The different JWST instruments and modes have specific capabilities, constraints, and drawbacks for observing moving targets.
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.
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 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 decrease 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.
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 through one channel. The same field of view can be observed through two different filters (short, 0.6–2.3 µm, and long, 2.4–5.0 µm, wavelengths) simultaneously. NIRCam has 2 channels 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 in one channel; RAPID readout mode does not result in data volume issues when using only one channel. Surveys would benefit from use of both apertures 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 occultations. 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. See the NIRISS section below for applications of slitless spectroscopy to solar system objects.
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).
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 and fixed slits spectroscopy. 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. 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 with the micro-shutter assembly (MSA) is not expected to be commissioned for moving target observations. As such, the proposed investigation in Norwood et al. (2016) to observe the Uranian satellites with the MSA is not feasible. However, there will eventually be an option for custom long slits in the MSA 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 performed by opening long slits in the MSA in sequence across the detector.
Moving target articles
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
Norwood, J., et al. 2016, arXiv
Solar System observations with the James Webb Space Telescope