Moving Target Recommended Strategies
A summary of general and instrument-specific recommended strategies for creating moving target observing programs.
The recommendations below are broken into general and instrument-specific categories. The general recommended strategies deal mostly with target definition, Solar System target windows, and the Visit Planner in APT. Instrument-specific recommended strategies cover the most commonly proposed instrument modes for solar system observations.
General recommended strategies for moving targets
Defining moving targets
The following minor bodies, in addition to the planets and their known satellites, are "standard targets" and should be selected from the standard target list: Ceres, 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 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.
Run the Visit Planner on a subset of no more than 6 observations at a time. This will help avoid timeouts with the front end server of the Visit Planner, as well as more serious timeouts in the underlying programs that will result in the calculation failing to complete. Once you have run the Visit Planner on all observations, you can run Smart Accounting.
Solar System Target Windows
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 absolutely necessary for your science goals, do not alter or delete the Default Windows.
*Bold italics style indicates words that are also parameters or buttons in software tools like the APT and ETC. Similarly, a bold style represents menu items and panels.
MIRI recommended strategies for moving targets
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 acceptable but not ideal for artifact rejection, proper spectral sampling, and proper spatial sampling. Do not 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.
Dedicated background observations are highly 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.
Proposers should always specify a TA for LRS observations. Even objects with low-uncertainty orbits may not be properly placed on the LRS slit (0.51" thickness) with the blind pointing accuracy of JWST.
NIRSpec recommended strategies for moving targets
Fixed slit observations
NIRSpec fixed slit observations will require a TA with the S1600A 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 acquisition. Since the IFU is the largest spectroscopic aperture on NIRSpec (3” × 3” FOV), it can be advantageous to select this mode when pointing uncertainties are a concern.
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 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; this will still allow light through the fixed slit, and for MSA observations through the MSA itself, allowing an image of the scene to be obtained.
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 stuck-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. 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.
NIRCam recommended strategies for moving targets
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 you 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 sensitivity 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.