- JWST Cycle 1 Proposal Opportunities
- JWST Cycle 1 Guaranteed Time Observations Call for Proposals
- • JWST Director's Discretionary Early Release Science Call for Proposals
- • JWST Call for Proposals for Cycle 1
- James Webb Space Telescope Call for Proposals for Cycle 1
- •JWST Cycle 1 Proposal Checklist and Resources
- •JWST Cycle 1 Proposal Policies and Funding Support
- JWST Cycle 1 Proposal Categories
- •JWST Cycle 1 Observation Types and Restrictions
- •JWST Cycle 1 Proposal Preparation
- •JWST Cycle 1 Single-Stream Proposal Process
- •JWST Cycle 1 Special Submission Requirements
- •JWST Cycle 1 Observation Mode Restrictions
- •JWST Cycle 1 Proposal Selection Process
- •JWST Cycle 1 Awarded Program Implementation
- •JWST Cycle 1 Proposal Science Categories and Keywords
- JWST General Science Policies
- • JWST Observing Overheads and Time Accounting Policy
- • JWST Duplicate Observations Policy
- • JWST Science Parallel Observation Policies and Guidelines
- • JWST Observing Program Modification Policy
- • Policies for the Telescope Time Review Board
- • JWST Target of Opportunity Program Activation
- NASA-SMD Policies and Guidelines for the Operations of JWST at STScI
- •Policy 1 - Limitations on the Use of Funds for the Research of General Observers and Archival Research
- •Policy 2 - Data Rights and Data Dissemination
- •Policy 3 - Data Requests and Facilities
- •Policy 4 - Post-Launch Commissioning of JWST
- •Policy 5 - Clarification of Extensions of Exclusive Access Data to Public Affairs Activities
- •Policy 6 - Distribution of JWST Science Data Obtained from Investigations Other Than Those Selected Through the Peer-review Process
- •Policy 7 - NASA Needs for Support for Other Missions
- •Policy 8 - Definition of Observing Time
- •Policy 9 - Allocation of Guaranteed Observing Time to Scientists Selected Under AO 01-OSS-05 and Through NASA-ESA-CSA Agreements
- •Policy 10 - Redistribution of Guaranteed Observing Time Among Observers
- •Policy 11 - Protection of Science Programs Associated With Guaranteed Time
- •Policy 12 - Education and Public Outreach
- Methods and Roadmaps
- JWST Imaging
- • JWST Slit Spectroscopy
- • JWST Slitless Spectroscopy
- JWST High-Contrast Imaging
- •Contrast Considerations for JWST High-Contrast Imaging
- •JWST Coronagraphic Observation Planning
- •JWST Coronagraphic Sequences
- •JWST Coronagraphy in ETC
- •JWST High-Contrast Imaging in APT
- •JWST High-Contrast Imaging Inner Working Angle
- •JWST High-Contrast Imaging Optics
- •JWST Small Grid Dither Technique
- •MIRI-Specific Treatment of Limiting Contrast
- •NIRCam-Specific Treatment of Limiting Contrast
- •NIRISS AMI-Specific Treatment of Limiting Contrast
- •Selecting Suitable PSF Reference Stars for JWST High-Contrast Imaging
- JWST Integral Field Spectroscopy
- JWST MOS Spectroscopy
- JWST Time-Series Observations
- •Overview of Time-Series Observation (TSO) Modes
- •Noise Sources for Time-Series Observations
- •Sensitivity of Time-Series Observation Modes
- •Bright limits of Time-Series Observation Modes
- •Preparing Time-Series Observations with JWST
- •Target Acquisition for Time-Series Observations
- •NIRCam-Specific Time-Series Observations
- •NIRISS-Specific Time-Series Observations
- •MIRI-Specific Time-Series Observations
- JWST Moving Target Observations
- •Field of Regard Considerations for Moving Targets
- •Instrument-Specific Considerations for Moving Targets
- •JWST Moving Target Calibration and Processing
- •JWST Moving Target Ephemerides
- •JWST Moving Target Observing Procedures
- •JWST Moving Target Policies
