See also: NIRCam Coronagraphic Imaging, JWST High-Contrast Imaging

The goal of coronagraphic target acquisition (TA) with NIRCam is to accurately align an astronomical point source—the "host"—on a coronagraphic mask (occulter). In cases where suppression of the light from the point source is adequate to detect nearby sources (e.g., planets) or emission (e.g., circumstellar disks), a single observation of the science target may be adequate. In order to achieve optimal limiting contrast between a bright host and the faintest detectable "companion," an observation of one or more PSF reference targets is necessary. Details about limiting contrast for NIRCam are available in this article: HCI NIRCam Limiting Contrast.

Coronagraphic target acquisition

In order for the coronagraph to achieve maximum suppression of unwanted host light, a small angle maneuver (SAM) must place the target accurately on the center of the coronagraphic mask (occulter).

A "target" may be any of 4 types:

1. A "host," meaning a bright, point source that may harbor a "companion" feature of primary scientific interest, such as an extrasolar planet, circumstellar disk, or quasar feeding zone; 
2. A "PSF reference," meaning a generic, bright, point source, observed to characterize the PSF, particularly in the wings and outside the inner working angle (IWA); or 
3. The target may be a "reacquisition" of a host or reference target that must be reacquired to reduce pointing errors. Re-acquisition is necessary after a roll maneuver, for example, but is not required after changing filters.
4. An offset target may also be used for target acquisition for either host or reference targets.

Coronagraphic target acquisition (TA) involves an initial slew of the telescope to place the target on a 4" × 4" subarray in the ~10″ vicinity of the selected coronagraphic mask. If the target is brighter than K ≈ 7, the subarray is located behind a neutral density square (nominally ND = 3). If fainter than K ≈ 6.5, the target is positioned behind a nearby, clear (ND = 0) region of the coronagraphic optical mount (COM). The first phase of TA is complete when the detector obtains an exposure of the target on an appropriate region of the COM (ND = 0 or 3) near the specified coronagraphic mask.

Coronagraphic TA images are always be taken in either the F210M filter and a 128² subarray, or the F335M filter and a 64² subarray, for short- or long-wavelength (SW, LW) coronagraphy, respectively. The subarrays are completely behind either a clear region of the COM (for faint targets) or an "ND = 3" square, for bright targets. 

Beginning with Cycle 2 (APT 2022.7 and beyond), science data will be collected simultaneously in the SW and LW channels, with the target behind either a SW or a LW mask for a given observation. If a SW mask is chosen, TA occurs in the SW channel with the F210M filter, and if a LW mask is chosen TA occurs in the LW channel with the F335M filter.

Subsequently, target acquisition is performed autonomously onboard, as described in the NIRCam Target Acquisition article, culminating with the host target being centered behind the selected coronagraph.

To eliminate the possibility of a latent image when the target is moved from the TA aperture to the science pointing, the "opaque" or "dark" position on the pupil wheel is placed into the beam before executing the final TA slew.

In cases where very precise measurement of the final host target position is needed, astrometric confirmation images may also be specified. These are 2 full frame exposures, the first acquired just after the completion of the on-board TA process, and the second after the slew that transfers the target to the coronagraph (at which point, it is obscured). Such images can be used to more precisely measure the offset between the host and a companion.

Once the target is positioned behind the coronagraphic mask, the specified science observations begin.