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 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 observatory blind pointing 0.10" (1-σ per axis) will not be accurate enough to place the target at the center of the 4QPM with the required accuracy of 5 mas (1-σ per axis). Therefore a target acquisition (TA) is required.
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, but this still requires a target acquisition.
- 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.
- 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 error 2–4 mas.
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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.
Temporary Restrictions on MIRI coronagraphic observations
Commissioning of the coronagraphs revealed some issues that will affect proposers; for details see MIRI Instrument Features and Caveats.
- The PSF for the TA filters shows some asymmetry for some combinations of coronagraph and TA filter.
- TA has only been verified for the neutral density (FND) and F1000W TA filters. The MIRI team continues to verify TA with the other 3 TA filters. If the proposed science requires a different TA filter, proposers should contact the JWST Help Desk.
- TA has only been verified in quadrant 1 (upper-right quadrant) of each 4QPM coronagraph, as well as quadrant 4 for the 4QPM-1550. Commissioning showed that latent images should not be a problem for most use cases. This also means that the automatic "repeat in opposing quadrant" option in APT should not be selected. Please contact the JWST Help Desk if science requires TA in a different quadrant. For the Lyot coronagraph, which is less sensitive to pointing, all quadrants are available but quadrant 1 is still recommended.
- Flight data revealed that there is light being scattered into the coronagraphs, which produces linear features at the 4QPM boundaries, at the Lyot occulting spot, and near the bottoms of all coronagraphic fields. Therefore, observers are required to obtain associated/dedicated backgrounds for each and every target observed (both the primary science target and the reference PSF target). The background specification must match exactly the specified parameters for the associated target, and the background observation must be repeated in an opposing quadrant using the automatic "repeat in opposing quadrant" option in APT. The MIRI team is investigating a noiseless mitigation scheme for future cycles.
Target acquisition process
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.
In-orbit performance of the preflight strategy has been verified during commissioning.
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
TA for the Lyot coronagraph follows the same process as for the 4QPMs.
Background images for MIRI coronagraphy
Cycle 2 - Temporary Restrictions on MIRI coronagraphic observations
- Fight data revealed that there is light being scattered into the coronagraphs, which produces linear features at the 4QPM boundaries, at the Lyot occulting spot, and near the bottoms of all coronagraphic fields. Therefore, observers are required to obtain associated/dedicated backgrounds for each and every target observed (both the primary science target and the reference PSF target). The background specification must match exactly the specified parameters for the associated target, and the background observation must be repeated in an opposing quadrant using the automatic "repeat in opposing quadrant" option in APT. The MIRI team is investigating a noiseless mitigation scheme for future Cycles.
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.
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.