- JWST Moving Targets in APT
- •JWST Moving Targets in ETC
- •JWST Moving Target Useful References and Links
- •Overheads for Moving Targets
- •Moving Target Recommended Strategies
- JWST Parallel Observations
- • JWST Target of Opportunity Observations
- • General Proposal Planning Workflow
- Observatory Functionality
- • JWST Position Angles, Ranges, and Offsets
- • JWST Instrument Ideal Coordinate Systems
- JWST Background Model
- • JWST Guide Stars
- • JWST Mosaic Overview
- • JWST Dithering Overview
- JWST Duplication Checking
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- •JWST Observing Overheads Summary
- •JWST Slew Times and Overheads
- JWST Instrument Overheads
- Observing Overheads for NIRCam Imaging
- • JWST Data Rate and Data Volume Limits
- Observatory Hardware
- • JWST Observatory Overview
- • JWST Observatory Coordinate System and Field of Regard
- • JWST Field of View
- • JWST Orbit
- JWST Spacecraft Bus
- • JWST Pointing Performance
- • JWST Telescope
- • JWST Wavefront Sensing and Control
- • JWST Momentum Management
- • JWST Integrated Science Instrument Module
- • JWST Solid State Recorder
- • JWST Target Viewing Constraints
- • Fine Guidance Sensor, FGS
- Astronomers Proposal Tool
- • JWST Astronomers Proposal Tool Overview
- • APT Proposal Information
- APT Targets
- • APT Observations
- • APT Visit Splitting
- JWST APT Coordinated Parallel Observations
- • JWST APT Pure Parallel Observations
- • APT Target Acquisition
- JWST APT Mosaic Planning
- • APT Special Requirements
- • APT Visit Planner
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- • JWST APT Target Confirmation Charts
- • APT Submitting Your JWST Proposal
- JWST APT Functionality Examples
- • JWST APT Help Features
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- Other Tools
- Mid Infrared Instrument
- • MIRI Overview
- MIRI Observing Modes
- MIRI Instrumentation
- MIRI Operations
- MIRI Target Acquisitions
- MIRI Dithering
- MIRI Mosaics
- •MIRI MRS Simultaneous Imaging
- MIRI Time Series Observations
- MIRI Predicted Performance
- MIRI APT Templates
- MIRI Observing Strategies
- MIRI Example Programs
- •MIRI Coronagraphy of GJ 758 b
- MIRI and NIRSpec Observations of SN1987A
- •MIRI and NIRCam Coronagraphy of the Debris Disk Archetype around Beta Pictoris
- •MIRI IFU and NIRSpec Observations of Cas A
- Near Infrared Camera
- • NIRCam Overview
- NIRCam Observing Modes
- NIRCam Instrumentation
- •NIRCam Field of View
- •NIRCam Modules
- •NIRCam Optics
- •NIRCam Dichroics
- •NIRCam Pupil and Filter Wheels
- •NIRCam Filters
- •NIRCam Coronagraphic Occulting Masks and Lyot Stops
- •NIRCam Filters for Coronagraphy
- •NIRCam Grisms
- •NIRCam Weak Lenses
- NIRCam Detectors
- NIRCam Operations
- NIRCam Dithers and Mosaics
- •NIRCam Coronagraphic PSF Estimation
- •NIRCam Coronagraph Astrometric Confirmation Images
- •NIRCam Apertures
- NIRCam Target Acquisition Overview
- NIRCam Predicted Performance
- NIRCam APT Templates
- NIRCam Observing Strategies
- NIRCam Example Programs
- NIRCam Imaging and NIRISS WFSS of Galaxies Within Lensing Clusters
- •NIRCam Coronagraphy of HR8799 b
- •NIRCam Deep Field Imaging
- NIRCam Grism Time-Series Observations of GJ 436b
- NIRCam Time-Series Imaging of HAT-P-18 b
- •NIRCam WFSS Deep Galaxy Observations
- •NIRCam and MIRI Coronagraphy of the Debris Disk Archetype around Beta Pictoris
- Near Infrared Imager and Slitless Spectrograph
- • NIRISS Overview
- NIRISS Observing Modes
- NIRISS Instrumentation
- NIRISS Operations
- NIRISS Predicted Performance
- NIRISS APT Templates
- NIRISS Observing Strategies
