MIRI Coronagraphic Recommended Strategies
Pre-launch recommendations for astronomers planning coronagraphic observations with JWST's Mid-Infrared Instrument.
See also: MIRI Observing Strategies, JWST High-Contrast Imaging Roadmap, as well as relevant science use case examples MIRI and NIRCam Coronagraphy of the Beta Pictoris Debris Disk.
Note that what follows are pre-launch recommendations that will be updated as the knowledge about the instrument's optimal usage and calibration progresses.
The Mid-Infrared Instrument (MIRI) onboard 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)?
The imaging channel on MIRI is equipped with four coronagraphs that provide high-contrast imaging (HCI), covering wavelength bands from 10 to 23 μm (Boccaletti et al. 2015).
In addition to the classical Lyot coronagraph, which provides an inner working angle (IWA) of ~3.3λ/D, MIRI also incorporates the 4-quadrant phase-mask coronagraph technology (4QPM; Rouan et al., 2000) to provide the smallest possible IWA of ~1 λ/D at 10 to 16 μm. These advantages might be used to investigate the environments near bright point-sources, including exoplanets and other faint circumstellar sources, plus the environments surrounding Active Galactic Nuclei (AGNs).
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 observations to 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 3.3 λ/D in projected radius at 23 µm, and is suspended in the focal plane by 2 supporting struts in the mounting bracket, which themselves block light in the field of view (FOV). The Lyot coronagraph works only in a single broad band (R ~ 4) centered at 23 μm, provided to maximize the sensitivity to colder and extended objects (cold silicates in circumstellar disks, for instance). The ~3.3 λ/D inner working angle (IWA) makes it useful for detecting objects, structures, or diffuse emission at an apparent separation of ≥2.16" from the bright object. 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 objects and structures 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 provide an IWA to near 1 λ/D, but at the price of strong sensitivity to optical aberrations and source misalignments. Another issue is the partial attenuation between two adjacent quadrants. The MIRI 4QPMs are quasi-monochromatic, working only within narrow bandpasses (R ~ 14–17 4QPM) 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 or at a specific date. It 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 is the roll flexibility for this observing period? Are there any restrictions that I need to place on my observations?
Utilizing the JWST Backgrounds Tool (JBT)
See also JWST Backgrounds Tool
The JWST Backgrounds Tool (JBT) is a simple command line tool that accesses JWST background models to return a plot of the total background intensity, and its components, as a function of time. It can be useful for estimating and visualizing the impact of the background on the schedulability of JWST observations, since it returns the number of days per year that a given target is observable at low background for a given wavelength and selectable threshold.
Selecting a PSF subtraction strategy
In order to maximize the contrast obtained using the JWST coronagraphs, it is critical to obtain at least one good reference point spread function (PSF) that matches the science PSF. This reference PSF will be "subtracted" form the science target utilizing various algorithms, including:
Referenced differential imaging (RDI)
The RDI uses an observation of a reference star to calibrate the coronagraphic point spread function (PSF), and subtract it from the image of the target object. The quality of RDI, however, is sensitive to several factors. This includes thermally-induced wavefront drifts of the observatory that cause changes to the PSF relative to the science target, and imperfect target acquisitions of the science and PSF reference targets.
Angular differential imaging (ADI)
Two coronagraphic observations of the target are obtained at different telescope roll angles, and the two are subtracted. Differencing on the same star introduces diversity between instrument artifacts from the astrophysical signal (i.e., diffraction pattern is fixed to the telescope, while the astrophysical signal is fixed to the sky). This technique is highly effective since it enables PSF subtraction at nearly the same observatory attitude (for wavefront stability), mitigates detector artifacts, and also eliminates color mismatch terms. However, ADI introduces self-subtraction biases, especially given the limited available roll (~10º max) of JWST. It can also degrade recovery of diffuse emission, such as debris disks or host galaxies of AGN.
Small grid dithers (SGDs)
This technique utilizes a defined set of sub-pixel dithered exposures to optimize coronagraphic PSF subtraction — it is 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 sample the PSF diversity close to the center of the coronagraph. Post-processing optimization algorithms (such as LOCI or KLIP) are then used to construct an optimal synthetic reference PSF to be subtracted from the target 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 SGD 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 used 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 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., using a non-interruptible sequence). The inference from this requirement is that the science target and PSF reference target (for RDI and SGDs) should be schedulable witin the same visibility windows (which can be verified using the CVT).
