MIRI Coronagraphic Imaging APT Template
Cycle 3: 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.
See also: MIRI Coronagraphic Imaging, JWST High-Contrast Imaging Roadmap, HCI APT Instructions, MIRI Coronagraphic Recommended Strategies, MIRI and NIRCam Coronagraphy of HR8799 b, MIRI and NIRCam Coronagraphy of the Beta Pictoris Debris Disk, Proposal Planning Video Tutorials
Coronagraphic imaging is one of 4 observing modes available with the Mid-Infrared Instrument (MIRI); the 4 coronagraphs in the imaging channel provide high-contrast imaging (HCI) capabilities covering photometric bands from 10 to 23 μm. In addition to a classical Lyot coronagraph (which provides an inner working angle (IWA) of ~3.3λ/D), MIRI incorporates three 4-quadrant phase masks (4QPM) to provide the smallest possible IWA of ~1λ/D at 10–16 μm.
An observer will have control over 3 primary parameters for MIRI coronagraphic imaging:
- coronagraph mask/filter combination
- dithering type
- detector read out mode and exposure time (via the number of frames and integrations).
Step-by-step APT instructions are provided below.
Words in bold are GUI menus/
panels or data software packages;
bold italics are buttons in GUI
tools or package parameters.
Mosaics are not available for MIRI coronagraphic imaging.
MIRI Coronagraphic Imaging tab
Target Acquisition Parameters
Target acquisition (TA) is required for coronagraphic imaging of primary science and reference PSF observations. TA is not required and is not available for associated background observations.
The necessary parameters, Target ACQ, Acq Exposure Time, and Acq Quadrant, are available under the MIRI Coronagraphic Imaging tab, in the Target Acquisition Parameters panel. If the target is a background target, the Target ACQ is set to NONE and the other target acquisition parameters are hidden.
For coronagraphic imaging, the Acq Target is the same as the science target. The user can specify one of these Acq Filters: F560W, F1000W, F1500W, and FND.
Acq Exposure Time
See also: Understanding Exposure Times
A TA must be completed by selecting a MULTIACCUM exposure configuration. Each exposure is specified by setting the readout pattern and characteristics parameters: Acq Readout Pattern and Acq Groups/Int.
Acq Readout Pattern
See also: MIRI Detector Readout Overview
This field specifies the readout pattern to be used to obtain the acquisition data. For TA exposures ≤ 23.5 s, the FAST readout mode should be used; for longer TA exposures (Texp ≥ 23.5 ≥ Texp) one of the FASTGRPAVG modes should be used. See the article MIRI Target Acquisition for more information.
Choices for the Acq Readout Pattern are
- FAST (the default)
See also: Understanding JWST Exposure Times
The MIRI readout timing pattern in the acquisition exposure is defined by only one of the MULTIACCUM parameters: Acq Groups/Int. It defines the number of groups during an integration, where a group is the product of cycling through all the pixels. The user can select one of the following options: 4, 6, 8, 10, 12, 22, 36, 44, 66, 86, or 98.
Acq Integrations/Exp, Acq Total Integrations, and Acq Total Exposure Time cannot be changed by the user.
See also: JWST Coronagraphic Visibility Tool
The user selects the target Acq Quadrant in which the initial target acquisition will be performed. Quadrants available are 1, 2, 3, and 4. On the detector, the first quadrant starts at the top right and, going counterclockwise, each consecutive quadrant number designation increases (note that these are displayed and labeled in the Coronagraphic Target Visibility Tool GUI when the MIRI coronagraphs are selected and displayed). The TA can be achieved in any of 4 locations concentrically distributed about the center of each coronagraphic subarray. As target acquisition on bright targets can produce temporary latent images on the array, the user has the option to repeat the observation with the target acquisition performed in the diagonally-opposite quadrant (see Repeat Observation).
See also: MIRI Coronagraphic Imaging
The Coron Parameters dialog box is used to create the entire imaging sequence for a single observation. Each MIRI coronagraphic imaging observation can consist of only a single set of images that all use the same filter, dither pattern, and exposure configuration.
The Background Quadrant parameter is only available when the target is a background target. The user selects the quadrant in which the background target will be placed. The available background quadrants are 1, 2, 3, and 4.
MIRI coronagraphic filters are associated directly with each coronagraph and are not interchangeable. The user selects the desired Coron Mask/Filter combination for the observation. The fixed coronagraphic mask and filter combinations are 4QPM/F1065C, 4QPM/F1140C, 4QPM/F1550C, and LYOT/F2300C.
