NIRCam Coronagraphic Target Acquisition

NIRCam coronagraphic target acquisition (TA) positions a bright target at the optimal location of the occulting mask.

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See also: NIRCam Coronagraphic ImagingJWST High-Contrast Imaging

To achieve the maximum suppression of diffracted light from the coronagraphic target, a small angle maneuver (SAM) from the initial telescope pointing is needed to place the target at the optimal location of the occulting mask. The accuracy of the SAM depends on how precisely the initial centroid of the target can be measured during the TA process. To avoid saturating the TA images of typically bright targets, the NIRCam coronagraph optical mount (COM) is equipped with a series 5″ × 5″ neutral density (ND) squares of optical density ~3 through which bright stars can typically be imaged with ~7.5 magnitudes of attenuation. Fainter targets can alternatively be imaged in a clear region of the COM. Once the centroids and subsequent SAMs are computed on-board, the target is then precisely moved to the appropriate location behind the occulting mask (Figure 1).

Figure 1. NIRCam coronagraphic occulting masks and neutral density squares for target acquisition

NIRCam module A coronagraph optical mount (COM), which contains 3 round and 2 bar occulting masks for coronagraphic imaging and several neutral density (ND) squares for bright target acquisition (TA). Acquisition of fainter targets is performed in clear regions of the COM between the large ND squares. Bright and faint TA images are recorded using 4″ × 4″ subarrays located within the large ND squares and adjacent clear regions, respectively. One ND square is associated with each of the round occulters, and two ND squares are used for each of the bar occulters. (One ND square is shared by both bars.) The arrows indicate the directions of the small angle maneuvers (SAMs) that move the bright targets from the ND squares to the corresponding occulters when either the SW channel (blue arrows) or LW channel (red arrows) is the primary coronagraphic channel. The ND square shared by the two bar occulters (shown with two arrows) is used when the science filter selected for the primary channel is F200W or F212N (blue arrow) or F460M or F480M (red arrow). Bright TA for other primary-channel science filters uses the ND squares that are closer to the narrow ends of the bars. Figure adapted from Krist et al. 2010.

Coronagraphic TA procedure

NIRCam coronagraphic TA begins with a slew of the telescope that places the coronagraphic target on a 4″ × 4″ subarray within ~10″ of the selected occulting mask. Depending on the brightness of the target, the subarray is either located within an ND square or in an adjacent clear region of the COM. The sizes of the TA subarrays depend on the choice of occulting mask, which determines whether the LW or SW channel is the primary coronagraphic channel.  The TA subarray images are always recorded with the F210M filter when the SW channel is primary, regardless of which filters are used during the subsequent coronagraphic science exposures. Likewise, F335M is the designated TA filter when the LW channel is primary. Table 1 lists the instrument configuration and approximate target brightness limits for bright and faint TA when either the SW or LW channel is primary.

Table 1.  Specifications for bright and faint TA in each channel



TA subarray

Target brightness limits



Bright TA
(ND square)

Faint TA
(clear aperture)

Too faint
(no TA)

SWF210M128 × 1284″ × 4″K < 6.3K > 6.3K > 19.0
LWF335M64 × 644″ × 4″K < 4.7K > 4.7K > 17.2

The maximum exposure time allowed for the TA images is 250 s, which corresponds to the largest number of groups (65) that can be recorded with the longest readout pattern (DEEP8) and the larger SW subarray.  It is very important to use the ETC to ensure that the TA image has SNR > 30 for accurate measurement of the image centroid.

After the TA images are recorded, the calculation of the image centroid and the SAM to the occulting mask are performed autonomously onboard, as described in the general article for NIRCam Target Acquisition.   The centroid calculation takes about 9 minutes for the larger SW TA subarrays, and about 4 minutes for the smaller LW TA subarrays. The FLAT (i.e., opaque) pupil-wheel element is rotated into the optical path during the SAM to avoid a latent (residual) image on the detector from the moving bright target.

Once the SAM is completed, the specified science observations can begin. However, if a precise measurement of the target's location behind the occulter is needed, full frame dual-channel astrometric confirmation images may be recorded before and after the SAM. The first image allows registration of the unocculted target with respect to fainter sources within the field of view, most of which will appear in the second image of the occulted target. These images can also be used to precisely measure the separation and position angle between the occulted target and any detected faint companions. The SW and LW confirmation images are recorded by default with the F210M and F335M filters, respectively. 

Coronagraphic TA is performed once per science observation, i.e., once per set of exposures constructed with a single instance of the NIRCam Coronagraphic Imaging APT Template. In other words, coronagraphic TA is needed for each coronagraphic target or telescope roll maneuver, but it is not repeated for filter changes within an observation.  


Girard, J. H., et al. 2022, Proceedings of the SPIE, 121803Q
JWST/NIRCam Coronagraphy: commissioning and first on-sky results

Beichman, C. A., et al. 2010, PASP, 122:162
Imaging Young Giant Planets from Ground and Space

Krist, J. et al. 2010, Proc. SPIE 7731, 3J 
The JWST/NIRCam coronagraph flight occulters

Perrin, M. et al. 2013, JWST-STScI-003472
Sample Target Acquisition Scenarios for JWST

Stark, C., et al. 2016, JWST-STScI-004707
How to Implement a JWST Coronagraphic Observation Sequence in APT

Latest updates
    • Streamlined and revised text to reflect dual-channel mode starting in Cycle 2.
    • Added Table 1 summarizing coronagraphic TA specs.
    • Simplified Figure 1 to emphasize SAM offsets.

    Minor updates
Originally published