MIRI Coronagraphic Imaging APT Template

The JWST APT MIRI coronagraphic imaging template is used to specify parameter settings for the 4 coronagraphic settings in JWST's Mid-Infrared Instrument (MIRI).

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Main article: MIRI Coronagraphic Imaging
See also: HCI RoadmapHCI APT Instructions, MIRI Coronagraphic Recommended StrategiesMIRI and NIRCam Coronagraphy of HR8799 bMIRI and NIRCam Coronagraphy of the Beta Pictoris Debris Disk

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. The design offers a very small IWA in a stable environment. 

An observer will have control over 3 primary parameters for MIRI coronagraphic imaging:  

  1. coronagraph mask/filter combination
  2. dithering type
  3. detector read out mode and exposure time (via the number of frames and integrations).  

Allowed values are documented and maintained in the MIRI Coronagraphic Imaging Template Parameters page, but also described below. 



Step-by-step APT instructions

Generic

The following parameters are generic to all templates, and are not discussed in this article: observation Number, observation Labelobservations CommentsTarget name, ETC Wkbk. Calc ID (in the Filters dialog box), Mosaic Properties, and Special Requirements.

Mosaics are not available for MIRI coronagraphic imaging.

Target Acquisition Parameters

Target acquisition (TA) is required for coronagraphic imaging. The necessary parameters, Target ACQAcq Exposure Time and Acq Quadrant, are available under the MIRI Coronagraphic Imaging tab, in the Target Acquisition Parameters panel.

Target ACQ 

Main article: MIRI Coronagraphic Imaging Target Acquisition

For coronagraphic imaging, the Acq Target is the same as the science target. The user can specify one of these Acq Filters: F560WF1000WF1500W, and FND.

Acq Exposure Time

Main article: Understanding Exposure Times

A Target ACQ must be completed by selecting a MULTIACCUM exposure configuration. Each exposure is configured by setting the readout pattern and characteristics parameters: Acq Readout Pattern and Acq Groups/Int

Users should use the Exposure Time Calculator (ETC) to determine the exposure configuration needed to obtain the required signal to noise for the TA source. A failed target acquisition results in a failure of the observation in question. 

Acq Readout Pattern

Main Article: 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 FASTGRPAVG Detector Readout Capability for details.

Choices for the Acq Readout Pattern are 

  1. FAST (the default)
  2. FASTGRPAVG
  3. FASTGRPAVG
  4. FASTGRPAVG16
  5. FASTGRPAVG32
  6. FASTGRPAVG64

Acq Groups/Int

Main Article: 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.

Acq Quadrant

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). 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).

Coron Parameters 

Main article: 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.

Coron Filter

Main article: MIRI Coronagraphic Filters and Dispersers

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, LYOT/F2300C.

Exposure Time

Users should use the Exposure Time Calculator (ETC) to determine the best exposure configuration to optimize the signal to noise.

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

Readout Pattern

Main article: MIRI Detector Readout Overview

The MIRI coronagraphic imaging template offers only one readout mode:

  1. FAST (only option available)

Number of groups and integrations

See also: Understanding JWST Exposure Times

The MIRI timing pattern per exposure is defined by only 2 MULTIACCUM parameters: 

  1. Groups/Int: the number of groups during an integration, where a group is the product of cycling through all the pixels.
  2. Integrations/Intthe number of integrations during an exposure, where integration is defined as the time between resets. 

Dither Type

Main article: MIRI Coronagraphic Imaging DitheringHCI Small Grid Dithers

MIRI coronagraphic imaging supports the following types of small grid dither patterns:

Dither typeNumber of dithers
NONE0
5-POINT-SMALL-GRID5
9-POINT-SMALL-GRID9

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

Repeat Observation

See also: HCI Coronagraphic SequencesHCI APT Coronagraphic Sequence Examples

If YES 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:

First Quadrant
Second Quadrant
13
24
31
42

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 pages). 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

Additional justification

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



Other Tabs

Special Requirements

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 select the 2 observations to offset in position angle, and specify the Min PA offset and Max PA offset (in degrees).

Comments

The Comments field (under the Comments tab) should be used for observing notes.



References

Lajoie, C-P et al., 2016, SPIE, 99045K
Small-grid dithers for the JWST coronagraphs




Published

 

Latest updates

  • "Second Exposure" renamed "Repeat Observation".

  •  
    Typos and errors - ready for Cycle 1