See also: NIRCam Coronagraphic Imaging, JWST 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).

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

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.