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The Solar System Targets form is used to specify moving targets in some JWST observations.  This article provides a description of the Level 2 and 3 specifications needed for the Astronomer’s Proposal Tool (APT).

Level 2 Position 

For the Level 2 position, you specify either a standard target name (from the list of Solar System Standard Targets) or one of six Target Reference Systems (TRSs), which are:

PLANETOGRAPHIC - coordinates relative to Level 1 target

PLANETOCENTRIC - coordinates relative to Level 1 target

POSITION ANGLE - coordinate offsets from Level 1 target

MAGNETO - position in magnetic coordinate system

TORUS - line-of-sight projected coordinate system

SATELLITE - orbital elements of a satellite

For the PLANETOGRAPHIC, PLANETOCENTRIC, MAGNETO, and TORUS coordinate systems, the north pole is defined to be the rotational pole in the northern celestial hemisphere. For planets with direct rotation, the angular momentum vector coincides with the north pole. For planets with retrograde rotation, the angular momentum vector coincides with the south pole.

Planetographic Coordinate System 

The following parameters define the PLANETOGRAPHIC coordinate frame:

  • LONGITUDE in degrees
  • LATITUDE in degrees (use – to denote south latitude)

The following optional values are available:

  • ALTITUDE above the reference ellipsoid, in kilometers
  • LONGITUDE RATE OF CHANGE, in degrees/day
  • LATITUDE RATE OF CHANGE, in degrees/day
  • ALTITUDE RATE OF CHANGE, in kilometers/day
  • EPOCH (the reference time for the temporal variation, in TDB, TDT, or UTC)

(warning) Note that if you specify a LONGITUDE, LATITUDE, or ALTITUDE RATE OF CHANGE, you must specify an EPOCH.

The PLANETOGRAPHIC TRS is the IAU planetographic coordinate system. It is a non-spherical coordinate system aligned with and rotating about the rotation axis of the Level 1 body, positive north, whose origin lies at the center of the reference body. Locations within this TRS are specified by longitude, latitude, and altitude above the surface. (The lambda(III) coordinate system defines the prime meridian in this coordinate system; (warning) if lambda(I) or lambda(II) coordinate systems are desired, note this in the COMMENTS field.)

Planetographic Latitude is defined as the angle between the equator and the normal to the surface of the reference ellipsoid at the point of interest.

By definition, the planetographic longitude of the sub-Earth point increases with time. For planets with direct rotation, the planetographic longitude increases in a left-handed direction (to the West). For planets with retrograde rotation, the planetographic longitude increases in a right-handed direction (to the East). Longitudes should be specified in degrees West for planets with direct rotation and degrees East for planets with retrograde rotation.

If ALTITUDE is omitted, then the surface of the reference ellipsoid is assumed.

If the coordinates are constant in time, then none of the other optional entries should be used. (warning) If any coordinate is given as a function of time, then EPOCH is required. The time-varying coordinate is interpreted as shown in the following example. For

LONGITUDE = 20

LATITUDE = -5

LONGITUDE RATE OF CHANGE = 45

and

EPOCH = 5-JAN-2012:15

the longitude at any time, T, is given by:

longitude = LONGITUDE + LONGITUDE RATE OF CHANGE * (T – EPOCH)

or, numerically,

longitude = 20 + 45 * (T – 5–JAN–2012:00:15:00)

(red star) The same interpretation for time-varying coordinates also applies to the other TRSs described below.

Planetocentric Coordinates 

The following parameters define the PLANETOCENTRIC coordinate frame:

  • LONGITUDE in degrees
  • LATITUDE in degrees (use – to denote south latitude)

The following optional values are available:

  • RADIUS in kilometers
  • LONGITUDE RATE OF CHANGE, in degrees/day
  • LATITUDE RATE OF CHANGE, in degrees/day
  • RADIUS RATE OF CHANGE, in kilometers/day
  • EPOCH (the reference time for the temporal variation, in TDB, TDT, or UTC

(warning) Note that if you specify a rate of change for the LONGITUDE, LATITUDE, or RADIUS RATE OF CHANGE, you must specify an EPOCH.

