JWST High-Contrast Imaging Roadmap

A roadmap to guide users, step-by-step, through the process of designing a JWST high-contrast imaging (HCI) observing program is provided in this article.

On this page

See also: Getting Started with JWST Proposing

High-contrast imaging (HCI) observations can be some of the most complex to schedule with JWST and for that reason, the workflow of this roadmap is considered iterative. When planning HCI observations many parameters come into play and for some science cases, it is not always initially apparent which HCI mode—if at all—will provide you with the best scientific results; users may find themselves returning to earlier steps and/or stages before "linearly" producing their proposal and Astronomer's Proposal Tool (APT) files. 

The roadmap points below are supplemented with links to articles for additional information.

Stage 1 – Become familiar with HCI capabilities and instrument-specific modes

In addition to the steps suggested in the article Getting Started with JWST Proposing (see the section "Become familiar with JWST capabilities, terminology, and documentation"), users should consider the following issues for high-contrast Imaging.

1. Which JWST observing modes enable HCI?

2. What HCI optical designs are offered by JWST?

Familiarize yourself with the advantages, limitations, and functionality of each HCI design, as well as which scientific investigations they are optimized for.

HCI optics:

3. What are the allowed mask-filter combinations for each of the HCI modes?

4. What are the primary performance metrics for HCI?

HCI MIRI Limiting Contrast
HCI NIRCam Limiting Contrast
HCI NIRISS Limiting Contrast

5. What are the predicted performances of the instrument-specific modes?

  • MIRI, achievable IWAs of 0.33–2.16":  (1 λ/D for 4QPMs, 3.3 λ/D for Lyot) typical contrasts achieve 10-4 to 10-5 for separations larger than 0.5″–1″.
  • NIRCam, achievable IWAs of 0.14–0.89": (round and bar-shaped occulters optimized for 6 λ/D and 4 λ/D, respectively) contrasts typically ∼10−6 or better at 1′′ IWA and beyond.
  • NIRISS AMI, achievable IWAs of 0.089–0.15": (1 λ/D) typical contrasts ~10-4 at separations between ~70–400 mas.
  • Imaging and IFU spectroscopy: (with non-coronagraphic PSF subtraction) achievable contrasts ~10-3 to 10-4 for IWAs somewhere between those of AMI and coronagraphy for a given filter.

6. What are the fundamental physical limits for detection?

  • Photon noise of the stellar point spread function (PSF):

MIRI Point Spread Functions
NIRCam Point Spread Functions
NIRISS Point Spread Functions

  • Detector noise:

MIRI Detector Performance
NIRCam Detector Performance
NIRISS Detector Performance

  • Background noise (zodiacal light + thermal emission), especially longward of ~15 μm.

7. What are the operations unique to HCI?

8. What are the recommended observing strategies pertaining to HCI?

JWST Coronagraphic Sequences
MIRI Coronagraphic Recommended Strategies  
NIRCam Coronagraphic Recommended Strategies 
NIRISS AMI Recommended Strategies

9. Are there any features and characteristics that observers should be aware of when planning, reducing, analyzing, and interpreting HCI observations? 

Cycle 3 - Temporary restrictions on MIRI coronagraphic observations

Commissioning of the coronagraphs revealed some issues that will affect Cycle 3 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 been verified for the neutral density (FND) TA and the F1500C filter. The MIRI team continues to verify TA with the other 2 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 coronagraph. 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 for Cycle 3. Please contact the JWST Help Desk if science requires TA in a different quadrant.

  • 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. 

* HCI can be carried out using basic imaging modes of the observatory (Rajan et al., 2015Durcan, Janson, and Carson, 2016), as well as using IFU strategies similar to Konopacky et al. (2013), however, these modes are not yet covered in the documentation.

‡ based on JWST/MIRI coronagraphic performances as measured on-sky (Boccaletti, A. et al. 2022)

Stage 2 – Compare your parameter space to the performance limits and capabilities of HCI observing modes

1. Identify wavelength range(s) of interest for your intended science.

How does this influence (or limit) your choice of science instrument(s), mask(s) and filter(s)?

2. Determine the apparent separations between host and companion source(s) at the time of observation.

Which instrument(s) and mask-filter combination(s) can achieve the required working angles?

3. Determine the companion contrast(s) at the wavelength(s) of interest.

Are your observations feasible given the contrast limits of the instrument(s)?

