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The JWST MIRI coronographic imaging mode requires target acquisition procedures.

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Introduction

MIRI coronagraphic imaging requires the placement of a bright source as close as possible to the center of the Lyot spot mask or 4QPMs with an accuracy of 22.5 and 5 mas (1s, 1 axis), respectively, for maximal point source suppression/cancellation (Soummer et al. 2012,  Lajoie et al. 2012,  Lajoie et al. 2013,  Lajoie et al. 2014a,  Lajoie et al. 2014b,  Soummer et al. 2014). This required accuracy limits the spacecraft move between the target acquisition (TA) region and the center of the coronagraphs to <20" and 5", respectively. The neutral density filter requirements for the target acquisition have ensured that Vega can be observed in the coronagraph's subarray mode.  

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Figure 1. Footprints of the coronagraph masks on the MIRI imager focal plane

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The footprints of the coronagraph masks on the MIRI imager focal plane. The 32 × 32 pixel region labeled TABLOCK is the location for placing the coronagraphic target when the telescope is initially slewed to the target. The 4 white boxes in each corongraphic subarray are the locations of the coronagraphic target acquisition (TA) regions of interest (ROIs, 64 × 64 pixels); reference points are indicated by a cross. The background image is an FM flood-illuminated image taken in F1065C.

Two effects make the TA process complex: (1) for the 4QPM coronagraphs, the phase mask can distort the image of a star close to its center and undermine the accuracy of the centroid determination; and (2) the detector arrays have latent images that could mimic planets or other exciting astronomical phenomena if the centroiding process left them close to the target star. These effects would make adequate TA very difficult at the nominal JWST offsetting accuracy specifications described above. Fortunately, it is projected that small-angle offsets up to 20" are expected to be accurate to 5 mas (1σ per axis). Simulations of the centering accuracy on the coronagraph using the projected performance and a fiducial distance of 2" from the coronagraph center indicate a scatter of ∼7 mas (rms) and average centering errors of 2–4 mas. The details depend on the particular strategy, i.e., whether one utilizes a single position for target acquisition, or uses more than one to acquire additional information about the pointing. None of the strategies quite reaches the desired centering performance for the 4QPM coronagraphs (the Lyot is much more relaxed in this area), so further optimization is expected during commissioning. 

The observer will have the opportunity to choose which of the 4 quadrants on the subarray for performing the initial TA.  Due to the fact that spacecraft roll orientations are very restricted, the observer is allowed to select which of the 4 locations within the cornagraphic subarray to perform the target acquisition (TA).  The observer will also have the option to repeat the entire observation, but with the TA performed within a region of the subarray that is diagonally opposed to the original TA. This ability ensures that the observer can mitigate confusion in the science images from persistent images from the TA process.

Software processing requirements for the target acquisition image include a flat field of the 64 × 64 pixel ROI surrounding the coronagraph sweet spots of which there will be 16 in the baseline strategy.  A centroiding algorithm will be needed for the targets in the sweet spots, and we adopt the strategy outlined in Lajoie et al. (2014a). These exposures will be normally short and thus cosmic rays should not be an issue. 

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In the case of longer exposures (specifically needed for faint source TA for low resolution spectroscopy), an algorithm to remove the cosmic rays before centroiding is required.

 


Lyot coronagraph target acquisition

For Lyot coronagraphy, the point source will be placed in one of the 4 target acquisition ROIs in the Lyot coronagraphic field of view (MASKLYOT, 304 × 320 pixels).  The readout times for each subarray in FASTMode is 0.324 s. Given the brightness of the sources, target acquisition may leave latent images in the target acquisition regions. To mitigate confusing the latent image with a nearby faint source, it may be optimal to take 2 coronagraphic observations: one with target acquisition using the 1st ROI and one with target acquisition using a second 2nd ROI that is diagonally opposed to the first one.  Any persistence images will be different between the two coronagraphic observations allowing for discrimination of faint sources and these persistence images.  Discrimination is possible since the observations taken with the 1st target acquisition region will not have persistence images in the 2nd target acquisition region, and the persistence images are variable in time such that the persistence images in the 1st ROI will have decayed by the time the 2nd ROI target acquisition observations are done. The goal is to have the ROIs located as close to the center of the Lyot spot (radius = 2.4''; Renouf 2006) as possible without being affected by any edge effects.  The accuracy of spacecraft small angle maneuvers from 2"–20" is expected to be <~4–6 mas (Lajoie et al. 2014a).

