The JWST Moving Target Visibility Tool (MTVT) is a Python command-line tool for calculating moving (solar system) target visibility windows as a function of time. It has similar functionality to that of the JWST General Target Visibility Tool (GTVT), with a few additional features specific to moving targets. The MTVT is packaged with the GTVT, and is automatically installed when users install the GTVT (no standalone installation process is provided for the MTVT). Both the GTVT and MTVT currently use the post-launch JWST ephemeris.
Unlike the GTVT, users input a solar system target designation, rather than a fixed (RA, Dec) sky position. The designation can be an official name (e.g., Saturn, Gaspra, Encke), number (e.g., 599, 20000), or provisional designation (e.g., 2002 MS4). MTVT uses the JPL Horizons system to resolve the designation, and retrieves the target ephemeris (RA, Dec) at one day intervals. At that point, the functionality of the MTVT is identical to that of the GTVT.
For a given RA and Dec, the MTVT provides aperture position angle (APA) information for all 4 science instruments and the FGS within the allowed visibility windows. It also outputs the V3-axis position angle (V3PA). Results are in the form of an ASCII file as well as one or more summary plots. A number of options are available from the command line for tailoring the output to your needs. Once the interactive plot is displayed, icons can be selected to pan and zoom in on the plot to see detailed information.
The allowable position angles output by the MTVT can, for example, be used to help users plan observations of giant planet satellites to ensure that the giant planet avoids falling on a nearby science or FGS aperture, or to determine the visibility windows and durations for Mars or fast moving near-Earth asteroids (NEAs). In order to visualize an observation in APT using Aladin, users should add a PA range special requirement that reflects the range of allowed position angles from MTVT, and create a fixed target proxy with coordinates consistent with those position angles. Once that is done, visualization can proceed as described in the Visualizing Dithers of a Solar System Observation in APT tutorial.
The schedulability of a given target observation is more complex that just its visibility. It also involves the availability of guide stars as a function of time and other constraints that may be set with special requirements and/or Solar System special requirements in APT. The MTVT is a "quick look" tool for pre-planning purposes, but the Astronomers Proposal Tool is the true arbiter of schedulability for a given proposed observation.
Installation and dependencies
The MTVT is currently incompatible with Windows operating systems.
The MTVT comes packaged with the GTVT. The user is referred to the "Installation and usage" section on the JWST General Target Visibility Tool Help article for instructions on how to install the GTVT/MTVT (or update it if an older version of GTVT/MTVT was installed).
Once successfully installed, MTVT is run from the command line, as described below.
To see the MTVT help information, typejwst_mtvt -h or jwst_mtvt --help
$ jwst_mtvt -h
jwst_mtvt <desg> [--start_date=<obs_start>] [--end_date=<obs_end>] [--instrument=<inst>] [--write_ephemeris=<write_path>] [--write_plot=<plot_path>] [--smallbody] [--silent]
<desg> Name of moving target.
[--start_date] Start date for plot
[--end_date] End date for plot
[--instrument] JWST instrument to plot individually
[--write_ephemeris] File name to write ephemeris to
[--write_plot] File name to write plot out to
[--smallbody] Smallbody designation for major body
--silent Boolean to print results to screen [default: False]
-h --help Show this screen.
--version Show version.
Basic use of the MTVT is shown with the example command below:
Note that the following command will produce the same results, i.e., the provided designation is not case-sensitive:
After running this command, a table will be output in the terminal and a plot will open in a new window that includes the allowable aperture position angles as a function of time for the 4 science instruments (NIRCam, MIRI, NIRSpec, NIRISS), the FGS, and the V3-axis. Examples of these outputs are shown in the "MTVT outputs" section below.
Official names such as "Ceres," "Jupiter," and "Chiron" are not case-sensitive. However, primary designations such as "2002 MS4" and "2003 AZ84" are case-sensitive. Specifically, provisional designations must use capital letters or else the MTVT will output an error.
Evaluating planet visibility
An example command for running the MTVT for a planet is shown below:
However, running the above command will result in the following message
ValueError: Ambiguous target name; provide unique id:
ID# Name Designation IAU/aliases/other
------- ---------------------------------- ----------- -------------------
5 Jupiter Barycenter
In order to obtain a result from the MTVT, the user must select Jupiter Barycenter as the target using ID# 5:
Or Jupiter as the target using ID# 599:
The ID# of the planet barycenters and the planets themselves used by JPL Horizons are presented in Table 1.
Table 1. JPL Horizons planet ID numbers
Evaluating minor body visibility
An example command for running the MTVT for a minor body using its name is shown below.
An example command for running the MTVT for a minor body using its provisional designation is shown below (quotation marks are required).
jwst_mtvt '2002 MS4'
An example command for running the MTVT for a minor body using its number and the --smallbody optional argument is shown below. The purpose of this optional argument is to remove ambiguity between low-numbered minor bodies and major bodies. The example below will return information on the asteroid 4 Vesta. But, without the --smallbody optional argument, the MTVT would return information for the Mars barycenter.
jwst_mtvt 4 --smallbody
For higher-numbered minor bodies, the --smallbody optional argument is not necessary, as shown below for the dwarf planet Haumea.
The special case of comets
An example command for running the MTVT for a comet using its name is shown below:
An example command for running the MTVT for a comet using its designation is shown below. Use of the --smallbody optional argument is not necessary for comets when using the designation as the identifier.
The commands above are equivalent and will both result in the following message:
JPL Horizons contains multiple orbital solutions for periodic comets, so the user must select one option to proceed. The user is less likely to receive this message for long-period comets. If the user does not know the "Record #" for the orbital solution of interest ahead of time, this is an easy way to see a list of possible values. If the user knows the "Record #" ahead of time, they can skip the step of viewing the list. If the user wants the most recent orbital solution for 2P/Encke, they would type the following command:
Note that the most recent orbital solution is not selected as a default when running the MTVT.
The default outputs for the MTVT are identical to the GTVT: a 6-panel plot showing the allowable position angles for the V3 axis, the 4 science instruments, and the FGS; and an ASCII table containing the information in the plot in table form. See the "Example plots from GTVT" section of the JWST General Target Visibility Tool Help page for instructions on how to output a plot for only one instrument.
The table below shows the dates and duration of each visibility window when the target is in JWST's field of regard, as well as the allowable position angles and the start and end RA and Dec values for these dates. The RA and Dec, along with the allowable position angles for the V3 axis, each of the 4 science instruments, and the FGS, are output for each date that the object is observable by JWST. The table written to the terminal is much longer than shown and has been truncated for the purpose of presentation in this article. The table can be scrolled horizontally to reveal the hidden columns.