Procedures and strategies for planning coronagraphic observations with JWST.
Coronagraphic and other high-contrast imaging (HCI) observations can be some of the most complex to schedule with JWST. Most HCI science requires the scheduling of a sequence of observations that collects data for both the primary science target and a reference star, to support point-spread function (PSF) subtraction. The JWST PSF is expected to be time-variable, which has important consequences, for example that both the science target and the PSF-reference star should be observed as closely together in time as is feasible, in order to minimize changes in the PSF in data that will be reduced together. This brings into play the issue of target visibilities, discussed below. Until on-orbit experience shows this restriction to contemporaneous imaging can be relaxed, the JWST project has imposed a strategy on HCI programs whereby observations of the science target and PSF-reference star must be planned and coordinated to execute together, in a back-to-back sequence of observations.
Ultimately, the limiting contrast is controlled by PSF variability.
Depending on the specific goals of your HCI science program, a number of planning options and issues may or may not apply to your particular program. For example, do your goals call for the default 10° roll dither on your science target? Is a larger position-angle offset needed to recover a part of the scene that would otherwise be blocked by the selected corona graphic mask (e.g., a face-on disk observed with a NIRCam bar occulter)? Is the highest-quality PSF matching needed for your science? If so, perhaps you should consider the small-grid dither (SGD) technique (article pending). In the case of NIRCam observations, do you need high-accuracy astrometry? If so, perhaps you should obtain images for full-field astrometry (FFA) in addition to the HCI science data (article pending).
Your most basic decision in planning HCI observations is your choice of wavelengths needed for the science. The choice of wavelength ranges will influence your choice of science instruments and the options for masks, filters, and detector operations. See the JWST High Contrast Imaging Overview article, which offers links to the details on individual instruments.
Estimating your exposure times is a science-critical aspect of observation planning. You must estimate exposure times for both for science observations and target acquisition, which is crucially important for the success of coronagraphic observations. You can estimate exposure times using the JWST Exposure Time Calculator. Some HCI-specific suggestions on using the ETC are available in JWST Coronagraphy in ETC.
Below, this article treats planning topics in more detail, providing links to other articles for more detailed information.
We provide a flow of planning considerations to help you through the process. We suggest checking target visibilities first, as running ETC calculations for a target that is not observable at the necessary position angle on the sky could waste effort, but of course ETC calculations can be run at any time. Also, some users may want to consider the strategy issues up front in their planning. Nevertheless, the following sequence is a reasonable flow of planning activities for HCI.
- Check your target visibilities. Target visibilities can enter planning in several ways. Even if you have no restrictions on the placement and orientation of the coronagraphic field of view, you still must find a suitable reference star to support PSF subtraction. You must check and verify that both the science and PSF-reference targets can be observed at the same time. In cases of known or expected structures around the science target—the companion sources of interest, such as disks or exoplanets—then it is desirable to place that structure optimally with respect to the masks in a given JWST coronagraph. In particular, position-angle flexibility for JWST is a strong function of the target's ecliptic latitude. Therefore, before investing a lot of effort in estimating exposure times, you should probably first check that your desired observations are even permitted. The Coronagraphic Visibility Tool (CVT) is an important resource for such pre-planning and observational strategizing. The CVT computes both visibility windows and available position angles as functions of time for MIRI and NIRCam coronagraphs. The CVT also 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.
- Make some general strategic decisions: Will you require astrometric images (NIRCam only)? Will you use the (default) roll-dither technique for your science target (or not)? Will you want to use the SGD technique for your observation(s)? Define the content and structure of your conronagraphic sequences, viz.
- which instruments and coronagraphs/filters/occulters are needed for each target?
- verify that the standard sequence applies, and that neither a roll dither nor a reference star is required
- decide how to handle sequences of multiple filters—or multiple coronagraphic masks in the case of MIRI
- decide if SGDs and/or full-field astrometry (FFA) are needed
- Once you have established visibility and adopted a strategy, you should use the JWST ETC to calculate the exposure parameters for all included types of observations, such as science, PSF-reference, target-acquisition (TA), and FFA.
- You will need to have in mind the systematic limit of coronagraphic performance, as expressed by the limiting contrast, which is a function of the apparent separation between host and companion. Evaluating limiting contrast may help you decide about such matters as whether the highest quality PSF-subtraction is needed, which might drive you to consider whether the extra time needed to employ the small grid dither technique is "worth it" for your science.
- When you have addressed these considerations, you are ready to enter your observations into APT and craft your actual observing program. You may wish to review the concept of coronagraphic sequences and their purpose. It will help organize your proposal if you collect together, in their own APT observation folder, all the relevant observations for a given coronagraphic sequence. For a general overview, see the companion article JWST High Contrast Imaging in APT. For detailed, step-by-step help with each relevant observing template, see the APT template guides for NIRCam Coronagraphic Imaging, MIRI Coronagraphic Imaging, or NIRISS Aperture Masking Interferometry.
MIRI Coronagraphic Imaging Template APT Guide
NIRCam Coronagraphic Imaging Template APT Guide
NIRISS Aperture Masking Interferometry Template APT Guide
JWST High Contrast Imaging Overview
JWST High Contrast Imaging in APT
JWST Coronagraphy in ETC
JWST Coronagraphic Visibility Tool Help
JWST Position Angles, Ranges, and Offsets