Originally published