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

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Coronagraphic target acquisition
Coronagraphic target acquisition

Introduction

The goal of coronagraphic target acquisition (TA) with NIRCam is to accurately align an astronomical point source—the "host"—on a coronagraphic mask (occulter).

The purpose of PSF subtraction is to achieve limiting contrast between a bright host and the faintest detectable "companion," which is the main source of scientific interest. See here for a NIRCam-specific discussion of limiting contrast.

The companion may be an extended source, such as a circumstellar disk, or a point source, such as an exoplanet. The PSF reference image may be a composite of multiple images obtained after pointing changes—either a roll or an offset. To achieve PSF subtraction, the reference image is scaled and subtracted from science images.

See here for an overview of NIRCam's coronagraphic imaging mode, and see here for a general overview of high-contrast imaging with JWST.

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 document the PSF, particularly in the wings and outside the inner working angle (IWA); or (3) the target may be a "reacquisition" of a previously observed target that must now be reacquired to reduce pointing errors, which as may have been introduced, for example, by a roll maneuver. Some activities do not call for a subsequent reacquisition, such as rotating the filter wheel to change the filter.

The first phase of 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 square of neutral density (ND, 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 image of the target on an appropriate region of the COM (ND = 0 or 3) near the specified coronagraphic mask.

In the second TA phase, an on-board centroiding algorithm estimates the target's precise position on the detector and computes the distance and direction of the small-angle maneuver (SAM) needed to move it to the center of the specified coronagraphic mask. This phase includes rejecting cosmic rays, subtracting the background, and flat-fielding.

The third TA phase is executed by a SAM that shifts the target to the center of the coronagraphic mask. Simultaneously, the image of a possible  companion target—if one is indeed present nearby but outside the IWA—may now be shifted onto an unobscured the detector for imaging.

To eliminate the possibility of a latent image, the "opaque" or "dark" position on the pupil wheel is placed into the beam before executing the SAM in the third TA phase.

The 3 TA phases are executed autonomously, with no interaction with the ground.

TA images are always be taken in either the F210M or F335M filter, for short- or long-wavelength (SW, LW) coronagraphy, respectively.

The detector read-out patterns for TA are restricted to provide 3 evenly spaced up-the-ramp images. The 3 images are used to reject cosmic rays, which could otherwise introduce error to the calculation of the location of the target.

TA images are taken using 642 or 1282 subarrays completely behind a clear region of the COM or an "ND=3" square, as appropriate. 

Once the target is relocated behind the coronagraphic mask, science observations can begin, starting with the selection of the requested science filter.

The NIRCam TA procedure estimates the centroid of the brightest target in the target-acquisition subarray. If coronagraphy is desired for a nearby target of interest that is less bright, users can insert a SAM from the TA source and the target of interest, which will place the target of interest behind the selected coronagraphic mask. 

Figure 1. Features of the NIRCam Module A Coronagraphic Optical Mount (COM), ND = 3 regions for target acquisition (TA), including the bar and round coronagrapic masks. 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.

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To summarize, the ten detailed events of any NIRCam coronagraphic TA are:

  1. Place the target behind the clear region or "ND=3" square associated with the selected coronagraphic mask (occulter).
  2. Rotate the filter wheel to the TA filter (F210M or F335M).
  3. Rotate the pupil wheel to the appropriate Lyot stop for the type of occulter (round or bar-shaped).
  4. Take the TA images using the default, 3-sample detector-readout pattern.
  5. Compute the centroid position of the target. This process includes rejecting cosmic rays, background subtraction, and flat-fielding.
  6. Rotate the pupil wheel to the opaque ("dark") position. 
  7. Rotate the filter wheel to the desired science filter.
  8. Execute a small-angle maneuver (SAM) to place the target on the selected coronagraphic mask. For the spot occulters, the SAM will place the target behind the center of the mask. For the bar occulters, the SAM will place the target at the position along the bar that is appropriate for the selected science filter.
  9. Rotate the pupil wheel to the appropriate Lyot mask (round or bar).
  10. Take science data.

The centroiding algorithm takes about 5 min. Additionally, the TA exposures themselves can take > 15 min in the following cases:

F210M (short-wavelength TA)

        7.1 < K < 8.2 (bright)

        14.2 < K < 15.3 (faint)

        15.3 < K (too faint)

F335M (long-wavelength TA):

        5.5 < K < 6.6 (bright)

        13.1 < K <  14.3 (faint)

        14.3 < K (too faint)

 



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Related links

JWST User Documentation Home
NIRCam Coronagraphic Imaging

NIRCam Coronagraphic Occulting Masks and Lyot Stops
NIRCam Filters for Coronagraphy
JWST High Contrast Imaging Overview
JWST High Contrast Imaging Optics
JWST High Contrast Imaging Inner Working Angle
Contrast Considerations for JWST High-Contrast Imaging
NIRCam-specific treatment of limiting contrast
JWST Coronagraphic Observation Planning
JWST Coronagraphic Sequences
JWST High Contrast Imaging in ETC
JWST High Contrast Imaging in APT

 

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References

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

Perrin, M., Stansberry, J., Beck, T., Hines, D., and Soummer, R., 2013 JWST-STScI-003472
Sample Target Acquisition Scenarios for JWST

Stark, C., Van Gorkom, K., & Pueyo, L., 2016, JWST-STScI-004707, SM-12
How to Implement a JWST Coronagraphic Observation Sequence in APT

JWST technical documents

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