- NIRISS Example Programs
- NIRISS WFSS and NIRCam Imaging of Galaxies Within Lensing Clusters
- NIRISS AMI Observations of Extrasolar Planets Around a Host Star
- NIRISS SOSS Time-Series Observations of HAT-P-1
- Near Infrared Spectrograph
- NIRSpec Overview
- NIRSpec Observing Modes
- NIRSpec Instrumentation
- •NIRSpec Optics
- •NIRSpec Dispersers and Filters
- NIRSpec Detectors
- •NIRSpec Micro-Shutter Assembly
- •NIRSpec Integral Field Unit
- •NIRSpec Fixed Slits
- NIRSpec Operations
- NIRSpec Dithers and Nods
- NIRSpec MOS Operations
- NIRSpec IFU Operations
- •NIRSpec FS Operations
- •NIRSpec BOTS Operations
- NIRSpec Target Acquisition
- NIRSpec Predicted Performance
- NIRSpec APT Templates
- NIRSpec Multi-Object Spectroscopy APT Template
- •NIRSpec MOS Proposal Checklist
- •NIRSpec MSA Planning Tool, MPT
- NIRSpec MPT - Catalogs
- •NIRSpec MPT - Planner
- NIRSpec MPT - Manual Planner
- •NIRSpec MPT - Plans
- •NIRSpec MPT - Parameter Space
- •NIRSpec MSA Spectral Visualization Tool Help
- •NIRSpec Observation Visualization Tool Help
- •NIRSpec IFU Spectroscopy APT Template
- •NIRSpec Fixed Slit Spectroscopy APT Template
- •NIRSpec Bright Object Time-Series APT Template
- •NIRSpec FS and IFU Mosaic APT Guide
- NIRSpec Multi-Object Spectroscopy APT Template
- NIRSpec Observing Strategies
- •NIRSpec Background Recommended Strategies
- •NIRSpec Bright Spoilers and the IFU Recommended Strategies
- •NIRSpec Detector Recommended Strategies
- •NIRSpec Dithering Recommended Strategies
- •NIRSpec MOS Recommended Strategies
- •NIRSpec MSA Leakage Subtraction Recommended Strategies
- •NIRSpec Target Acquisition Recommended Strategies
- NIRSpec Example Programs
- NIRSpec and MIRI Observations of SN1987A
- •NIRSpec and MIRI IFU Observations of Cas A
- NIRSpec Bright Object Time Series Observations of GJ 1214b
- NIRSpec MOS Deep Extragalactic Survey
- •NIRSpec MOS Observations of NGC 346
- Understanding Data Files
- Obtaining Data
- Data Processing and Calibration Files
- JWST Data Reduction Pipeline
- • Primer and Tutorials
- • Pipeline User's Guide
- • Software Reference Documentation
- Algorithm Documentation
- • Obtaining and Installing Software
Pre-launch recommendations are available for astronomers planning coronagraphic observations with JWST's Mid-Infrared Instrument.
Note that what follows are pre-launch recommendations that will be updated as the knowledge about the instrument's optimal usage and calibration progresses.
Parent page: JWST Observing Guidance → JWST Recommended Strategies → MIRI Observing Strategies
See also: MIRI Observing Strategies, JWST Coronagraphic Observation Planning,
as well as relevant science use case examples MIRI Coronagraphy of GJ 758 b and MIRI and NIRCam Coronagraphy of the Debris Disk Archetype around Beta Pictoris.
The Mid-Infrared Instrument (MIRI) on board JWST is equipped with 4 coronagraphs that will enable cutting-edge science at small inner working angles (IWAs). In order to prepare for such high-contrast imaging (HCI), observers will have to decide on the choice of filter and focal plane mask, target acquisition strategy, and order of observations, among others.
Planning a coronagraphic observation with MIRI
Which coronagraph (or no coronagraph)?
MIRI incorporates two coronagraphic imaging architectures, each with their own advantages and drawbacks. The coronagraphic focal plane mask assembly comprises of three 4-quadrant phase masks (4QPM) and a traditional coronagraphic occulting Lyot spot. Each occulting mask has a corresponding wavelength range and associated IWA.
