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.
Stage 1 – Becoming familiar with the HCI capabilities and instrument-specific modes of JWST
In extension to the steps suggested in the Getting Started Guide ("Become familiar with JWST capabilities and terminology"), users should consider the following in particular to high-contrast Imaging:
Which of the JWST observing modes enable HCI?
What HCI optical designs are offered by JWST?
What are the allowed mask-filter combinations for each of the HCI modes?
What are the primary performance metrics for HCI?
What are the predicted performances of the instrument-specific modes?
What are the fundamental physical limits for detection?
What are the operations unique to HCI?
What are the recommended observing strategies pertaining to HCI?
Stage 2 – Comparing your parameter space to the performance limits and capabilities of the HCI observing modes.
Identify the 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)?
Determine the apparent separations, between your host and companion source(s) at the time of observation. Which instrument(s) and mask-filter combination(s) can achieve the required working angles?
Determine the companion contrast(s) at the wavelength(s) of interest. Are your observations feasible given the contrast limits of the instrument(s)?
For coronagraphic observations, how important is the azimuthal coverage around your science target?
Is it possible that your scientific goals can be achieved with non-coronagraphic PSF subtraction?
Stage 3 – Selecting 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 out the PSF of the central source.
Consider the degrading factors that may limit the PSF calibration and what steps you will take to mitigate them.
Which observing technique(s) will you include in your PSF subtraction strategy?
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
Which observing technique(s) will you include in your PSF calibration strategy?
Stage 4 – Assessing target visibilities and allowed position angles
The following steps should be used in conjunction with those outlined in the Getting Started Guide ("Determine if your targets can be observed").
Familiarize yourself with JWST position angles, coordinate systems, and related nomenclature to understand the telescope’s pointing constraints.
Determine the viewing constraints placed on your target(s).
Using at least one of the JWST target visibility tools, assess your target visibilities and allowed position angles versus time.
In the case of known companions, consider whether your observations require any restrictions on the orientation of the instrument field of view (FOV)/ detector being referenced.
Determine the aperture position angle(s)/ date(s) at which your companion(s) are nominally visible.
If implementing the ADI technique in your PSF calibration strategy (Stage 3):
Check how the instantaneous roll flexibility changes over the the particular visibility period.
Assess how potential rolls of the telescope will change the positions of the companion(s) relative to any instrumental obstructions.
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.
Stage 5 – Using the Exposure Time Calculator (ETC)
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 Guide, the following are advisable for HCI:
Define your Scenes and Sources.
Initiate calculations for each of your planned observations.
Adjust the exposure time via the NUMBER OF GROUPS, INTEGRATIONS, and/or EXPOSURES until you obtain the desired SNR and contrast on your target.
Check your individual calculations for detector saturation.
Initialize Target Acquisition (TA) calculations for each of your observations.
Run your TA calculations and examine the output information.
Does any saturation occur?
Does your exposure specification allow you to obtain the minimum required SNR for the TA procedure of the instrument mode?
Stage 6 – Selecting a suitable PSF calibrator
If you have established the need for a PSF reference target according to your PSF calibration strategy designed in Stage 3, this section is relevant. Otherwise, continue to Stage 7.
Select a PSF reference calibrator with consideration of the following criteria:
Well-known: Is the target a known good PSF reference star?
Schedulability: Do the visibility windows of the science target and PSF calibrator overlap at the time of the desired observation?
Proximity: Is the PSF calibrator in relative proximity to the science target?
Avoidance of Binary: Is the PSF calibrator a single and unresolved source?
Spectral Type: Does the PSF calibrator the share the same spectral properties as the science target?
Brightness: Is the PSF calibrator similar in magnitude to the science target?
Return to your previous workbook the ETC (created during Stage 5) to amend the spectral properties of the reference PSF source and finalize the exposure parameters of your calculations.
Stage 7 – Finalizing 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
Consider the total number of observations you will require for your observing program.
At the observation level: consider how you will organize (group) your observations.
If your sequence of observations involves the use of multiple filters and/or occulters, you should consider following the optimal efficiency scheduling strategy.
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?
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:
For all coronagraphic imaging programs: it is highly recommended to perform the standard coronagraphic sequence, or a derivative of it.
Do your science goals call for high accuracy astrometry?
Stage 8 – Using the Astronomer's Proposal Tool (APT)
Aside from the steps described in the Getting Started Guide roadmap, consider the following particular to HCI:
Organize science and PSF calibrator observations into sequences (to be scheduled back-to-back).
Use the PSF Reference Observations section to indicate which observations produce PSF references and to specify to which science observations they should be linked. The PSF reference star must be in the same FILTER and SUBARRAY.
Are NIRCam full frame astrometric (FFA) images are needed?
For Coronagraphic Imaging modes: Do any of your observations require the small grid dithering (SGD) technique?
Add any the necessary Special Requirements:
Verify your observation set-up.
Using the Smart Accounting Reports are you able to identify the trade-offs in efficiency (science time/total time) for different observation strategies?
1 HCI can be carried out using basic imaging modes of the observatory (Rajan et al., 2015; Durcan, 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.
2 Based on performance simulations and contrast predictions based on the latest information on the as-built telescope and instrument properties, including both static and dynamic contributions to wavefront error (Perrin et al. 2018)
3 We report all contrasts as 5σ post-processing contrasts after single reference star subtraction.
4 KL image projection (KLIP) algorithm (Soummer et al. 2012)
5 “locally optimized combination of images” or LOCI algorithm (Lafrenière et al. 2007)
4 Bold italics font style is used to indicate parameters, parameter values, and/or special requirements that are set in the APT GUI.
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