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 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 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.
Figure 1. Footprints of the coronagraphs on the MIRI imager focal plane
This figure shows the footprint of the coronagraphs on the imager FOV. The detector subarrays associated with each corongraph are shown in the red boxes. Note that each subarray is oversized on the detector to achieve proper frame times, but light only falls on the square portions that are not blocked by the support structure (black in this figure). The 32 × 32 pixels region labeled "TABLOCK" is the location for placing the coronagraphic target when the telescope is initially slewed to the target to the target. The larger four 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) of each corongraph. The smaller four white boxes, closer to the center of each coronagraph, indicate the second coronagraphic TA ROIs in each quadrant (e.g., 1b, 2b). Reference points are indicated by a red cross. The background image is a flight model flood-illuminated image taken with the F1065C filter/mask during cryo-vacuum testing.
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.
Temporary Restrictions on MIRI coronagraphic observations
Commissioning of the coronagraphs revealed some issues that affect proposers; for details see MIRI Known Issues.
- 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 at this time. 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. This repeat provides a 2-point dither to mitigate against residual cosmic rays, detector artifacts and contamination of the background by astronomical sources. The MIRI team is investigating a noiseless background subtraction scheme for future cycles.
Note that background observations have zero proprietary period.
Target acquisition process
Figure 2. The MIRI coronagraphic target acquisition (TA) process
Click on the figure for a larger view.
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. 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 Coronagraphs, HCI Optics
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 behind the mounting bracket that holds the coronagraphic masks and the LRS slit. This is necessary, because the filter currently in place was the last used in the previous imaging or coronagraphic observation. Without the TABLOCK position, cycling through the filter wheel could expose the detector to very bright light.
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 then 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
See also: MIRI Coronagraphs, HCI Optics
TA for the Lyot coronagraph follows the same process as for the 4QPMs.
Offset Target Acquisition
Typically the primary/science target is used for TA (i.e., a self-TA). However, the procedure can also be carried out with an offset TA target, such as a nearby bright star, which should be within 60" of the science target. Use of an offset TA target is advisable in the following cases:
- The science target is too bright and saturates even the FND TA filter;
- The science target requires a long (~100s of seconds) integration to reach the required SNR of ≥ 20 (but recommend ≥ 50) for accurate TA.
If the observer is using an offset TA target, care should be taken with the coordinates (including proper motions) used in the APT for both the TA target and the science target. Self-TA will correct for small errors in position, provided that the target is the brightest object in the TA region of interest (ROI). However, for an offset TA, any error in the position of either the offset TA target or the science target will result in misplacement of the science target in the coronagraph.
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.