NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids
This example science program presents an application of the Moving Target Roadmap, using NIRSpec IFU observations of Near-Earth Asteroids as an example. This article covers selection of fast moving targets and appropriate observing modes based on the targets' positional uncertainties. Proper determination of exposure parameters in the ETC and construction of an APT file are covered in separate linked articles.
Main article: JWST Moving Target Observations
See also: Step-by-Step ETC Guide for NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids
Step-by-Step APT Guide for NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids
Near-Earth asteroids (NEAs) represent a population of asteroids that have escaped the main belt and cross the orbit of the Earth. These objects could be potentially hazardous to life on Earth, but on the other hand are also good targets for robotic exploration due to their proximity. JWST will enable unprecedented study of their surface composition in the near-infrared (~1-5 μm). This includes comparison of NEA surface compositions to that of main belt asteroids (MBAs) to identify the source population, as well as studies of rotational variability. The volatile content and effects of space weathering can also be studied with near-infrared spectra.
In this Example Science program, we will obtain near infrared spectra of a sample of NEAs that are visible during JWST Cycle 1 (October 1, 2021 - September 30, 2022). These spectra will help determine the main belt asteroid families that these objects originated from, providing valuable information about the dynamical processes that produce NEAs and the current collisional environment in the main belt. The goal is therefore to obtain spectra of a handful of NEAs with signal-to-noise-ratios (SNRs) >20 over the largest fraction of the spectral range.
Step 1: Familiarize yourself with policies related to moving targets
There are two policies that you should be aware of that affect moving and fixed targets equally: target of opportunity (ToO) policies and the observation duplication policy. Since this is not a ToO proposal, there is no need to consider those additional restrictions.
Let's assume this is an example proposal for Cycle 1. In terms of the duplication policy, we need only consider the Guaranteed Time Observations (GTO) and Early Release Science (ERS) programs (proposals for later cycles will also need to consider Cycle 1, etc.). A "duplication" is any observation of the same target with the same instrument and mode.
There are no ERS programs that will observe Near-Earth Objects (NEAs), but there is one GTO program that will observe NEAs: "Near-Earth Objects", GTO 1245. The "Public PDF" under the "Program Contents" heading provides the list of targets and instrument modes on the first page. For this particular program, the NEAs (3200) Phaethon and (3552) Don Quixote cannot be observed in Cycle 1 using the instrument modes and instrument parameters specified in the APT file for GTO 1245. All other targets, and different instrument modes and/or instrument parameters for these two targets, are fair game. See the duplications policy for additional information.
Step 2: Familiarize yourself with apparent motion and pointing constraints for moving target observations with JWST
In this example program, the targets will be drawn from the population of NEAs, which typically have high apparent rates of motion and are only in the JWST field of regard for a short period of time. JWST is able to track moving targets with apparent rates of motion up to 30 mas/s (108"/hour), so the targets selected for this program must have apparent rates below this threshold. Targets moving faster than this threshold will be streaked across the detector; targets moving slower than this threshold can be tracked exactly by JWST.
See Moving Target Acquisition and Tracking for details on how moving target tracking is initiated. This page also contains a plot showing that the fraction of NEAs that can be observed by JWST on any given day is ~91%.
The pointing constraints on moving targets are the same as they are for fixed targets: Targets can only be observed when they fall within JWST's field of regard (FOR). Along the ecliptic, the FOR covers two separate windows (one preceding and one following JWST) from 85°-135° solar elongation. See the links above for additional information.
The JWST Moving Target Visibility Tool (MTVT) can be used to determine when a particular target is within JWST's FOR. See JWST Moving Target Visibility Tool Help for download and usage instructions.
Now that we are aware of all the constraints on moving targets, let's identify appropriate NEAs for this example proposal. Consider the following list of potential NEA targets (perihelion distance less than 1.3 AU):
- (433) Eros
- (1221) Amor
- (1915) Quetzalcoatl
- (1916) Boreas
- (3838) Epona
- (3908) Nyx
We can first use the MTVT to evaluate whether or not each of the targets is within the JWST FOR during the Cycle 1 period (assumed to be October 1, 2021 through September 30, 2022). The plots returned for each of the targets show that (433) Eros and (1916) Boreas are not within JWST's field of regard for the Cycle 1 date range, so we remove them from the list of potential targets. But we are not done yet.