The intended science will ultimately determine the choice of coronagraph, depending on the wavelength(s) of interest and, to some extent, on contrast and separation of the science target. When assessing feasibility, you may need to use models to extrapolate shorter wavelength measurements in the 3 to 23 µm range to determine companion contrasts at MIRI wavelengths.
The MIRI Lyot spot has a radius of 2.16", which is 3 λ/D in projected radius at 23 µm, and is suspended in the focal plane by two supporting struts in the mounting bracket, which themselves block light in the field of view (FOV). The Lyot coronagraph works only in a broad band centered at 23 μm, provided to maximize the sensitivity on colder and extended objects (cold silicates in circumstellar disks, for instance). It provides an inner working angle (IWA) of ~3 λ/D, making it useful for detecting objects, structures, or diffuse emission at an apparent separation of ≥2.16" from their host. This could include the outer regions of protoplanetary and debris disks, extended structures around post-AGB stars or galaxies, and scattering/ionization "cones" of AGN. For companions very close to their host (within 3 λ/D), observers should consider using the 4QPMs.
Four-quadrant phase masks (4QPMs)
MIRI's 4QPMs are able to reduce the IWA to near 1 λ/D, but at the price of strong sensitivity to optical aberrations and source misalignments. Another defect is the partial attenuation of a point source when it lies in-between two adjacent quadrants. The MIRI 4QPMs are monochromatic, working only in narrow bands (R = 20) centered at 10.65 μm, 11.4 μm, and 15.5 μm. These masks are optimized for detecting mid-IR features in gas giant and terrestrial atmospheres: searching for absorption in the Ammonia (NH3) band at 10.65 µm; measuring the adjacent continuum at 11.4 µm and the longer wavelength continuum at 15.5 µm. They are also useful for studying a variety of other objects, structures, and emissions very close to bright point sources such as the inner regions of debris disks, very tight binary systems, or the near-nuclear environments of AGN.
Utilizing the Coronagraphic Visibility Tool (CVT)
The JWST Coronagraphic Visibility Tool (CVT) was created as a resource to assist in the pre-planning and observational strategizing of MIRI (and NIRCam) coronagraphic observations. The GUI-based tool provides the visibility and allowed position angles (PAs) for a given target across the year and is also particularly useful for visualizing the coronagraphic field of view and its physical limitations (as mentioned above). Observers should use the CVT to evaluate the feasibility of their observations and address the following questions: will scheduling be an issue for my target? What are the restrictions on the allowed PAs and time for my observations? What about the roll flexibility for this observing period? Are there any restrictions that I need to place on my observations?
Selecting a PSF subtraction strategy
There are 3 complementary PSF subtraction strategies supported by MIRI:
Referenced differential imaging (RDI)
The RDI strategy uses an observation of a reference star to calibrate the coronagraphic point spread function (PSF) and subtracting it from the image of the target star that contains the astrophysical signal. The quality of RDI, however, is sensitive to several factors. This includes thermally-induced wavefront drifts of the observatory that cause the PSF to change from when the science target was acquired, and imperfect target acquisitions of the science and PSF reference targets.
Angular differential imaging (ADI)
Here, two coronagraphic observations of the target are taken that differ by a telescope roll (i.e., taken at different spacecraft orients). Differencing the PSF on the same star as the astrophysical signal rotates introduces diversity between instrument artifacts from the astrophysical signal. This technique is highly effective as it allows PSF subtraction at nearly the same attitude (for wavefront stability), mitigates detector artifacts and also eliminates stellar color mismatch terms. However, ADI is sensitive to self-subtraction biases, especially given the limited available roll (~10º max) of JWST.
Small grid dithers (SGDs)
This technique utilizes a defined set of sub-pixel dithered exposures to optimize coronagraphic PSF subtraction–it's recommended only to be applied to the PSF reference target in cases when the highest suppression of the target star light is needed. SGDs essentially provide a small library of reference images that effectively samples the PSF diversity close to the center of the coronagraph mask. Post-processing optimization algorithms (such as LOCI or KLIP) are then used to construct an optimal synthetic reference to be subtracted from the target star image.