Choosing a reference PSFs target
See also HCI PSF Reference Stars.
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 remove residual (diffracted) light from the bright target. By STScI policy, observers are required to define at least one 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 PSF reference(s):
- Choose a known "good" PSF reference—"good references" are usually stars that are not astrophysically contaminated (e.g., 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 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 should be 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 you to select suitable, non-resolved calibrators using a number of search criteria.
The roll capability of JWST is limited; 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 (ADI)
This type of roll was mentioned previously as a technique 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 "effective" IWAs. For MIRI at 10.65 μm, the minimum separation to avoid self-subtraction of a point source is obtained for an "effective" IWA of 1.9" with the typical ±5° roll. At 15.5 μm, this "effective" IWA becomes 2.8". For all science observations at separations below this "effective" 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 away from PSF structures and coronagraph mask features
These include diffraction spikes, the axes of the 4QPM masks, and the supporting struts of the Lyot occulting spot. However, this typically requires rolls larger than 10° and therefore an observation at a different date, albeit with more significant changes in the wavefront error compared to back-to-back observations. As such, we recommend the second observation obtain 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.
Background images for MIRI coronagraphy
Due to thermal emission from the telescope, Zodiacal dust, and other (galactic) sources, it may be advantageous to acquire a background measurement independent from the primary coronagraphic image. Observers should carefully assess their science program to determine whether dedicated background observations are required to successfully accomplish their science objectives.
In general, when performing reference PSF subtraction, both the science and reference PSF images need to have had a background subtracted prior to the PSF subtraction. Because the reference PSF will usually have a different peak brightness than the science target, it will have to be multiplied by a scale factor before subtraction. If the backgrounds have not been previously removed, this scaling will cause subtraction artifacts, and compromise the resulting photometry; ADI is the possible acceptation, since the reference and science target are the same object. Subtraction artifacts from background mis-match will be particularly deleterious for science targets with extended structure, such as debris disk and host galaxies around active galactic nuclei.
MIRI includes the capability to obtain such background observations for each coronagraphic filter/mask combination. While primary coronagraphic images of science and reference targets always require target acquisition, observations of a background do not require Target Acquisition (TA). However, the background observation must be associated with at least one primary target, but can be associated with multiple targets (e.g., both the science and PSF reference target.)
Standard dithers and Small Grid Dithers are not allowed for associated background observations. However, The center of the background image can be positioned within one of the four primary TA regions of Interest (ROIs). In addition, the option to repeat the associated background in a second, diagonally-opposed, ROI is available. This allows for additional mitigation of residual cosmic ray hits and instrumental artifacts.
Because the associated background observations do not require a TA, they are significantly less time-consuming than the primary observations. However, the exposure time for the background observations should be sufficient to obtain a signal-to-noise that is equal to or greater than the background level in the primary images.
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In general, users should consider using the FAST readout mode; however, for TA of faint targets (where longer 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 coronagraphic imaging TA, the minimum required integrated (within the extraction aperture) signal-to-noise ratio (SNR) is 30; this minimum is defined to ensure good centroiding. 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. We recommend selecting the quadrant in which the companion source of interest does not reside. If this is not an option, or if you're performing TA on bright targets, performing a Repeat observation 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 2 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 (persistant images
Slowly decaying latent images may produce residual signals that will interfere with the centroid measurement algorithm, and further limit the accuracy of any TA procedure around bright stars. To mitigate this, observers can consider using the Repeat observation strategy. Here, 2 observations are obtained: one with TA using the 1st ROI and second with TA using a 2nd ROI that is diagonally opposed to the first ROI. This will allow for discrimination of latent images because they are time variable.
Utilizing the Exposure Time Calculator (ETC)
Estimatingexposure timesis a science-critical aspect of MIRI coronagraphic observation planning; exposure times must be determined for all coronagraphic observations and target acquisition procedures. To that end, ETC workbook and specific calculation ID#s are a required field for coronagraphic observations specified within the Astronomer's Proposal Tool (APT).
