MIRI Coronagraphic Imaging Target Acquisition
The JWST MIRI coronographic imaging mode requires target acquisition (TA) procedures for the target and reference point source. It is possible to obtain an associated background image, and this does not require a target acquisition.
MIRI coronagraphic imaging observations require precise and accurate positioning of a bright source at the location of maximum attenuation by the Lyot spot mask or 4QPMs: for the 4QPM, this is the apex between the four quadrants; for the Lyot, it is at the center of the occulting spot.
For the 4QPM, the required absolute accuracy of placing a star at the apex is 10 mas (1-σ per axis), but the ultimate positioning of the object on the mask requires a repeatable precision of 5 mas (1-σ per axis).
For the Lyot coronagraph, the pointing accuracy and precision are less stringent due to the 3.3 λ/D spot size. This relaxes the requirements to 22.5 mas. The neutral density filter requirements for the target acquisition have ensure that Vega can be observed in the coronagraph's.
- 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.
- The detector arrays have persistence that could result in residual artifacts that can mimic planets or other astronomical phenomena if the centroiding process leaves them close to the target star.
These effects could make adequate TA 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 multiple acquisitions to acquire additional information about the pointing (see MIRI Coronagraphic Recommended Strategies). 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.
There are four acquisition filters available for MIRI TA: F560W, F1000W, F1500W and a neutral density filter FND 1 (which is needed for TA in the case of very bright sources; see MIRI Coronagraphic Recommended Strategies).
Due to the fact that spacecraft roll orientations during a given visibility period are very restricted, the observer is allowed to select which of the four locations within the coronagraphic subarray to perform the target acquisition (TA). They 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 (i.e,. a SECOND EXPOSURE). This ability enables the observer to mitigate against the effect of latency due to the acquisition of successive images.
Software processing requirements for the target acquisition image include a flat field of the 48 × 48 pixel region of interest (ROI) surrounding the coronagraph sweet spots of which there will be 16 in the baseline strategy. A centroiding algorithm for the targets in the sweet spots is outlined in Lajoie et al. (2014a). These exposures will normally be short; therefore, cosmic rays should not be an issue.
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4QPM target acquisition
There are several possible strategies for 4QPM TA, which are discussed in detail by Lajoie et al. (2012, 2013, 2014a, 2014b). The baseline approach is described as follows on the assumption that offset slew accuracy is consistent with NASA’s pre-launch estimates.
First, the target is initially placed at a fiducial location behind the mounting bracket that holds the coronagraphic masks and the LRS slit. Then, the TA filter is put into the optical path, and the target is moved into an ROI in one of the four quadrants. An exposure is obtained, a centroid is found for the target, and the offset necessary to move the target to the inner ROI is calculated and a small angle maneuver (SAM) is performed. The process is repeated to then move the target to the center of the apex of the 4QPM. For 4QPM coronagraphy, there are specific readout subarrays defined for each mask/filter (F1065C, F1140C, F1550C).
In scenarios where the potential contribution of latent images in the science observations pose concerns for the coronagraph science goals, a SECOND EXPOSURE can be performed. Here, the target acquisition (followed by a science exposure) will be repeated in the quadrant diagonally opposed to the quadrant in which the initial TA was performed. This allows for discrimination between latent images and faint sources because the latents are variable in time: the 1st observation will not not have latents present in the ROI of the 2nd TA and latents in the 1st TA ROI will have decayed by the time the 2nd TA observation is completed.
Lyot coronagraph target acquisition
TA for the Lyot coronagraph follows the same process as for the 4QPMs. the target is initially placed at a fiducial location behind the mounting bracket that holds the coronagraphic masks and the LRS slit. Then, the TA filter is put into the optical path, and the target is moved into an ROI in one of four "quadrants" in the Lyot field of view ( MASKLYOT a.k.a. LYOT, 30" x 30"). Given the brightness of some sources, it is possible that the target acquisition will leave a latent image in the TA region of interest, which will persist in the science image.
To mitigate confusing the latent image with a nearby faint source, two coronagraphic observations can be obtained: 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 latent images will be different between the two coronagraphic observations, allowing for discrimination of faint sources and these latent images. Discrimination is possible since the observations taken with the 1st target acquisition region will not have latent images in the 2nd target acquisition region because the latent images are variable in time; that is, the latent images in the 1st ROI will have decayed by the time the 2nd ROI target acquisition observations are performed.
The goal is to have the ROIs located as close to the center of the Lyot spot as possible without being affected by any edge distortion effects. The accuracy of spacecraft small angle maneuvers from 2"–20" is expected to be <~4–6 mas (Lajoie et al. 2014a).
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, SPIE, 9143, 91433R
Simulations of JWST MIRI 4QPM Coronagraphs Operations and Performances
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.