Step 3: Generate an ephemeris in JPL/Horizons to determine the best time to observe the target
Main article: Moving Target Ephemerides
The MTVT provides a quick look at target observability, but does not determine if the target's apparent motion is below the 30 mas/s (108"/hour) threshold. For most solar system targets, this will not be an issue, but for NEAs it is. We must therefore use the JPL/Horizons tool to further evaluate our list of potential NEA targets:
- JWST can be selected as the "Observer Location" by typing "@jwst" in the box (see the link above for this step).
- Option #3 (Rates; RA & DEC) should be selected from "Table Settings."
- The "solar elong. cut off" and "angular rate cutoff" options under "Optional observer-table settings" should be set to 85-135 and 108, respectively. This will return an empty ephemeris for any target with a total apparent rate of motion greater than 108"/hour when it is within JWST's FOR (if it is ever within the FOR).
Running ephemerides for the remaining targets, we find that (3838) Epona is moving too quickly to be observed by JWST without streaking. Our final list of NEA targets is therefore:
- (1221) Amor
- (1915) Quetzalcoatl
- (3908) Nyx
Step 4a: Determine the instrument and observing mode for your moving target observation
Main article: Moving Target Observing Strategies
Determine the instrument and observing mode
We can envision that the science case for this program is to obtain spectra of NEAs in the near-infrared (~1-5 μm) in order to place constraints on their surface compositions. We can rule out the following instruments
- MIRI, which covers wavelengths from 5-28 μm.
- NIRISS provides slitless spectroscopy options in the proper wavelength range, but slitless spectroscopy is not a great option for any moving target observations. Tracking on the target will cause the stars (and their spectra) to streak across the detector; choosing not to track on the target would result in the target and its spectrum streaking across the detector. Neither situation is ideal and would be made even worse considering the high apparent motions of NEAs in this example proposal.
- NIRCam is primarily used for imaging in the near-infrared wavelength regime, but there are slitless spectroscopy options. However, these would be unusable for the reasons that NIRISS cannot be used.
That leaves NIRSpec as the best option for this program. NIRSpec actually provides three options for near-infrared spectroscopy: multi-object spectroscopy (MOS), fixed slit spectroscopy, and integral field unit (IFU) spectroscopy. MOS spectroscopy makes use of the micro-shutter assembly (MSA) to mimic a machined mask that would be used on a ground-based telescope. The MSA contains thousands of very small shutters (0.20" x 0.46") and the user must select which to open for a particular field. For moving targets, conventional MOS observations (e.g., made using the MSA planning tool) are not possible. However, MOS mode can be used with the MSA configured to a long-slit, primarily to observe extended targets such as comets, Mars, and the giant planets. Hence, fixed slit, MOS, or IFU spectroscopy are all viable modes for observing these objects. The fixed slits provide a higher SNR than the IFU or MOS, but extended targets are best observed with the IFU. Our NEAs are point sources, so the MOS long-slit is unnecessary, and higher SNR can be achieved with the fixed slits, but there is an additional consideration that must be taken into account.
Are there any special considerations for my chosen instrument mode for moving targets?
Proposers interested in observations of minor bodies should always be aware of their proposed targets' positional (RA & Dec) uncertainties. A reasonable estimate of the positional uncertainty (3σ) can be obtained from JPL/Horizons by selecting option #38 (POS uncertainty (RSS)) in "Table Settings." It is not safe to assume that numbered objects will always have small positional uncertainties. This is not an issue for planets and major satellites.
Proposers should also be aware of the pointing performance of JWST of 0.1" (1σ). The 3σ pointing accuracy is therefore 0.3".