Note: when referring to a companion, the term "contrast" corresponds to the ratio of the companion's observed flux to that of its host. An observation is estimated to be feasible if the companion-to-host flux ratio is greater than the "limiting contrast" Climit(s).

Modeling may be required to extrapolate shorter wavelength measurements in the 3–23 μm regime for these predictions (e.g., to determine companion contrasts at MIRI wavelengths from far infrared or submillimeter data). 

Contrast considerations for JWST HCI

HCI MIRI Limiting Contrast
HCI NIRCam Limiting Contrast
HCI NIRISS Limiting Contrast

4. For coronagraphic observations, how important is the azimuthal coverage around your science target? 

MIRI coronagraphic imaging:

  • with the 4QPMs, the linear boundaries between adjacent quadrants attenuate light, reducing sensitivity in the field along the 4 edges of the mask.
  • with the Lyot coronagraphthe Lyot spot is suspended in the focal plane by 2 supporting struts in the mounting bracket, which themselves block light in the FOV.

NIRCam coronagraphic imaging:

  • the round occulting masks provide 360azimuthal coverage around the bright object.
  • the bar occulting masks sacrifice some FOV in the direction along the bar, as a function of azimuth around the bright object.

5. Is it possible that your scientific goals can be achieved with non-coronagraphic PSF subtraction? 

 For moderate contrasts (~ 10–3 to 10–4) and/or point source detections well in the background limited regime, it might be wise to opt for one of the standard imaging modes.

JWST Imaging

Stage 3 – Select a PSF calibration strategy

All HCI observations with JWST require the measurement and calibration of stellar point spread functions (PSFs) in some way for post-processing contrast reduction. For any PSF calibration strategy, the observing and data processing techniques are interdependent.

Coronagraphic PSF subtraction strategies

In order to achieve the necessary high contrast and recover faint sources surrounding the science target, one must calibrate and subtract the PSF of the central source

1. Consider the degrading factors that may limit the PSF calibration and what steps you will take to mitigate them. 

These include wavefront drifts of the observatory, PSF star color differences, self-subtraction biases (especially for disks), imperfect target acquisitions, line-of-sight jitter, and dynamic wavefront error.

2. Which observing technique(s) will you include in your PSF subtraction strategy?

Each PSF calibration and subtraction method has a corresponding observing strategy. The imaging techniques are combined with post-processing optimization algorithms (such as LOCI§ and KLIP) to generate an optimal synthetic reference PSF, to be subtracted from the science target image. 

Will you employ the "referenced differential imaging" (RDI) technique? (Required)

In this techniquethe observation of a nearby star is used to generate an unresolved, high signal-to-noise (SNR) PSF to subtract from the science target.

(minus) The RDI technique is sensitive to wavefront drifts and PSF star color differences.

By scheduling the science and PSF reference observations in a non-interruptable sequence, the effect of wavefront drifts should be minimized.

Will you employ the "angular differential imaging" (ADI) technique? (Recommended) 

In this technique, the science target is observed at 2 different roll angles and is used as a self-reference for PSF subtraction. 

(plus)  ADI allows for PSF subtraction at nearly the same spacecraft attitude (for wavefront stability) and helps mitigate detector artifacts.

(minus)  However, ADI comes at the cost of self-subtraction biases, especially given the limited available roll (~10°) of JWST.

For robustness, observations using both RDI and ADI PSF calibration techniques are currently recommended. You can deviate from this plan if desired, but you must explain your alternate PSF subtraction strategy in your proposal.

More information here: standard coronagraphic sequence.

Will you employ the "small grid dithering" (SGD) technique? (Optional)

This technique involves performing subpixel dithers of the PSF reference target, to build a mini library of reference images that effectively samples the PSF diversity close to the center of the coronagraphic mask.

(plus) The SGD technique can be used to mitigate possible misalignments between science and reference images, reducing the contrast loss from TA (pointing offset) residuals to a negligible level (10x contrast improvement for MIRI, 3-5x improvement for NIRCam).

(minus) However, SGDs come at the cost of (5-9x) longer PSF star exposure times. 

The SGD technique is optional. SGDs have proven to be very useful for the PSF reference star, and is recommended. SGD on the science target can also improve contrast, but experience has shown that it should only be used when the highest quality PSF subtraction is needed.