In general, SGDs improve contrast at the coronagraph's IWA by a factor of more than 10 for MIRI. At larger separations, the gain subsides to between 5 and 10. However, while the small grid dither technique enhances performance, it comes at a non-negligible cost; the exposure time is increased by a factor approximately equal to the number of dithered observations. Thus we recommend that the feature is only requested when it is scientifically justified. The 4QPMs, which are very sensitive to alignment differences, are the most likely to benefit most from this option.
Note that because of the limitations and biases associated with both ADI and RDI, for robustness, we advocate for PSF subtraction calibration via both techniques for standard coronagraphic observations, especially now at pre-launch, when predictions are necessarily imperfect. Furthermore, because the JWST PSF will vary in time from wavefront thermal evolution, all science and PSF calibration observations are required to be taken back-to-back (i.e., in a non-interruptible sequence). The inference from this is that the science target and PSF reference target (for RDI and SGDs) must be schedulable in the same visibility windows (you will need to use the CVT).
Choosing a reference PSFs target
The JWST PSF is expected to be time variable, which has important consequences on the choice of PSF reference targets. Their observation is crucial in MIRI coronagraphic observations and will be used to calibrate wavefront errors and other uncertainties in the TA process. By STScI policy, observers are required to define a reference PSF within every program. To ensure effective PSF subtraction, whilst also minimizing overheads and potential contrast losses, the observer should follow these guidelines when selecting their PSF reference(s):
- Choose a known "good" PSF reference – "good references" are usually stars that are not astrophysically contaminated (i.e., without additional astrophysical signal from a debris disk or companion.)
- To mitigate thermal changes, choose a reference star in as close temporal and physical proximity to the science target as is feasible. A general guideline is to find a reference star within about 20° of your science target.
- Choose a reference star that is spectro-photometrically similar to the science target. Choosing a star that is of similar brightness or greater than the science target will ensure an equal or greater signal-to-noise for the reference star, in a similar or shorter amount of total integration time.
- The PSF reference star must observable within the same visibility window as the science target, at the desired time of the observation. Observers can confirm this by utilizing the CVT.
A useful tool to aid in the selection of a good PSF reference target is the Jean-Marie Mariotti Center (JMMC)'s SearchCal, a GUI that allows to select suitable, non-resolved calibrators using a number of search criteria.
The roll capacity of JWST is limited at a given time; typically 10° total (±5° off nominal), but larger rolls can be obtained between different epochs depending on the target position on the sky. In coronagraphic observations, telescope rolls are used in two applications:
Angular differential imaging
This type of roll was mentioned previously to obtain a self-reference for PSF subtraction; it also provides better sampling and increased robustness against detector defects or artifacts by rotation of the PSF. In this case, the roll angle must be sufficient to prevent self-subtraction of astrophysical targets of interest (e.g., for point source, the rotation must move the point source by at least one PSF FWHM). This technique is wavelength dependent, thus longer wavelengths are associated with larger IWAs. For MIRI at 10.65 μm, the minimum separation to avoid self-subtraction of a point source is obtained for an IWA of 1.9" with the typical ±5° roll. At 15.5 μm, this IWA becomes 2.8". For all science observations at separations below this IWA, the rolled science target cannot be used as a self-reference in PSF subtraction unless the second observation is taken at a roll difference greater than 10° (and, therefore, at a significantly later epoch).
Moving astrophysical targets of interest off of PSF structures
These include structures such as diffraction spikes, or axes of the 4QPM masks, obtained at two orientations. However, this typically requires rolls larger than 10° and therefore an observation at a later date, albeit with more significant changes in the wavefront error compared to back-to-back observations. As such, we recommend the second observation require its own PSF reference star to calibrate these wavefront changes. The CVT can be useful in planning for larger roll offsets to assess the availability of multiple position angles and to estimate the time separation between observations.
Target acquisition will be required for all coronagraphic observations with MIRI to achieve the pointing requirements for optimal contrast and performance. The acquisition filters available to users for all coronagraphic imaging modes are F560W, F1000W, F1550W, and the neutral density filter (FND). The FND provides the strongest flux attenuation and is recommended (especially for the 4QPMs) to avoid saturation and persistence when observing bright targets.