The purpose of the JWST Exposure Time Calculator tool is to provide observers with the information necessary to design a proposal that will yield successful observations (i.e., that the astronomical observables sought by a given program will be accessible at the level of the SNR required to achieve the science objectives).
For MIRI coronagraphic observations, the current implementation of the ETC is useful for the following tasks: (1) investigating detector readout patterns and associated speeds and saturation; (2) computing the SNR of an off-axis source under the ideal contrast assumption (e.g., perfect centering and scaling); and (3) visualizing and/or downloading the resulting scene after background and PSF subtraction.
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(s). 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 (see JWST ETC Coronagraphy Strategy). It is also here that the observer can specify the Scene rotation, which allows for rotation of the scene with respect to the axes of the MIRI coronagraphs. When used in conjunction with the CVT, this feature enables the observer to determine the optimal Aperture Position Angle Special Requirements for the scene of interest.
Precise, more realistic (e.g., introducing a centering error and the small grid dithers) coronagraphic calculations in the vicinity of the masks (<1” for the 4QPM) is not implemented in the ETC. This requires a call to WebbPSF “on the fly”, which can be done by scripting Pandeia.
Because the PSF stability is unknown until launch, the ETC does not support ADI or SGD—only RDI.
Implementation into APT programs
A few special requirements below are in the form of proposal parameters, and are represented a bit differently in APT. For such instances, there will be tips on how to find them in APT.
To enter special requirements, click on the Special Requirements tab in the active GUI window to show the Special Requirements parameter field. To add a special requirement, click on Add … at the bottom of the Special Requirements parameters pane.
Example: for "APERTURE PA RANGE", select Position Angle, then PA Range in the Special Requirements parameters pane. This will be noted below as (Position Angle → PA Range).
In the Astronomer's Proposal Tool (APT, a standard coronagraphic sequence involves a set of linked observations in a non-interruptible sequence. Coronagraphic programs with several observations bundled in a non-interruptible sequence will benefit largely from the APT Smart Accounting tool, which will aid in reducing the total slew times charged for such sequences.
The APT MIRI Coronagraphic Imaging template provides sections for entering information on a variety of parameters such as those for target acquisition, exposure times, special requirements for linking observations, and observations of PSF reference stars. Best practices are to always decide ahead on the observation strategy and observation sequence that will be used for a given pair or set of targets. The HCI APT Instructions article provides helpful tips on how to organize observation folders and the observations to which they pertain.
Setting the appropriate special requirements (SRs) is important for MIRI coronagraphic observations. At the very least, users should place the "SEQUENCE NON-INTERRUPTIBLE" (Timing → Group/Sequence Observations Link) timing SR on their coronagraphic sequence. For observations with the 4QPMs, in particular, observers will likely require an enforced "APERTURE PA RANGE"(Position Angle → PA Range) on their observations to place the source of interest optimally with respect to the mask and to ensure they are not coincident with any of the 4QPM axes (see Figure 1). The ideal orientation of a single companion or feature of interest on the mask is usually ~45° from the 4QPM axes (if the target visibility allows this orientation). However, this can differ for targets with multiple features/sources of interest. The CVT can be used to assess available position angles.
In order to define a roll dither between science target observations, the appropriate "APERTURE PA OFFSET" SR (Position Angle→PA Offset Link) must be used. If a second sequence at a larger PA offset is needed, the "APERTURE PA OFFSET" SR must be set between the 2 sequences; and so forth. For programs requiring this level of attention to detail, observers should use the observing constraints determined from the CVT.
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. Assuming the observer requires both a roll dither of angle α (where α < 10°) and a larger roll offset of angle θ (where θ > 10°), the following strategy can be used:
- Create a coronagraphy sequence with 3 observations, with timing special requirements, where Obs 1 is a science target, Obs 2 is a science target, and Obs 3 is the reference star.
- Add a Special Requirement in the sequence such that Obs 2 has an "APERTURE PA OFFSET" (Position Angle→PA Offset Link) of +α° with respect to Obs 1.
Create a second 3-observation "NON-INTERRUPTIBLE" (Timing → Group/Sequence Observations Link) sequence such that Obs 4 is a science target, Obs 5 is a science target, and Obs 6 is 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°].