A coronagraphic imaging sequence must be completed by selecting a MULTIACCUM exposure configuration (in the Coron Parameters dialog box). Each exposure is configured by setting the readout pattern and characteristics parameters: Readout Pattern, Groups/Int, and Integrations/Exp. The number of exposures is set by Exposures/Dith.
See also: MIRI Detector Readout Overview
The MIRI coronagraphic imaging template offers only one readout mode: FASTR1.
Number of groups and integrations
See also: Understanding JWST Exposure Times
The MIRI timing pattern per exposure is defined by only 2 MULTIACCUM parameters:
- Groups/Int: the number of groups during an integration, where a group is the product of cycling through all the pixels. Groups/Int = 2 requires permission from STScI, which can be obtained with a request through the Help Desk.
- Integrations/Int: the number of integrations during an exposure, where integration is defined as the time between resets.
The MIRI coronagraphic template offer 2 subarray options:
- A "mask" subarray that corresponds with the field of view (FOV) provided by the associated mask/filter (F1065C, F1140C, F1550C, F2300C). This subarray yields frame times that are appropriate for brighter primary targets.
- A 'full" array that includes the "mask" subarray FOV plus the imager FOV. This selection is appropriate for fainter primary targets that require longer frame times. The imager FOV also provides the ability to constrain the astrometry of sources within the "mask" FOV by measuring the positions of sources with the imager FOV.
MIRI coronagraphic imaging supports the following types of small grid dither patterns:
|Dither type||Number of dithers|
Small grid dithers for MIRI are very small, fast, and precise pointing offsets of a target image (currently 10 mas per step). Note that small grid dithers do not provide the same functionality as the dither patterns for MIRI imaging, MIRI LRS, or MIRI MRS. Small grid dithers are used to obtain multiple images of a reference star to optimize the PSF subtraction with the science target, whereas the other dither patterns can provide optimal sampling, bad pixel mitigation, and background subtraction. Depending on the desired contrast, the user may wish to use small grid dithers. With appropriate PSF subtraction using data post-processing techniques, highest contrast is achieved with the 9-POINT-SMALL-GRID, while 5-POINT-SMALL-GRID will yield a higher contrast than NONE.
When the target is a background target, DITHER TYPE is set to NONE (or BACKGROUND if Repeat Observation is set to YES) and cannot be changed.
The user specifies whether a repeat acquisition and observation are needed. If an Acq Target is selected, the target acquisition (followed by a science exposure) will be repeated starting in the quadrant diagonally across the central spot from the initial target Acq Quadrant, as follows:
If Acq Target is set to NONE, 2 exposures are executed: the first in the Background Quadrant and a second exposure in the diagonal quadrant as shown in Figure 1.
PSF Reference Observations
See also: HCI PSF Reference Stars
PSF reference observations can be specified in the PSF Reference Observations panel; these observations allow the user to build a PSF reference library for PSF subtraction. Ideally, PSF reference stars are similar to the user's science target and are known to be "good references," i.e., stars without additional astrophysical signal from a debris disk or companion. The observation for a PSF reference star should mirror the observation for the user's science target. Note that reference stars do not have a proprietary period.
This is a PSF Reference Observation
Checking this box indicates that the observation is a PSF reference observation to be used for PSF subtraction (a detailed discussion on the choice of a good PSF reference star can be found in the high-contrast imaging article). The PSF reference star observation should mirror that of the science target. Note that reference star observations have no proprietary period.
PSF Reference Observations
For a science target, uncheck the box mentioned above. This shows the PSF Reference Observations field that lists PSF reference observations that were previously specified in the same proposal; click on one of them to associate it with the science observation.
If the user does not need any PSF Reference Observations (e.g., for a survey of many targets, some of the science targets may serve as PSF references for one another), they must check this box and explain their reasoning with additional text in the science justification section of a submitted proposal.
See also: HCI Coronagraphic Sequences
A variety of observatory-level Special Requirements may be chosen under the Special Requirements tab.
As is often the case with coronagraphic observations, the user may specify in the Special Requirements panes that the observations be in sequence and non-interruptible. To add these special requirements, select Add... in the Special Requirements field, then select Timing followed by Group/Sequence Observations Link. In the pop-up window, select observations from the Observation List and check the Sequence and Non-interruptible boxes.
Additionally, the Special Requirements parameter is where the user could specify a roll dither for their science target. To do this, click on Add... under the Special Requirements parameters field, then select Position Angle followed by PA Offset Link. From there, the user selects the 2 observations to offset in position angle and specifies the Min PA offset and Max PA offset (in degrees).
The Comments field (under the Comments tab) should be used for observing notes.
Lajoie, C-P et al., 2016, SPIE, 99045K
Small-grid dithers for the JWST coronagraphs