The PLANETOCENTRIC TRS is the IAU planetocentric coordinate system. It is a right-handed spherical coordinate system aligned with and rotating about the rotation axis of the Level 1 body, positive north, whose origin lies at the center of the Level 1 body. Locations within this TRS are specified by longitude, latitude, and radius from the origin. (The lambda(III) coordinate system defines the prime meridian in this coordinate system; if lambda(I) or lambda(II) coordinate systems are desired, note this in the COMMENTS field.)

Planetocentric longitude increases in a right-handed direction for all planets. For planets with direct rotation, the planetocentric longitude of the sub-Earth point does not increase with time.

If RADIUS is omitted, then RADIUS is assumed to be the equatorial radius of the Level 1 body. Note that in general, if RADIUS is omitted, the point specified will not necessarily be on the visible surface of the planet.  This is of special concern for oblate planets, e.g. Jupiter and Saturn, where a point at high latitude at the equatorial radius can appear above the limb of the planet in projection. When using this coordinate system for surface features on Jovian planets, it is best to specify the radius explicitly.

For spherical planets, planetographic and planetocentric latitudes are identical. For significantly non-spherical objects, there is no simple conversion between the two latitude systems.

For planets with retrograde rotation, the planetocentric and planetographic longitudes of a point are identical. For planets with direct rotation, the planetocentric and planetographic longitudes of a point have opposite sign.

Position Angle Coordinate System 

The following parameters define the POSITION ANGLE coordinate frame:

  • RADIUS, in arcseconds
  • POSITION ANGLE relative to the reference axis, in degrees
  • REFERENCE AXISNORTH (celestial north) or SUN (the apparent direction to the Sun as projected on the sky)

The following optional values are available:

  • RADIUS RATE OF CHANGE, in arcseconds/sec
  • ANGLE RATE OF CHANGE, in degrees/day
  • EPOCH (the reference time for the temporal variation, in TDB, TDT, or UTC)

(warning) Note that if you specify a rate of change for the RADIUS or POSITION ANGLE, you must specify an EPOCH.

The POSITION ANGLE TRS is a position-angle coordinate system (i.e. a two-dimensional polar-coordinate system). This TRS is useful for pointing at targets whose positions are known only in terms of an offset in projected celestial coordinates from another body. The origin of the system lies at the center of the Level 1 body. Locations are specified by giving the apparent distance from the origin (in projected celestial coordinates as viewed from the Earth) and the position angle from some reference axis to the target point. For REFERENCE AXIS = NORTH, angles are measured from celestial north (positive angles are measured in the same sense as rotating from celestial north through east). For REFERENCE AXIS = SUN, angles are measured from the direction to the Sun as projected on the sky (positive angles are measured in the same sense as rotating from celestial north through east).

Magneto Coordinate System 

The following parameters define the MAGNETO coordinate frame:

  • Magnetic LONGITUDE, in degrees
  • Magnetic LATITUDE, in degrees (use - to denote south latitude)
  • Magnetic RADIUS, in kilometers

The following optional values are available:

  • Cartographic LONGITUDE OF THE POLE, in degrees
  • Cartographic LATITUDE OF THE POLE, in degrees
  • Cartographic LONGITUDE OF THE ORIGIN in degrees
  • Cartographic LATITUDE OF THE ORIGIN in degrees
  • Cartographic RADIUS OF THE ORIGIN, in kilometers

The MAGNETO TRS is intended to support observations fixed with respect to a planetary magnetic field. It is a spherical coordinate system rotating with the Level 1 body around the rotation axis, with a specified offset of the coordinate origin and inclination of the coordinate pole. The MAGNETO coordinate system is defined in the following manner:

  • Define a “cartographic” reference frame identical to the planetographic TRS, except use spherical latitudes.
  • Rotate the new coordinate system relative to the cartographic frame so the new pole is located at LATITUDE OF THE POLE and LONGITUDE OF THE POLE.
  • The final step is to translate the origin of the new system to the specified cartographic latitude, longitude, and radius (LATITUDE OF THE ORIGIN, LONGITUDE OF THE ORIGIN, and RADIUS OF THE ORIGIN, respectively).

