NIRCam Coronagraphic Imaging Target Acquisition

The JWST NIRCam coronagraphic target acquisition (TA) positions the bright "host" on the center of the coronagraphic mask.

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See also: NIRCam Coronagraphic ImagingJWST 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 three 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 ≈ 7, 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 or F335M filter, for short- or long-wavelength (SW, LW) coronagraphy, respectively.

Coronagraphic TA images are taken using 1282 or 642 subarrays, for SW or LW, respectively. The subarrays are completely behind eithera clear region of the COM (for faint targets) or an "ND = 3" square, for bright targets. 

Subsequently, target acquisition is performed autonomously on board, 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 two 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.

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

NIRCam coronagraphic occulting masks and neutral-density squares for target acquisition

Features of the NIRCam module A coronagraphic optical mount (COM), including the neutral density squares (ND = 3) regions for bright target acquisition (TA), and the bar and round coronagrapic masks. Acquisition of fainter targets uses subarrays placed at positions between the ND squares. The three lines of information at the bottom are the mask name, the inner working angle (IWA)—the smallest observable apparent separation—and the wavelength range. TA images are taken using subarrays matched to the regions of the COM used for the first phase of TA. The ND square between the two bar occulters (with both a blue and a red arrow) is used for bright TA when the first SW science filter is F200W or F212N, or when the first LW science filter is F460M or F480M; bright TA for the bar occulters in other filters uses the ND squares nearer the narrow ends of the bars.

The centroiding algorithm takes about 9 minutes for SW coronagraphy, and about 4 minutes for LW (due to the difference in subarray sizes).

The TA exposures themselves are limited to be < 250 seconds, using the maximum number of groups (65), the longest readout pattern (DEEP8), and the SW subarray (1282).

It is very important to use the ETC to confirm SNR estimates for target aquisition, but the approximate limits for bright and faint coronagraphic TA on a typical star are:

F210M (short wavelength TA)

  • Bright TA (ND square): K < 6.3
  • Faint TA (clear aperture): K > 6.3
  • Too faint: K > 19.0

F335M (long wavelength TA):

  • Bright TA (ND square): K < 4.7
  • Faint TA (clear aperture): K > 4.7
  • Too faint: K > 17.2



References

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

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




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