HCI Small Grid Dither
NIRCam Small Grid Dithers
JWST Coronagraphic Sequences: Use of the small grid dithering technique

Non-coronagraphic PSF subtraction

Using the same PSF subtraction methods, it is also possible to achieve high performance with non-coronagraphic imaging modes, such as direct imaging in filters that may not have coronagraphs available, or using one of JWST’s integral field spectrographs in NIRSpec or MIRI. The contrasts achieved with such modes, even with careful PSF calibration, will not equal the contrasts achieved with the coronagraphs, but even “moderate” contrasts can still offer compelling science capabilities. Such observations are already planned for Cycle 1 by both GTO and ERS teams.

Interferometry calibration strategies 

For NIRISS AMI, the observations of calibrator stars are used to measure, then remove, instrument systematics. The PSF (or interferogram) produced by the NRM has a narrow central core which is surrounded by an extended skirt created by interference between pairs of apertures. These fringes in the outskirts of the NRM PSF are easily measurable due to their relative brightness and wider angular extent, making instrumental effects easier to calibrate out of science data.

Which observing technique(s) will you include in your PSF calibration strategy?

Will you observe a PSF reference calibrator?

The observation of a calibrator star allows for the instrumental systematics (affecting interferometric observables) to be measured. The reference PSF is used to calibrate out the instrumental contributions to closure phases (CP; the sum of three phases around a closed triangle of baselines) and squared visibility amplitudes (SqV), during the third stage in the calibration pipeline (calwebb_ami3).

Near-contemporaneous acquisition of target and point source calibrator data is desirable, except for very low contrast needs: if contrast limits are not very demanding, a reference star from an unrelated observation, or possibly an analytically generated reference PSF can be used.

HCI NIRISS Limiting Contrast
NIRISS AMI Recommended Strategies

Will you employ the Kernel phase imaging technique?

In this technique, one or more direct images using the CLEARP aperture and the same suite of FW filters as those used for the NRM images are obtained for PSF characterization.

§ “locally optimized combination of images” or LOCI algorithm (Lafrenière et al. 2007)

KL image projection (KLIP) algorithm (Soummer et al. 2012)

Stage 4 – Assess target visibilities and allowed position angles 

The following steps should be used in conjunction with those outlined in the Getting Started with JWST Proposing (see the section, "Determine if your targets can be observed").

1. Familiarize yourself with JWST position angles, coordinate systems, and related nomenclature to understand the telescope’s pointing constraints.

JWST Position Angles, Ranges, and Offsets
JWST Observatory Coordinate System and Field of Regard
JWST Field of View
JWST Instrument Ideal Coordinate Systems

2. Determine the viewing constraints placed on your target(s).

JWST Target Viewing Constraints

3. Using at least one of the JWST target visibility tools, assess your target visibilities, as well as allowed position angles versus time.

JWST Target Visibility Tools
JWST General Target Visibility Tool (for all instrument modes)
JWST Coronagraphic Visibility Tool (for coronagraphic modes)
Synergy between the target visibility tools

4. In the case of known or expected companions, consider whether your observations require any restrictions on the orientation of the instrument field of view (FOV)/ detector being referenced.

This is especially in relation to any instrumental obstructions, such as the cross pattern for the MIRI FQPM, bars for NIRCam, or outside of gaps in the uv-pane coverage for NIRISS.

For coronagraphy, the CVT provides visualizations of the focal plane projected onto the sky, which is useful for evaluating the placement and orientation of known science sources on the coronagraphic masks.

Using the CVT: Adding companions to the primary target

Determine the aperture position angle(s)/ date(s) at which your companion(s) are nominally visible.

JWST Position Angles, Ranges, and Offset

5. If implementing the ADI technique in your PSF calibration strategy (during the Select a PSF calibration strategy stage):

Check how the instantaneous roll flexibility changes over the the particular visibility period.

This instantaneous roll flexibility is approximately ±5° from nominal, but varies with time and look direction between ±3.5° to ±7°.

JWST Target Viewing Constraints.

Assess how potential rolls of the telescope will change the positions of the companion(s) relative to any instrumental obstructions.

JWST Position Angles, Ranges, and Offsets

6. In the case of coronagraphy, consider whether your goals call for a larger roll offset on the science target than can be obtained instantaneously in a single visibility period.

