Moving Target Observing Strategies

A summary of general and instrument-specific recommended strategies for creating moving target observing programs is presented in this article.

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The diversity of JWST observing modes present many opportunities for new and innovative solar system science. Below are a list of recommendations for the JWST instruments and features that proposers should keep in mind when developing observing programs. The recommendations are broken into general and instrument-specific categories. The general recommended strategies deal mostly with target definitionSolar System target windows, and the Visit Planner in APT. Instrument-specific recommended strategies cover the most commonly proposed instrument modes for solar system observations. This list is not exhaustive.

General recommended strategies for moving targets

Defining moving targets

Words in bold are GUI menus/
panels or data software packages; 
bold italics are buttons in GUI
tools or package parameters.

The following minor bodies, in addition to the planets and their known satellites, are "standard targets" and should be selected from the standard target listCeres, Pluto (Charon, Styx, Nix, Kerberos, Hydra), Makemake, Haumea (Hi'iaka, Namaka), and Eris (Dysnomia).

Minor body ephemerides should be checked in JPL Horizons for positional uncertainties; in rare cases, even numbered objects can have uncertainties large enough to impact their observability. This is particularly true of observing modes that have small fields of view, which include NIRSpec IFU, NIRSpec WATA, NIRSpec fixed slits, MIRI MRS, MIRI LRS (slit), NIRCam SUB64P subarray, and NIRISS AMI.

Use the JPL Horizons search tool provided in APT when defining non-standard targets whenever possible. Avoid typing in the orbital elements by hand to minimize the chance of making an error.

Solar System Target Windows

A full list of Solar System Target Windows can be found in the Solar System Special Requirements article.

The Tutorial on Creating Solar System Observations in APT provides clarification on specific Solar System Target Windows, such as:

  • the difference between Separation and Distance windows;

  • the difference between Transit and Occultation windows;

  • the different options for Eclipses; and

  • when Central Meridian Longitude constraints can be applied.

Unless necessary for your science goals or to increase schedulability, do not alter or delete the Default Windows that are automatically populated for observations of specific targets.

MIRI recommended strategies for moving targets

See also: MIRI Observing Strategies

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

Target acquisition

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 offset from the center of the slit could result in shifted wavelength solutions (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.

MRS observations

Avoid target acquisition (TA) for MRS observations, if possible. This will save time and should not be used for extended objects or objects with low-uncertainty orbits, such as standard targets (planets, dwarf planets, and their satellites) and most numbered minor bodies.

Choose a 4-point dither for MRS observations. A 2-point dither is not recommended due to issues with artifact rejection, proper spectral sampling, and proper spatial sampling. Never choose less than 2 dither positions, as this will almost certainly result in data quality issues.

Simultaneous imaging will incur some small overheads, but in general, it is a good idea to select this option in APT when observing with the MRS. Simultaneous observations with the MIRI imager in a field adjacent to your target could yield serendipitous detections of minor bodies, especially asteroids, which have peak thermal emissions at MIRI wavelengths. However, it is not recommended that simultaneous imaging be selected when observing near giant planets, as this will likely saturate the detector (see MIRI MRS Recommended Strategies).

Dedicated background observations are recommended when observing giant planets (or other extended objects, like comets) that fill the smallest MRS field of view (3.3" × 3.7"). These background observations should be placed a few arcminutes from the science target using the Level 2 Type APT parameter's Position Angle setting, in the Solar System Targets form. For these dedicated background observations, no TA should be specified and dithering is not necessary. Simultaneous imaging may be selected, but is not necessary.

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.

LRS observations

Proposers should always specify a TA for LRS observations. Even objects with lower uncertainty orbits may not be properly placed on the LRS slit (0.51" in width) with the blind pointing accuracy of JWST, which is 0.19" (1-σ, radial) without TA but 0.10" (1-σ, radial) with TA.

Coronagraphic imaging

Three of the MIRI coronagraph apertures 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, and keep it there for the duration of the observations, which is likely to be more difficult for moving targets. Additionally, it is unlikely that observations of extended objects with the 4PQMs will yield useful results. If you are still considering MIRI coronagraphic imaging observations of moving targets, a TA is required.