While the origin and coordinate axes may differ from those of the cartographic system, the rotation axis and rotation rate are identical to those of the cartographic system. Locations in the MAGNETO TRS are specified by longitude, latitude, and radius from the origin of the defined coordinate system.

Torus Coordinate System 

The following parameters define the TORUS coordinate frame:

  • Torus LONGITUDE, in degrees
  • Torus LATITUDE, in degrees; use - to denote south latitude
  • Torus RADIUS, in kilometers
Figure 1. Definition of the torus

Let \hat{E} (black dashed arrow) be the vector from the observer to the apparent center of the torus frame (as defined in Figure 3). The {y}-axis of the torus frame is \hat{z}× – \hat{E} and the {x}-axis of the torus frame is \hat{y}×\hat{z}. Let P be a point in space (red dot), and let \vec{v} be the position vector from the origin of the torus frame to P (green arrow). The LONGITUDE (ɸ) of P (top illustration) is the angle between the {x}-axis and the projection of \vec{v} onto the \hat{x}-\hat{y} plane. LONGITUDE is positive from \hat{x} toward \hat{y}. The LATITUDE (θ) of P (bottom illustration) is the angle between the equator (red dashed line) and \vec{v}, where north is positive and south is negative. The RADIUS of P is the distance from the origin of the torus frame to P (the magnitude of \vec{v}). If the {z}-axis of the torus frame and the rotation axis of the reference body are not aligned, the {z}-axis will rotate with the reference body around the rotation axis. Since the {x}-axis and {y}-axis are defined with relation to the {z}-axis, both axes will vary in response to changes in the {z}-axis, but they do not rotate around the reference body (i.e., the {x}-axis will be in the general direction of the observer and the {y}-axis will be in the plane of the sky, favoring the side of the reference body as indicated by \hat{z} × − \hat{E} ).

The following optional values are available:

  • Cartographic LONGITUDE OF THE POLE, in degrees
  • Cartographic LATITUDE OF THE POLE, in degrees
  • Cartographic LONGITUDE OF THE ORIGIN in degrees
  • Cartographic LATITUDE OF THE ORIGIN in degrees
  • Cartographic RADIUS OF THE ORIGIN, in kilometers

If the optional fields above are left blank, they default to the nominal values for the Jupiter magnetic coordinate frame:

LONGITUDE OF THE POLE = 202

LATITUDE OF THE POLE = +83

LONGITUDE OF THE ORIGIN = 0

LATITUDE OF THE ORIGIN = +0

RADIUS OF THE ORIGIN = 0

Figure 2. Direction of the pole

The LATITUDE OF THE POLE and LONGITUDE OF THE POLE, if specified, define a direction vector in the cartographic frame attached to the reference body (e.g., Jupiter, Saturn, etc.). The LATITUDE OF THE POLE (θ) is positive in the Northern hemisphere and negative in the Southern hemisphere of the reference body. It is measured from the equator of the reference body to the point where the direction vector penetrates the surface (green dot). The LONGITUDE OF THE POLE (ɸ) increases with the reference body’s longitude sense (usually opposite to the direction of rotation) as measured from the body’s prime meridian (yellow curve).It is measured from the prime meridian to the point where the direction vector penetrates the surface.
Figure 3. Definition of the origin

The LATITUDE OF THE ORIGIN, LONGITUDE OF THE ORIGIN, and RADIUS OF THE ORIGIN, if specified, define a point within the cartographic frame of the reference body. The torus frame is centered at this point (represented by the red dot, with the {z}-axis of the torus frame represented by the green arrow). Specifying these quantities results in a torus frame that is not fixed with respect to the apparent disk of the reference body. (However, the torus frame does precess around the polar axis of the reference body.) The LATITUDE OF THE ORIGIN (θ) is positive in the Northern hemisphere and negative in the Southern hemisphere of the reference body. The LONGITUDE OF THE ORIGIN (ɸ) increases with the reference body’s longitude sense (usually opposite to the direction of rotation) as measured from the body’s prime meridian (yellow curve). The RADIUS OF THE ORIGIN (R) is measured in kilometers from the center of the reference body.

