The JWST MIRI coronographic imaging mode requires target acquisition procedures.
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.
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.
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).
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.
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.
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
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.