Certain science cases may require a follow-up at some more substantial angular offset (e.g., 30° offset) relative to the first observations—for instance, to recover part of the scene that may be blocked by the selected mask. In such cases, the observation will have to be scheduled at a significantly later time. 
Coronagraphic observers will want to assess their potential targets carefully, and when possible, select targets above 45° ecliptic latitude if they require large offsets in PA between observations.

The Coronagraphic Visibility Tool (CVT) can be used to assess the availability of multiple position angles, and estimate what the time separation will be. It is expected that each sequence will contain an observation of a relevant PSF reference star, since the PSF will likely change between the two epochs.

Stage 5 – Use the Exposure Time Calculator to determine observing parameters 

Estimating exposure times is a science-critical aspect of HCI observation planning. Once target visibility is confirmed and a PSF calibration strategy adopted, the JWST Exposure Time Calculator (ETC) should be used to determine the exposure parameters needed to achieve the desired signal to noise (SNR) on your target(s). Aside from the directions in the Getting Started with JWST Proposing article, the following are advisable for HCI:

1. Define your scenes and sources. 

Create a science scene in the ETC and populate it with the source targets.

The science scene should contain the source targets for observation and all other nearby sources that could contribute to both the observed target and background fluxes. The bright ("host") source should be placed in the center of the scene and the reference PSF source(s) at a significant offset (e.g., 10").

At this stage, a copy of the bright target can be used as a proxy for the reference PSF source—that is, until a physical PSF calibrator has been identified (see Select a suitable PSF calibrator). 

Faint sources must be placed within a square centered on the scene center for each instrument/mode pairing.

  • for MIRI the scene is a 8.91" square centered on the coronagraphic masks.
  • for NIRCam the scene is a 3.2" (for SW) or 6.4" (for LW) square centered on the coronagraphic masks.
  • for NIRISS the scene is a 5.31" square centered on the NRM.

Create a reference scene and populate it with the reference PSF source.

In order to facilitate target acquisition calculations for the reference PSF source, a dedicated "reference" scene—containing only the reference PSF target—is required. The reference PSF source should be positioned at the center of the scene (offset 0,0).

2. Initiate calculations for each of your planned observations.

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

Calculations are performed from the Calculations page where the user specifies the desired input parameters in the Configuration pane: (1) instrument and mode, (2) scene, (3) background model parameters, (4) instrument configuration, (5) detector setup, and (6) the strategy for calculation of the SNR (and contrast for coronagraphic modes). 

JWST ETC Creating a New Calculation
HCI ETC Instructions
JWST ETC Coronagraphy Strategy
JWST ETC Imaging Aperture Photometry Strategy

3. Adjust the exposure time via the number of groups, integrations, and/or exposures until you obtain the desired SNR and contrast on your target.

Understanding Exposure Time
MIRI Generic Recommended Strategies: How many groups and integrations should I use?
NIRISS AMI Recommended Strategies: Estimating number of groups for an exposure

For NIRISS AMI, there is an advanced Python tool and precomputed contrast curves found at https://github.com/agreenbaum/ami_simenter image description here and hbp://maestria.astro.umontreal.ca/niriss/AMIcontrast/index.phpenter image description here, respectively.

4. Check your individual calculations for detector saturation.

Check for saturation using the Saturation tab in the Images pane on the Calculations page in the ETC.

While some saturation may be tolerable, only partially-saturated pixels will be recoverable. In saturated regions, the photometric accuracy will be suboptimal and the contrast will most likely be affected at or close to the IWA; consequently, faint portions of the astronomical scene that overlap with saturated pixels may not be properly detected.

JWST ETC Reports
JWST ETC Calculations Page Overview
JWST Coronagraphy in ETC: avoiding saturation

Using the Saturation Map, check if any saturated pixels overlap with faint sources/features in the astronomical scene.

In the event of saturation, it may be possible to recover pixels at the expected position of the companion by modifying the exposure parameters: proceeding by trial and error, vary the readout pattern, Ngroups and/or Nints until all pixels at the expected position of the companion are no longer saturated.
JWST ETC Images and Plots

5. Initialize target acquisition (TA) calculations for each of your observations.

All HCI observations will require a science instrument assisted TA procedure—this includes both science and reference PSF observations.

JWST ETC MIRI Target Acquisition
JWST ETC NIRCam Target Acquisition
JWST ETC NIRISS Target Acquisition

6. Run your TA calculations and examine the output information.

JWST ETC Outputs Overview

Does any saturation occur?