NIRSpec recommended strategies for moving targets

See also:: NIRSpec Observing Strategies

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

Target acquisition

The primary NIRSpec modes for solar system observations are IFU and fixed slits (FS) 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 the IFU and FS 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. MSATA is not available for moving targets.

Fixed slit observations

NIRSpec fixed slit observations will require a TA with the S1600A1 slit (1.6” × 1.6” FOV), referred to as wide aperture target acquisition (WATA), and will add 8–15 minutes of overhead to each visit. If your science goals can be accomplished with the IFU and can tolerate the uncorrected initial pointing errors, it may be possible to avoid this overhead by skipping the WATA acquisitionSince the IFU is the largest spectroscopic aperture on NIRSpec (3” × 3” FOV), it can be advantageous to select this mode over a fixed slit when pointing uncertainties are a concern.

IFU observations

If you are worried about a target falling in the IFU (3” × 3” FOV) with blind pointing, then it will likely not fall within the WATA FOV (1.6” × 1.6”) either. The best advice is to get more astrometric measurements of your target prior to the cycle you are proposing for.

Regarding TA with NIRSpec, the MSATA option is not available for moving targets. Observers can choose between performing a WATA acquisition in the 1.6" × 1.6" aperture followed by a slew to place the target in the IFU, or they can rely on the initial JWST blind pointing accuracy and skip the target acquisition. In the latter case, the user can choose whether or not to obtain a verification image through the IFU aperture prior to the spectroscopic observations (TA Method  = VERIFY_ONLY or TA Method = NONE). For moving targets, it will usually make sense to take any verification image with the MSA (PV MSA Configuration) set to ALLCLOSED to reduce light leakage.

The choice of dithers is inherently tied to the accuracy of the target acquisition, as is quantified in the NIRSpec Target Acquisition Recommended Strategies article. When in doubt, choose a CYCLING dither type with SMALL size for NIRSpec IFU observations. This is especially important for extended objects, since the 2-POINT NOD and 4-POINT NOD have large throw distances. Observations of minor bodies with higher uncertainty orbits should use the SMALL size dithers as well to ensure they stay in the FOV between dithers.

Observations of targets in the NIRSpec IFU are subject to stray light from both failed open shutters and light leakage through the MSA. The latter can cause a significant increase in the background, due to the "pile-up" of dispersed background (similar to slitless spectroscopy modes). In general, leakage calibration exposures ("leak cals"), which can measure and subtract this signal, are not recommended for moving targets, since the leaking astronomical background will have moved between the science and leak cal exposure.

For observations of giant planet satellites, it would be best to avoid placing the planet in the MSA FOV (see Moving Target Scattered Light Considerations). The Moving Target Visibility Tool can be used to evaluate the allowable position angles of the NIRSpec FOV over particular dates, in order determine the appropriate Position Angle special requirement in the APT. Finally, we note that, for light leaking through open shutters, the contaminated spaxels can be removed with dithering; in this case, 4 dithers are preferable to 2.

Multi-object spectroscopy (MOS)

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. 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.


NIRCam recommended strategies for moving targets

See also: NIRCam Observing Strategies

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


For most targets, observations using Module B only (as opposed to Module = ALL) will provide sufficient field of view (approximately 2.2 × 2.2 arcmin), and will give more flexibility for choosing exposure parameters.

Choosing a Subarray other than FULL will restrict the field of view further, but allow for more Groups in each integration ramp for bright sources. Generally, the SNR is better using more Groups for a given integration time.

For the planets and their major satellites, use of subarrays is frequently required in order to avoid saturation.

Choose at least 2 dither positions to allow for bad pixel replacement. For sharper images, particularly at wavelengths below 2 μm in the short wavelength channel, and 4 μm in the long wavelength channel, use at least 4 dither positions.