Positional uncertainties are especially important to consider for spectroscopic observations with JWST, where the target must be accurately placed within a thin slit or small field of view. For instance, the NIRSpec fixed slits are either 0.2" or 0.4" in width, while the IFU is a 3" x 3" box. In terms of which fixed slit is best to use, that depends on the science case and the positional uncertainty of the target. The 0.2" slits (S200A1 and S200A2; S200B1 is a redundant slit that would only be used in the case of detector failure) would provide the spectral resolution in Table 2 of NIRSpec Dispersers and Filters but would only be useful for targets with very well-constrained orbits. The 0.4" slit (S400A1) would provide lower resolution but also have more tolerance for positional uncertainties. Target acquisition (TA) could be used to place objects in these small apertures; NIRSpec's Wide Aperture Target Acquisition (WATA) makes use of the S1600A1 aperture, which is a square aperture 1.6" on each side (MSATA, using the MSA to perform target acquisition, is not an option for moving targets).
While WATA can be useful for ensuring that targets with relatively small positional uncertainties are accurately placed in the center of the IFU aperture or a slit, it is less useful for targets with larger positional uncertainties. For example, if you are worried about placing a target in the IFU aperture with blind pointing, effective placement in the smaller S1600A1 aperture will be even more of a problem. For this reason, only objects with well-known positions should be observed with the fixed slits (3σ positional uncertainties less than half the slit width). Any object with a 3σ positional+pointing uncertainty at the time of observation greater than about 0.8" may be difficult to place in the WATA aperture. Blind pointing in the IFU aperture is only recommended for targets with positional+pointing uncertainties up to 1.5". Targets with larger positional uncertainties may not be appropriate for IFU observations. These are general guidelines and not hard-and-fast rules; in practice, the positional uncertainty must be considered together with the dither pattern and the spatial extent of the source. Further information can be found in NIRSpec Target Acquisition Recommended Strategies.
The proposer should always use their own discretion in whether or not to observe an object with a relatively large uncertainty and whether or not to use target acquisition. Just be aware that if TA is unsuccessful in acquiring the target in the aperture, the observation will not be executed.
In regards to the positional uncertainties for our NEA targets in Cycle 1:
- (1221) Amor has a 3σ positional uncertainty of ~0.1". After considering the 0.3" (3σ) pointing accuracy, we can observe this target with the S200A1 slit after acquiring with WATA (all of the 0.2" NIRSpec fixed slits are interchangeable, so the S200A1 aperture is appropriate).
- (1915) Quetzalcoatl has a minimum (3σ) positional uncertainty of ~0.6" in JWST Cycle 1. Taking into account the pointing accuracy, this NEA can be expected to fall within 0.9" (3σ) of the center of the NIRSpec IFU with blind pointing; this is less than half the IFU aperture width, so it is reasonable to expect that the target will not fall outside the IFU field of view. However, this is larger than half the WATA aperture field of view, so target acquisition is not recommended. Additionally, blind pointing may place the target within the IFU field of view, but would not guarantee a reasonable amount of space for necessary dithering. If encountering this situation for real observations, the only recommendation that can be made is to obtain astrometry of the target and submit it to the Minor Planet Center (MPC) prior to the observations being made. For the purposes of this example science program, we will move forward with observations of (1915) Quetzalcoatl, with the assumption that additional astrometry would be obtained prior to program execution.
- (3908) Nyx has a 3σ positional uncertainty of ~0.05". This is low, but the 0.3" (3σ) pointing accuracy is still applicable, so observations of (3908) Nyx with the S200A1 slit would still require the use of WATA.
Step 4b: Familiarize yourself with the dither patterns for the chosen instrument mode
The NIRSpec Dithering Recommended Strategies article suggests a 2-, 3-, or 5-point primary dither pattern for fixed slits. Sub-pixel dithers provide an additional means to increase the number of total positions that the source is observed at and are not strictly necessary. We will not use sub-pixel dithers in this program. There are many different options for NIRSpec IFU observations, depending on whether or not the target is a point source or an extended source. See NIRSpec IFU Dither and Nod Patterns for additional details.
See Tutorial on Visualizing Dithers of a Solar System Observation in APT for a description of how to create a fixed target as a stand-in in order to visualize the dithers in the Aladin viewer.
Step 5: Calculate the required exposure time and detector readout parameters using the Exposure Time Calculator (ETC)
Main article: Moving Target ETC Instructions
Please see the Step-by-Step ETC Guide for NIRSpec IFU and Fixed Slit Observations of Near-Earth Asteroids article to continue with the tutorial.
Step 6: Fill out the Astronomers Proposal Tool (APT) for your observation
Main article: Moving Target APT Instructions