Saturation should be avoided during target acquisition for optimal performance. If any fully or partially saturated pixels are present in the TA exposure, the ETC will issue a warning. The recommendation is to adjust your exposure parameters (e.g., by decreasing the number of groups) to avoid saturation.

MIRI Outputs for TA
NIRCam Saturation limits for TA
NIRISS Outputs for TA

Does your exposure specification allow you to obtain the minimum required SNR for the TA procedure of the instrument mode?

Stage 6 – Select a suitable PSF calibrator

If you have established the need for a PSF reference target according to your PSF calibration strategy designed (see Select a PSF calibration strategy), this section is relevant. Otherwise, you may skip this stage and finalize your observing strategy.

1. Select a PSF reference calibrator with consideration of the following criteria:

Well-known: is the target a known good PSF reference star? 

Selecting a reference PSF source that has been previously observed interferometrically/ coronagraphically (or from the ground with adaptive optics) and found to be single, is recommended. "Good references" are usually stars that are not astrophysically contaminated (i.e., without additional astrophysical signal from a debris disk or companion).

There exist a handful of external resources that can help in the search for a known PSF reference target. See Selecting PSF reference stars with Simbad and Selecting PSF reference stars with SearchCal for more information.  

Schedulability: do the visibility windows of the science target and PSF calibrator overlap at the time of the desired observation?

Unless on-orbit experience shows that the need for contemporaneous imaging can be relaxed, the JWST project requires observations of the science target and PSF reference star to be executed together, in a non-interruptable sequence of observations.

Users should aim to observe the science and PSF reference observations as close together in time as possible, in order to minimize changes in the PSF and obtain the lowest possible limiting contrast.

Checking visibility of PSF reference stars   
Coronagraphic Visibility Windows  
JWST Wavefront Sensing and Control

Proximity: is the PSF calibrator in relative proximity to the science target? 

This is needed in order to limit thermal changes and minimize telescope overheads. 

HCI PSF Reference Stars
JWST Slew Times and Overheads

Avoidance of Binary: is the PSF calibrator a single and unresolved source?

This can be addressed by selecting a known good PSF reference star. If the PSF reference star is not well known and/or has not been previously observed with high spatial (e.g.,  < 0.1") resolution imaging or interferometry, performing further archive checks or seeking another PSF reference star is recommended.

HCI PSF Reference Stars

Spectral Type: does the PSF calibrator share the same spectral properties as the science target? 

This has a stronger impact at shorter wavelengths and with wider filters. Spectral mismatch may generate extra noise during the process of photometrically rescaling the reference and causing possible under- and over-subtraction of the PSF. 

Effect of spectral "mismatch"
High-Contrast Imaging Inner Working Angle

Brightness: is the PSF calibrator similar in magnitude to the science target? 

Whenever possible, the use of a reference PSF that is brighter than the science target is recommended because the process of flux rescaling also scales the noise. Selecting a calibrator that is as bright as (or brighter) will help achieve the same signal-to-noise ratio in comparable exposure times. 

Effect of spectral "mismatch"

2. Return to your previous ETC workbook to amend the spectral properties of the reference PSF source and finalize the exposure parameters of your calculations.

Stage 7 – Finalize your observing strategy

In previous stages, you have made a series of choices concerning the content of your observing program—in this stage, you will decide on an observing strategy with which to structure this content. This observing strategy should be designed to mitigate performance degradation and yield the best possible scientific results, with the least possible overheads.

MIRI Observing Strategies
NIRCam Observing Strategies
NIRISS Observing Strategies

1. Consider the total number of observations you will require for your observing program.

Note that PSF reference observations should be observed using the same telescope optical configuration so that no wavefront correction should occur between any of the observations. 

The inference of the above is that observations in different filters require individual PSF reference observations.

2. At the observation level: consider how you will organize (group) your observations.

Observations that need to be executed together in time should be grouped together in "sequences." Details of these sequences will depend on the science goals of your program. 

For all HCI modes, science and PSF reference star observations must be grouped into sequences—the goal is to minimize changes in the optics that might alter the PSF between observations. 

Science and reference PSF observations

HCI Coronagraphic Sequences 
NIRISS AMI Recommended Strategies: Observing calibrator(s)

3. If your sequence of observations involves the use of multiple filters and/or occulters, you should consider following the optimal efficiency scheduling strategy.