The primary NIRCam mode for solar system observations is imaging with one module (Module B).The same field of view can be observed through 2 different channels simultaneously (short wavelength: 0.6–2.3 µm; long wavelength: 2.4–5.0 µm). NIRCam has 2 modules providing 2 separate fields of view; however, for many observations (i.e., a single target) only a single module is necessary. The RAPID readout mode will 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. Brightness limits can be found in the NIRCam imaging article.

Time-series imaging

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 demonstrated 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 Centaurs 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.

NIRISS recommended strategies for moving targets

See also: NIRISS Observing Strategies

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)

Aperture Masking Interferometry (AMI)

Of NIRISS's modes, AMI may present the most unique opportunities for solar system observers. AMI uses a 7-hole aperture mask with 21 unique baselines to provide high spatial resolution imaging of up to 75″, with a contrast sensitivity of ~10 mag at 4.6 µm. Keep in mind that the 7 holes in the aperture mask correspond to 7 mirror segments; this means that light from 11 mirror segments is blocked, resulting in throughput of only ~15%.

Faint objects, such as KBO binaries, are therefore not ideal AMI targets. However, AMI's performance observing Io's volcanoes and binary asteroids has been extensively investigated. When observable by JWST, Io's diameter will, on average, cover 16 pixels on NIRISS, with a plate scale of ~65 mas/pixel. Simulations of AMI images of active Io eruptions show it is possible to measure emission from individual events and to resolve bright events separated by only 88 mas; see Thatte et al. (2015) and Keszthelyi et al. (2016). The accepted Solar System ERS proposal (program 1373) makes use of the AMI mode for observations of volcanos on Io; its APT file is available to download in APT and data are available in MAST. To download the APT file, select FileRetrieve from STScIRetrieve using proposal ID, then type 1373 in the pop-up window. Binary asteroids with separations under 75 mas and contrasts less than 10 mag, such as Ida/Dactyl (70 mas and 6.7 mag, respectively), will be resolvable using AMI (Rivkin et al., 2016).


Compared to NIRCam, NIRISS has a smaller pixel scale and higher sensitivity to extended emission <2.5 µm (Norwood et al., 2016). The pixel scales of the 2 instruments are comparable at wavelengths >2.5 µm. The imaging mode contains similar filters (0.9–5 µm) to NIRCam two channel imaging, and in effect NIRISS imaging becomes a "third channel" when operated in parallel.


The single object (SOSS) and wide field (WFSS) slitless spectroscopy modes, provided by grisms GR150C, GR150R, and GR700XD, may not be the most optimal modes for solar system observers; however, such programs are not without precedence. For instance, the G141 grism on Hubble's WFC3 has been used to observe the Pluto system (HST program 13667, PI M. Buie). 



Boccaletti, A., et al. 2015, PASP, 127, 633
The Mid-Infrared Instrument for the James Webb Space Telescope, V: Predicted Performance of 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, 127, 623
The Mid-Infrared Instrument for the James Webb Space Telescope, IV: The Low-Resolution Spectrometer

Keszthelyi, L., et al. 2016, PASP, 128, 959 
Observing Outer Planet Satellites (Except Titan) with the James Webb Space Telescope: Science Justification and Observational Requirements

Norwood, J., et al. 2016, PASP 128, 960
Solar System Observations with the James Webb Space Telescope

Rivkin, A. S., et al. 2016, PASP, 128, 959
Asteroids and the James Webb Space Telescope

Thatte, D., Sivaramakrishnan, A., Stansberry, J. 2015, STScI Newsletter, Volume 32, issue 02 (PDF)
Vulcanism on Io with Aperture Masking Interferometry on Webb's NIRISS

Wells, M., et al. 2015, PASP, 127, 646
The Mid-Infrared Instrument for the James Webb Space Telescope, VI: The Medium Resolution Spectrometer

Latest updates
    Updated for Cycle 2.

    Wording clarifications. Removed the Visit Planner recommendation because it is no longer valid. Combined with Instrument Specific Considerations page. Renamed.

  • Fixed formatting issues

    Changes made to NIRSpec section

    Changed the title of the article. Updated some bullet points based on recent APT work.

  • Added wording about the Moving Target Visibility Tool (MTVT) to the NIRSpec section.
Originally published