The TORUS TRS is defined primarily to support observations of Jupiter’s plasma torus and is closely related to the MAGNETO TRS. The difference between the two systems is in the definition of the prime meridian. For the TORUS TRS, the prime meridian is defined by the instantaneous longitude of the sub-Earth point.

TORUS is also useful for observers who want to observe in a coordinate system that is fixed relative to the apparent disk of the Level 1 body, e.g. central meridian observations. If the TORUS center is defined to be the same as the center of its associated Level 1 body:

LATITUDE OF THE POLE = +90

LONGITUDE OF THE ORIGIN = 0

LATITUDE OF THE ORIGIN = +0

RADIUS OF THE ORIGIN = 0

then, in that case, the TORUS TRS will not rotate with the Level 1 body. If the TORUS center is defined differently from the Level 1 body center, then the polar axis of the TORUS TRS will precess with the Level 1 body's rotation.

A typical observation would be of the east or west ansa (point of maximum elongation) of an equatorial circle whose radius is roughly five times the equatorial radius of Jupiter (in this case, LONGITUDE = 270 (90 for the west ansa), LATITUDE = 0, RADIUS = 3.57E05). This coordinate system can also be used to support observations of a planetary ring ansa.

Satellite Elements Coordinate System 

The following parameters define the SATELLITE coordinate frame:

  • A (Semi-major axis of satellite orbit), in km
  • EPOCH of the elements (Osculation Time), in TDB, TDT, or UTC
  • N (Mean motion of satellite), in degrees/day
  • L (Mean longitude at Epoch), in degrees

The following optional values are available:

  • E (Eccentricity of satellite orbit)
  • I (Inclination of satellite orbit to the planetary equator), in degrees
  • O (Longitude of ascending node of the satellite orbit), in degrees
  • W (Longitude of periapse), in degrees
  • RATE OF CHANGE OF LONGITUDE OF ASCENDING NODE, in degrees/day
  • RATE OF CHANGE OF PERIAPSE, in degrees/day
  • POSITION OF PARENT BODY POLE at EPOCH (Right Ascension and Declination)
  • EQUINOX (B1950 or J2000)

When the target is a satellite of the object defined in the Level 1 field, but the satellite itself is not among the standard objects, then orbital elements must be specified. These elements refer to the motion of the satellite around the Level 1 object.

The “reference” axis for the angles defined above is the intersection of the Earth’s equator at the standard epoch implied by the EQUINOX with the parent planet’s equator at the EPOCH of the elements. The positive X-axis for the coordinate system used in the orbit calculation is obtained by taking the cross product of the Z-axis of the standard system (i.e. the system defined by the standard equator and equinox given by EQUINOX) with the pole of the planet. If E, I, O, W, RATE OF CHANGE OF LONGITUDE OF ASCENDING NODE, and POSITION OF THE PARENT PLANET POLE are not supplied, then the standard IAU values are used. If POSITION OF THE PARENT PLANET POLE are supplied, then they should be referred to the standard equator and equinox given by EQUINOX. If EQUINOX is not provided, J2000 Is assumed.

STScI maintains its ephemeris data base with the best available elements, and you should use the STANDARD TARGET option for objects in list of Solar System Targets unless there is compelling scientific justification for specifying orbital elements.

Note: It is the responsibility of the observer to supply accurate orbital elements to STScI when specifying TYPE=SATELLITE.

Level 3 Position 

For Level 3, the TRSs are the same, except that “Level 3” should be substituted wherever “Level 2” occurs, and “Level 2” should be substituted wherever “Level 1” occurs.

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Keywords

Solar System Targets

Solar System Targets Position Level 2 description

Solar System Targets Position Level 3 description

Format definitions

Boldface italics type indicates the name of an APT parameter or a value for a parameter.

(warning) Red text indicates restrictions on a parameter.

(red star) Black text indicates an important note.

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