With the optimal efficiency strategy, observations for a given target are organized—in each filter and occulter—to minimize the number of rolls and slews. This strategy increases the time between the observation of a target in a given filter and the corresponding reference PSF star observation in the same filter, but it results in more efficient use of the observatory.

With the optimal wavefront stability strategy, standard sequences are executed consecutively in each filter to minimize the chance of any wavefront changes, which comes at the cost of an increased number of slews and rolls for the telescope. 

JWST Coronagraphic Sequences: Standard sequence implementation for multiple filters and occulters

Linking too many observations together into a sequence can make the total execution time long, to the point that the observations cannot be scheduled. Therefore, you should seek to strike a balance between efficiency and the pragmatic aspects of scheduling observatory activities.

You may find it instructive to inspect the reports that are generated by running Smart Accounting in APT. These files provide a more detailed breakdown of where various overheads are being charged and will help you understand the tradeoffs in efficiency for the different models. See the articles on the APT Visit Planner and APT Smart Accounting for more information.

4. Do your observations call for a more substantial position angle offset (e.g., 30° offset) on the science target than can be provided instantaneously in a single visibility window?

For instance, this could be needed to recover a part of the scene that would otherwise be blocked by a selected mask. The possibility of such an offset depends strongly on the ecliptic latitude of the target and must be scheduled at a significantly later time. The CVT can be used to help assess this. The special requirements for such an offset should only be used when truly necessary for the science.

JWST Coronagraphic Sequences: Larger roll offset case

5. If your program consists of a set of science targets that are clustered on the sky in close proximity and schedulable at the same time, is it possible to implement the "shared reference survey strategy"?

This is where multiple science targets are paired with an individual PSF observation, in the normal coronagraphic sequence.

JWST Coronagraphic Sequences: Shared reference survey case
Coronagraphic Visibility Tool

Is it possible to incorporate the "self-referenced survey strategy"?

This is under the assumption that some science targets will be for science, but others—those not showing surrounding structure—will be used for PSF reference observations.

JWST Coronagraphic Sequences: Self-referenced survey case

6. For all coronagraphic imaging programs: it is highly recommended to perform the standard coronagraphic sequence, or a derivative of it.

JWST Coronagraphic Sequences

7. Do your science goals call for high-accuracy astrometry? 

If so, perhaps you should obtain NIRCam images for full-field astrometry (FFA) in addition to your HCI science data.

NIRCam Coronagraph Astrometric Confirmation Image

8. Do your science observations require associated background observations? 

Cycle 3 - Temporary Restrictions on MIRI coronagraphic observations

Commissioning of the coronagraphs revealed some issues that will affect Cycle 3 proposers; for details see MIRI Instrument Features and Caveats.

  • 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. 

MIRI Coronagraphic Recommended Strategies

Stage 8 – Prepare your proposal in the Astronomers' Proposal Tool

Aside from the steps described in the Getting Started with JWST Proposing roadmap, consider the following particular to HCI:

1. Organize science and PSF calibrator observations into sequences. 

You may find it useful to collect all observations that pertain to a particular coronagraphic sequence into a single observation folder, and additional folders for other sequences. 

HCI Coronagraphic Sequences

2. Use the PSF reference observations section to indicate which observations produce PSF references and to specify which science observations they should be linked to.

The PSF reference star must be in the same FILTER and SUBARRAY.

You may find it very helpful to use designations in the Name in the APT proposal field (Fixed Targets form) to clearly indicate which targets are intended for science and which are PSF reference stars, as appropriate. These designations will show up in the pull-down menus in other parts of APT, to help you build up your observation sequences. Furthermore, if you have a large number of science targets and PSF stars to keep track of, you may find it useful to do so using the the comment box.

JWST HCI in APT: Setting links between PSF reference and science observations

3. If excluding the observation of a PSF reference target, justification must be reflected in the Additional Justifiction section.

NIRISS AMI PSF reference observation

4. Do any of your targets require a separate background target (and observation)?

If so, specify a separate background target and observation, and link it to your target observation. 

Specifying APT Background Targets

Cycle 3 - Temporary Restrictions on MIRI coronagraphic observations

Commissioning of the coronagraphs revealed some issues that will affect Cycle 3 proposers; for details see MIRI Instrument Features and Caveats.

  • 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. 

5. Are NIRCam full frame astrometric (FFA) images needed? 

If so, indicate Yes in the Astrometric Confirmation Image parameters template panel and enter the appropriate exposure information for these images.

