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.

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See also: MIRI Coronagraphic Recommended Strategies

MIRI coronagraphic imaging observations of science and reference targets require precise and accurate positioning of a bright source at the location of maximum attenuation by the Lyot occulting spot or 4QPMs: for the 4QPM, this is the apex between the 4 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. 

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

The 32 × 32 pixels region labeled "TABLOCK" is the location for placement of the coronagraphic target when the telescope is initially slewed. The larger 4 white boxes in each coronagraphic subarray indicate the initial coronagraphic target acquisition (TA) regions of interest (ROIs; 48 × 48 pixels) in each quadrant (e.g., 1a, 1b). The smaller 4 white boxes, closer to the center of each coronagraphic subarray, indicate the second coronagraphic TA ROIs in each quadrant (e.g., 1b, 2b). Reference points are indicated by a cross. The background image is a flight model flood-illuminated image taken with the F1065C filter/mask during cryo-vacuum testing. Note that the red boxes indicate the size of the subarrays as defined on the detector, which are larger than the area that is exposed to light each coronagraph to achieve proper frame times.
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 centroid determination.
  2. 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 pointing accuracy. Fortunately, it is projected that small angle offsets up to 20" are expected to be precise to 5 mas (1σ per axis). Simulations of the centering precision on the coronagraph using the projected offsetting 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.

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There are 4 TA filters available for MIRI: F560W, F1000W, F1500W and a neutral density filter FND* (which is required for TA in the case of very bright sources to avoid saturating the subarrays; see MIRI Coronagraphic Recommended Strategies).

Spacecraft roll orientations during a given visibility period are very restricted, so the observer is allowed to select which of 4 locations within the coronagraphic subarray to perform the 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



Target acquisition process

Figure 2. The MIRI coronagraphic target acquisition (TA) process


The coronagraphic target acquisition (TA) process. To minimize mechanism usage, the MIRI filter wheel is left in the last position used in a previous MIRI observation, so the current MIRI filter is unknown prior to a new observation, and there is no way to set filters/masks prior to the slew to target. Therefore, for MIRI coronagraphic TA, the target is first placed within a 32 × 32 pixels region labeled "TA BLOCK", which is located behind the mounting bracket that holds the coronagraphic masks and LRS slit preventing light from the target to saturate the subarray. This step ensures the proper filter/mask is inserted into the optical path before the telescope is slewed to the TA ROIs on each coronagraph subarray.


4QPM target acquisition

See also: MIRI CoronagraphHCI Optics

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 (TABLOCK) 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 4 quadrants in the coronagraph field of view. An exposure is obtained, a centroid is found for the target, the offset necessary to move the target to the inner ROI is calculated and a small angle maneuver (SAM) is performed. The process is then repeated to move the target to the center of the apex of the 4QPM.

In scenarios where the potential contribution of persistent images in the science observations pose concerns for the coronagraph science goals, a SECOND EXPOSURE can be performed. Here, the TA (followed by a science exposure) will be repeated in the region diagonally opposed to the location where the initial TA was performed. This allows for discrimination between persistent images and faint sources.

Lyot coronagraph target acquisition

See also: MIRI CoronagraphsHCI Optics

TA for the Lyot coronagraph follows the same process as for the 4QPMs.


Background images for MIRI coronagraphy

Due to thermal emission from the telescope, zodiacal dust, and other (galactic) sources, it may be advantageous to acquire a background measurement independent from the primary coronagraphic image. MIRI includes the capability to obtain such background observations for each coronagraphic filter/mask combination. While primary coronagraphic images of science and reference targets always require target acquisition, observations of a background do not require TA. However, the background observation must be associated with at least one primary target, but can be associated with multiple targets (e.g., both the science and PSF reference target).

Because the associated background observations do not require a TA, they are significantly less time-consuming than the primary observations. However, the exposure time for the background observations should be sufficient to obtain a signal to noise that is equal to or greater than the background level in the primary images.



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, 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. 




Latest updates
  •  
    Added text about background images; 
    Removed redundant text in the Lyot TA section
Originally published