In general, users should consider using the FAST readout mode; however, for TA with the Lyot coronagraph concerning fainter stars (where longer than the shortest integration times are needed), FASTGRPAVG should be used. Users must determine the exposure time required to obtain a sufficient signal-to-noise for the TA procedure to achieve the desired centroid accuracy. In order to achieve the centroid accuracy requirements for MIRI target acquisition, the minimum required integrated (within the extraction aperture) signal-to-noise ratio (SNR) must be ≥30. Saturation can also affect the accuracy of the centroiding procedure, and should be avoided.
The TA can be achieved in any of 4 locations concentrically distributed about the center of each coronagraphic subarray. It is generally recommended to select the quadrant in which the companion source of interest does not reside. If this is not an option, or if performing TA on bright targets, performing a "SECOND EXPOSURE" may be considered (see below). If selected, the observation will be repeated with the target acquisition performed in the diagonally-opposite quadrant (i.e. 1→3, 2→4, 3→1 and 4→2).
There are two effects that make target acquisition with MIRI’s coronagraphs complex: (1) the phase masks can distort the image of a star close to its center, undermining the centroid determination; and (2) the detector's arrays have latent images that could mimic astronomical phenomena.
Avoiding latent images
Slowly decaying latents can produce residual signals that will interfere with the centroid measurement algorithm and further limit the accuracy of any TA procedure around bright stars. If the user decides to utilize a single position for TA, we recommend performing TA as far out as possible and keeping in mind the corresponding slew accuracy (see below), in order to avoid strong latent images close to science targets. Estimates indicate that this position can be as far as 4 arcseconds away and suffer minimal performance degradation. Performing TA at smaller angular separations may imprint long-duration latents and undermine the entire rationale for the MIRI coronagraphs of observing expolanets down to λ/D from their stars. To mitigate this, observers can consider using the SECOND EXPOSURE strategy; here two observations are taken: one with TA using the 1st ROI and second with TA using a 2nd ROI that is diagonally opposed to the first one. This will allow for discrimination of persistence images because they are time variable, but at the cost of presicion in the position repeatability.
Avoiding centroid error
In order to minimize the effect of centroid error on TA induced by the 4QPM, it is recommended for observers to avoid the 4QPM axes (which can introduce errors as large as 100 mas on the centroid measurement), and aim for regions with the smallest errors, such as on the diagonal.
The error on the observatory slews is also an important factor during TA. Observers must therefore try to find a compromise between large slews, which have inherently larger errors, and close-in TA, with inherently larger error on the centroid induced by the 4QPM and possible contamination from latent images at the location of scientifically interesting objects. In general, we recommend performing target acquisition no closer than 500 mas to the 4QPM center and no further than ~750 mas.
If using SECOND EXPOSURE TA, the centroid measurements of the two symmetric acquisitions are averaged and used to make the final slew to the center of the coronagraph. This results in smaller mean offsets when compared to scenarios involving a single TA, especially for the 4QPMs since the two acquisitions help average out the symmetrical effect of the 4QPM on the centroid error. However, this technique requires more slews, which translate in a larger final dispersion of the pointings. Performing a single acquisition yeilds the best repeatability, albeit less accurate centering, making it the favourable TA procedure for satisfying the pointing requirements for the MIRI 4QPMs. We recommend restrincting the use of the SECOND EXPOSURE strategy to cases where possible latent images will compromise coronagraph science (e.g. searching for planet companions at small angular seperations).
Utilizing the Exposure Time Calculator (ETC)
See also JWST Coronagraphy in ETC.
Estimating your exposure times is a science-critical aspect of observation planning. It is crucial to estimate exposure times for both science observations and target acquisition; in fact, ETC workbook and specific calculation ID #s are now a required field for coronagraphic observations specified within the Astronomer's Proposal Tool (APT, version 25.4.2). For MIRI coronagrpahic observations, the ETC should be used for: (1) investigating detector saturation and (2) computing the SNR of a faint companion source under the ideal contrast assumption. Doing so will provide the observer with the information necessary to design and determine the best observation strategy for their science case.
PSF subtraction in the ETC
See also ETC Coronagraphy Strategy.
Currently for PSF subtraction, the ETC quantifies the shot noise in the wings of either the host or reference sources, as appropriate, at the position of the faint companion source. It estimates the PSF using one or multiple reference images, calling for the choices of reference target and PSF calibration method in the "strategy" tab. It is also here that the user can specify the "Scene rotation", which allows for rotation of the scene with respect to the axes of the MIRI FQPMs. When used in conjunction with the CVT, this feature allows the user to determine the optimal Aperture Position Angle Special Requirements for the scene of interest.