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Double check this number above with Dean 20 mas

 


4QPM target acquisition

There are several possible approaches to 4QPM TA, which are discussed in detail by Lajoie et al. (2012, 2013, 2014a, 2014b)The baseline approach to 4QPM TA, described below, assumes that offset slew accuracy is consistent with NASA’s pre-launch estimates.

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If the observatory pointing performance is different (i.e., worse), then we will implement one of the alternative strategies.

First, TA ROI approximately in the center of one of the 4 quadrants is used to locate the target.  Then a spacecraft move is used to place the source in a second TA ROI closer (~1"–5") to the center of the coronagraphic field of view.  The target is located again and then moved into the center of the coronagraphic field of view (i.e., at the apex of the 4QPM) using the most precise small spacecraft move.  For 4QPM coronagraphy, there are specific readout subarrays defined for each mask (MASK1550, MASK1140 & MASK1065, each 216 × 216 pixels).  

Due to persistence images, this procedure can be done twice where the center is approached from 2 directions, 180° apart.  The persistence images will be different between the 2 coronagraphic observations allowing for discrimination of faint sources and persistence images. Such discrimination is possible because the observations taken with the 1st target acquisition ROIs (1a & 1b in Figure 2) will not have latents in the 2nd target acquisition ROIs (2a & 2b in Figure 2), and the latents are variable in time such that the latents in the 1st ROIs will have decayed by the time the 2nd ROIs target acquisition observations are completed. The “a” ROIs are 64 × 64 pixels and the “b” ROIs are 7 × 7 pixels.  The uncertainty in the position of the source in the “b” ROIs is approximately 20 mas (8" spacecraft move from “a” to “b” ROIs).  Thus, the sizes of the “b” subarrays (0.77" × 0.77") are large enough so the source will always be in the “b” ROIs after the “a” target acquisition.

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Figure 2. Target acquisition with the 4QPM


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The large dashed boxes are where the regions of interest (ROIs) for the “a” coronagraphy 4QPM target acquisition should be located and the small solid boxes are the approximate proposed locations. The required spacecraft offsets are shown using arrows.

 


 

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JWST User Documentation Home
MIRI Coronagraphic Imaging

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References

Lajoie, C.-P., Soummer, R., Hines, D., 2012, JWST-STScI-003065,
Simulations of Target Acquisition with MIRI Four-Quadrant Phase Mask Coronagraph (II). 

Lajoie, C.-P., Hines, D., Soummer, R., and The Coronagraphs Working Group, 2013, JWST-STScI-003546,
Simulations of MIRI Four-Quadrant Phase Mask Coronagraph (III): Target Acquisition and CCC Mechanism Usage

Lajoie, C.-P., Soummer, R., Hines, D., and The Coronagraphs Working Group, 2014a, JWST-STScI-003712:,
Simulations of Target Acquisition with MIRI Four-Quadrant Phase Mask Coronagraph (IV): Predicted Performances Based on Slew Accuracy Estimates

Lajoie, C.-P., Soummer, R., Hines, D.C., & Rieke, G.H. 2014b,
Simulations of JWST MIRI 4QPM Coronagraphs Operations and Performances, SPIE, 9143, 91433R

Rieke, et al. 2006 (JWST-STScI-001012).  

Soummer, R., Hines, D.C. & Perrin, M. 2012, JWST-STScI-003063,
 Simulations of Target Acquisition with MIRI Four-Quadrant Phase Mask Coronagraph (I).

Soummer, R. et al. 2014, JWST-STScI-004141,
Coronagraphic Operations Concepts and Super-Template Definition for the Astronomer’s Proposal Tool. 

JWST technical documents

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Last updated

Updated May 18, 2017

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