Instrument-Specific Considerations for Moving Targets

The different JWST instruments and modes have specific capabilities, constraints, and drawbacks for observing moving targets.

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The diversity of JWST observing modes present many opportunities for new and innovative solar system science. However, there are also special considerations and drawbacks to particular modes. Below are a list of the JWST instruments and features that the proposer should keep in mind when developing observing programs. This list is not exhaustive.


Main articles: MIRI Imaging, MIRI Low Resolution Spectroscopy, MIRI Medium Resolution Spectroscopy

  • The Mid-Infrared Instrument (MIRI) can be operated in 4 different observing modes: (1) imaging, (2) low-resolution spectroscopy, (3) medium-resolution spectroscopy, and (4) coronagraphy

  • The primary MIRI modes for solar system observations are imaging, low-resolution spectroscopy with the LRS, and medium-resolution spectroscopy with the MRS. Both the LRS and MRS require targets with well-constrained orbital solutions for accurate placement in the science aperture. The LRS requires that the target be kept centered in the slit as accurately as possible because even small deviations will result in inaccurate wavelength solutions. Placing the source 4.6 mas from the center of the slit (the maximum deviation based on the expected pointing accuracy of JWST) could result in the wavelength solution shifted by 30 nm (Kendrew et al., 2015). Target acquisition (TA) is therefore recommended for placing point sources on the LRS slit. The LRS in slitless mode does not support dithering.

  • The FWHM of the MRS PSF roughly corresponds to the slice width, which varies with wavelength. The PSF FWHM is ~0.176" at the shortest wavelengths and expands to ~0.645" at the longest wavelengths (Wells et al., 2015). Pluto and Charon (maximum angular separation of ~0.8"), for instance, will be easily separable at the shorter wavelengths, but will possibly be blended at the longer wavelengths. As another example, spatial resolution on Titan will significantly degrade with increasing wavelength.

  • The MIRI coronagraphs are 4-quadrant phase masks (4QPMs), meaning they do not have Lyot stops and can provide much smaller inner working angles (IWAs) (Boccaletti et al., 2015). A possible application of this mode is the detection of satellites around minor bodies in the solar system (primarily asteroids and KBOs). A drawback is the need to place the target exactly on the center of the phase mask, which may be more difficult for moving targets. Additionally, it is unlikely that observations of extended objects with the 4PQMs will yield useful results.


Main articles: NIRCam ImagingNIRCam Coronagraphic Imaging

  • The Near Infrared Camera (NIRCam) can be operated in 5 different observing modes: (1) imaging, (2) coronagraphic imaging, (3) wide field slitless spectroscopy, (4) time-series imaging, and (5) grism time series

  • The primary NIRCam mode for solar system observations is imaging with one module. The same field of view can be observed through two different channels (short, 0.6–2.3 µm, and long, 2.4–5.0 µm, wavelengths) simultaneously. NIRCam has 2 modules providing 2 separate fields of view, but it is suggested that only one module be used if observing one specific target because almost all targets, including comets, can be observed using one module; RAPID readout mode does not result in data volume issues when using only one module. Surveys would benefit from use of both modules combined with NIRISS imaging in parallel (see below). Brightness limits can be found in the NIRCam imaging article.

  • NIRCam time-series imaging provides a maximum frame rate of ~20 Hz in conjunction with the smallest subarray (64 × 64 pixels, or 2.0″ × 2.0″). A possible application is for observing stellar occultations and mutual events. Brightness limits for this mode can be found in the NIRCam imaging article. The NIRCam coronagraphs (using traditional Lyot stops) provide IWAs of 0.4″, 0.63″, and 0.81″ (in radius) at wavelengths of 2.1, 3.35, and 4.1 µm, respectively. Possible application of the coronagraphs to solar system science are the study of Centaur and distant comet comae, observations of the satellites and rings of Uranus and Neptune, and the discovery of faint, widely separated satellites of the largest Kuiper Belt Objects.


Main article: NIRISS Aperture Masking Interferometry

  • The Near Infrared Imager and Slitless Spectrograph (NIRISS) can be operated in 4 different observing modes: (1) imaging, (2) single object slitless spectroscopy (SOSS), (3) wide field slitless spectroscopy (WFSS), and (4) aperture masking interferometry (AMI)


Main articles: NIRSpec IFU Spectroscopy, NIRSpec Fixed Slits Spectroscopy

  • The Near Infrared Spectrograph (NIRSpec) can be operated in 4 different observing modes: (1) multi-object spectroscopy (MOS), (2) integral field unit (IFU) spectroscopy, (3) fixed slits spectroscopy, and (4) bright object time-series (BOTS) spectroscopy

  • The primary NIRSpec modes for solar system observations are IFU, fixed slits spectroscopy, and MOS configured into a long slit. The fixed slits have no covers and are always open; the ALLSLITS subarray can be used to simultaneously record spectra through all the slits, if desired. Both modes require targets with well-constrained orbital solutions for accurate placement in the science aperture. Any observations with the 0.2" fixed slit, and likely the 0.4" fixed slit, will require target acquisition (TA). Wide aperture target acquisition (WATA) for moving targets will use the S1600A1 fixed slit, so the uncertainties on the ephemeris must be small enough to first place the object in a 1.6" × 1.6" box.

  • Multi-object spectroscopy (MOS) with the micro-shutter assembly (MSA) is available for moving target observations with a "pseudo long slit." These custom long slits are available in quadrant 4 (Q4) of the MSA and may be ideal for observations of comets, Mars, and the giant planets. Scanning of the entire disks of Mars and giant planets can be made by allowing the object to rotate and leaving the long slit fixed in the MSA. Scanning across the disk of Mars and the giant planets in a shorter period of time, or scanning across a comet, can be specified in the NIRSpec MOS template starting in APT 28.0. It should be noted that the proposed investigation in Norwood et al. (2016) to observe the Uranian satellites with the MSA is not feasible at this time.


Boccaletti, P.-O., et al. 2015, PASP
The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted performance for the MIRI coronagraphs

Buie, M. et al. HST program 13667
Observations of the Pluto System During the New Horizons Encounter Epoch 

Kendrew, S., et al. 2015, PASP
The Mid-Infrared Instrument for the James Webb Space Telescope, IV: The Low-Resolution Spectrometer

Kesthelyi, L., et al., 2016, arXiv
Observing outer planet satellites (except Titan) with the James Webb Space Telescope: Science justification and observational requirements

Norwood, J., et al. 2016, arXiv
Solar System observations with the James Webb Space Telescope

Rivkin, A. S., et al. 2016, arXiv
Asteroids and the James Webb Space Telescope

Thatte, D., Sivaramakrishnan, A., & Stansberry, J. 2015, STScI Newsletter, Volume 32, issue 02
Vulcanism on Io with aperture masking interferometry on Webb's NIRISS

Wells, M., et al.b2015, PASP
The Mid-Infrared Instrument for the James Webb Space Telescope, VI: The Medium Resolution Spectrometer



Latest updates
    Changes made to NIRSpec section

    Fixed nomenclature issue in NIRCam section. Fixed units in NIRISS section.

    Fixed incorrect link.