Because the PSF stability is unknown until launch, the ETC does not support ADI or SGD—only RDI.
While the JWST web-based ETC is perfectly adapted and trustworthy for the SNR calculations occuring in the background limited regime, typically at 1" separation and beyond, below 1" and down to the IWA of the coronagraph, the Pandeia-Coronagraphy extension of the ETC's Pandeia engine is recommended for more realistic contrast and detection limit calculations. Note that the IWA (in arcseconds) of the 10.65, 11.4, and 15.5 4QPMs are 0.33", 0.36", 0.49", respectively.
Implementation into APT programs
The APT MIRI Coronagraphic Imaging Template provided for coronagraphic imaging observations with MIRI provides sections for entering information on a variety of parameters including target acquisition, exposure times, special requirements for linking observations, observations of PSF-reference stars, and full-frame astrometric images, if needed. Best practice is to always decide ahead on the observation strategy and observation sequence that will be used for a given pair or set of targets to be observed. The JWST High-Contrast Imaging in APT article provides helpful tips on how to organise observation folders and the observations to which they pertain.
Setting the appropriate Special requirements (SRs) is a crucial step for MIRI coronagraphic observations. At the very least, users will need to place the "SEQUENCE NON-INTERRUPTIBLE" timing SR on their coronagraphic sequence. For observations with the 4QPMs, observers will likely require an enforced APERTURE PA RANGE on their observations to place the companion source of interest optimally with respect to the mask, especially to ensure they are not coincident with any of the 4QPM axes. The ideal orientation of a single companion on the mask is usually ~45° from the 4QPM axes (if available), however this can differ for targets with multiple companions. The CVT can be used to assess the available position angles.
In order to define a roll dither between science target observations, the appropriate APERTURE PA OFFSET SR must be placed on them. If a second sequence at a larger PA offset is needed, the APERTURE PA OFFSET SR must be set between the two sequences; and so forth. For programs requiring this level of attention to detail, observers should use the observing constraints determined from advanced work with the CVT. This will avoid the disappointment of later discovering that the needed angles are impossible.
The implementation of a multi-roll coronagraphic sequence is a non-trivial process and requires an understanding of the roll angles accessible by JWST for a given target. In light of this, we offer the following recommendation: assuming the user requires roll dither of angle α (where α < 14°) and a larger roll offset of angle θ (where θ > 15 °)
- Create a three-observation, NON-INTERRUPTIBLE coronagraph sequence such that: Obs. 1 is of the science target, Obs. 2 is of the science target and Obs. 3 is of the reference star.
- Add a Special Requirement on the sequence such that Obs. 2 has an APERTURE PA OFFSET of +α° with respect to Obs. 1
Create a second three-observation, NON-INTERRUPTIBLE sequence such that: Obs. 4 is of the science target, Obs. 5 is of the science target and Obs. 6 is of the reference star.
If the above is not schedulable, switch the APERTURE PA RANGE on Obs. 4 w.r.t. Obs.1 to [θ−180°,−θ°].
Observation Tool Links
JWST Exposure Time Calculator
JWST Coronagraphy in ETC
JWST ETC Coronagraphy Strategy
JWST ETC MIRI Target Acquisition
JWST ETC MIRI Target Acquisition Sample Workbook
JWST Astronomers Proposal Tool, APT
MIRI Coronagraphic Imaging Template APT Guide
MIRI Coronagraphic Imaging Template Parameters
MIRI Coronagraphic Imaging
MIRI Coronagraph Masks
JWST High-Contrast Imaging Overview
JWST High-Contrast Imaging Optics
JWST High-Contrast Imaging Inner Working Angle
Contrast Considerations for JWST High-Contrast Imaging
High Contrast Imaging with MIRI Coronagraphy
JWST Coronagraphic Observation Planning
JWST Coronagraphic Sequences
MIRI Coronagraphic Imaging Target Acquisition
JWST Small Grid Dither Technique
Select suitable PSF reference stars for JWST High Contrast Imaging
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