NIRCam Coronagraphic Imaging APT template: Astrometric Confirmation Image Parameters

6. For coronagraphic imaging modes: Do any of your observations require the small grid dithering (SGD) technique?

This selection can be made in the observation template by choosing the appropriate dither type, in the MIRI template, or dither pattern in the NIRCam template.

Specifying SGDs in AP

7. Add any the necessary special requirements:

Timing Special Requirements
Aperture Position Angle Special Requirements

SEQUENCE OBSERVATIONS... NON-INTERRUPTIBLE to force the Visit Planner to look at the collective schedulability of the entire set. 

Note that APT will execute the observations in a Sequence Observations ... Non-interruptible grouping in the order of increasing observation number. If you drag and drop the order of your observations in the APT tree editor, make sure the desired sequence of observations is still in increasing order of observation number. If it is not, edit the observation numbers so that ordering is achieved.

APT Visit Planner

APERTURE PA OFFSET ... for roll-dithered science target observations. 

Set the offset angle, or offset angle range between two roll-dithered observations. If a second sequence at a larger PA offset is needed, the Aperture PA Offset ... special requirement must still be set between the two sequences. The cases needing this level of attention to detail should be investigated ahead of planning, with a visibility tool (See Assessing target visibilities and allowed position angles).

APERTURE PA RANGE ... fix the allowed degree range of absolute PA on an observation.

This is only necessary if a known structure around a given target (say a disk or known planet) needs to be positioned to avoid structures in the instrument's field of view. 

Users should only constrain the requested angles when necessary to support their science goals. When an angular constraint is placed, the larger the range that can be allowed the better (from the standpoint of allowing scheduling flexibility). As an exercise, the user can try editing the special requirement that sets the allowed range of angles on the first observation and re-run the Visit Planner to see how the allowed time window changes.

8. Verify your observation set-up.

i. The APT Aladin Viewer can be used to visualize the field of view on the sky for planned JWST observations. 

Verify that the position angle(s) and roll dither(s) of an observation/ visit has been specified as intended.

JWST APT Aladin Viewer

Video tutorials are available for: Aladin Overview in APT: part 1Aladin Overview in APT: part 2Using Aladin and APT Visit Planner Together

ii. Run the APT visit planner.

Check the schedulability of observations, check constraints, and see whether guide stars are available to support the observations. 

APT Visit Planner

Video tutorials are available for: Reviewing Errors and Warnings

iii. Create Target Confirmation Charts.

 This is needed to verify the input target coordinates that will position the telescope in the correct place.

APT Target Confirmation Charts

9. Using the Smart Accounting Reports are you able to identify the trade-offs in efficiency (science time/total time) for different observation strategies? 

Note: programs that minimize the number of major slews and the number of visits will typically achieve higher efficiency than programs with large numbers of slews and visits.

APT Smart Accounting 
JWST Slew Times and Overheads
JWST Observing Overheads Summary
JWST Instrument Overheads


Boccaletti, A. et al., 2022, A&A, in press
JWST/MIRI coronagraphic performances as measured on-sky

Durcan, S., Janson, M., Carson, J. 2016, ApJ, 824, 58
High Contrast Imaging with Spitzer: Constraining the Frequency of Giant Planets out to 1000 AU separations 

Konopacky, Q. M., Barman, T. S., Macintosh, B. A., Marois, C., 2013, Science, 339, 1398 (Science link)
Detection of carbon monoxide and water absorption lines in an exoplanet atmosphere

Lafrenière, D. et al. 2007, ApJ, 660, 770L
A New Algorithm for Point-Spread Function Subtraction in High-Contrast Imaging: A Demonstration with Angular Differential Imaging

Perrin, M. D., et al. 2018, Proc. SPIE 10698, 1069809
Updated optical modeling of JWST coronagraph performance contrast, stability, and strategies

Rajan, A., et al. 2015, ApJ 809, L33
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Soummer, R., Pueyo, L., Larkin, J. 2012, ApJ, 755, L28
Detection and Characterization of Exoplanets and Disks Using Projections on Karhunen-Loève Eigenimages

Notable updates
    Updated warnings for Cycle 3

  • Updated MIRI performance metrics
    Added step to check for features and caveats
    Included cycle 2 background and TA warnings for MIRI in the appropriate places
Originally published