Phase 2 Proposal Instructions for Cycle 10
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Phase 2 Proposal Instructions for Cycle 10 | |||
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4.4 Target Position
Target pointing Specification (TPS) and ÒLevelsÓ
Three fields are used to describe the targetÕs position, referred to here as the Target Pointing Specification (TPS). The TPS has been defined using a hierarchical structure.
- Level 1 refers to a target in orbit about the Sun. Examples of Level 1 targets include planets, asteroids, and comets. When a Level 1 object is the desired target for observation, complete the Level 1 field and leave the other two target position fields blank.
- Level 2 refers to a target whose motion is normally described with respect to a Level 1 object. Examples of Level 2 targets include planetary satellites, surface features on planets or asteroids, and non-nuclear positions in the coma of a comet. When a Level 2 object is the desired target for observations, the Level 1 field contains information on the parent body, and the Level 2 field gives positions relative to this body. In this case, leave the Level 3 field blank.
- Level 3 refers to a target whose motion is normally described with respect to a Level 2 object. Examples are a surface feature on a planetary satellite or a pointing which is offset from the center of a planetary satellite. When a Level 3 object is the desired target for observation, then all three fields must be completed, with Level 1 giving the parent of the body described in Level 2, and Level 3 giving the position of the observed target with respect to the body in Level 2.
No more than three levels are allowed. If you believe that your target cannot be described in this form, contact your Program Coordinator.
Describing Targets
The targets specified in the target position fields can be described in up to four ways:
- By a name selected from a list of targets
- By orbital elements
- By coordinates with respect to another object
- Via target selection during a real-time observing session
Table 4.1 gives the list of valid names for solar system targets. PIs are responsible for obtaining up-to-date orbital elements for bodies not in this table. Objects must be denoted by their IAU-adopted name. A good reference for object names can be found in the Astronomical Almanac, and in the Marsden comet catalog (Marsden, B. G., Catalog of Cometary Orbits, Enslow Publishers, Hillside, NJ, 1983). If you are uncertain whether or not your target can be referenced by name, contact your Program Coordinator.
In those cases where the targetÕs position is given with respect to one of the standard objects, the latest available data from JPL on the bodiesÕ physical dimensions, orientation, and rotation rates are used in calculating the targetÕs position. In those cases where all or part of the TPS for your target can be described using standard names, we strongly recommend that you do so. Generally, this will result in the most accurate ephemeris generation for your target.
Specifying Time
Whenever there is an entry involving ÒtimeÓ (anywhere on the form), the format for that entry must be:
where DD is the day of the month, MMM is the first three letters of the month name, YYYY are the full four digits of the Gregorian calendar year, hh is the hour, mm is the minute, and ss.s are the decimal seconds. Only the necessary precision need be specified.
Two different systems of time are used in this document. TDB refers to Barycentric Dynamical Time and can be considered synonymous with ET (Ephemeris Time), which was used before 1984. UTC refers to Coordinated Universal Time. The precise interpretation of each time value depends on the context in which it is used.
4.4.1 Target Position Level 1 [Level_1]
Specify your target in one of the following ways:
- STD = <object name>, where the name must be from Table 4.1, or
- TYPE = <name>.
The TYPE = <name> target description allows the specification of non-standard targets in a variety of formats and must be the first entry in the field if it is used.
- If COMET is chosen, then a set of 2-body orbital elements in the IAU Circular format must be supplied for the target.
- If ASTEROID is chosen, then a set of 2-body orbital elements in the Minor Planet Circular format must be supplied.
For all cases, the required input data are described below. If the data are valid only over a specific period of time, then specify this time interval in the Window field according to the rules given later.
COMET and ASTEROID Positional Parameters
Table 4.3: Positional Parameters for TYPE = COMET
Table 4.4: Positional Parameters for TYPE = ASTEROID
The elements given above refer to the mean ecliptic and equinox of either B1950 or J2000 depending on which ÒvalueÓ is specified for EQUINOX. An example of TYPE = COMET is shown in the ÒExample Target List BlocksÓ below for Example 3. However, notice in this example that the Level 1 target (Comet Halley) is a standard object and could have been listed on the form by name.
It is the responsibility of the observer to supply accurate orbital elements to STScI when specifying TYPE=COMET or TYPE=ASTEROID.
4.4.2 Target Position Level 2 [Level_2]
Five Target Reference Systems (TRSs) are described in the following paragraphs. Please pay careful attention to the definitions of each TRS. Specify your target in one of the following ways:
STD = <object name> or TYPE = <name>
In this case <object name> is from Table4.1: Solar System Standard Targets, or the Type is:
PGRAPHIC
planetographic coordinates relative to Level 1 targetpolar coordinate offsets from Level 1 target
position in magnetic coordinate system
line-of-sight projected coordinate system
orbital elements of a satellite
For the PGRAPHIC, 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
Table 4.5: Parameters for TYPE = PGRAPHIC
LONG = <value> planetographic longitude in degrees, LAT = <value> planetographic latitude in degrees; + or Ð are required to denote north and south latitude, respectively, [ALT = <value>] planetographic altitude above the reference ellipsoid, in kilometers, [R_LONG = <value>]1 rate of change of LONG, in degrees/day, [R_LAT = <value>]1 rate of change of LAT, in degrees/day, [R_RAD = <value>]1 rate of change of RAD, in kilometers/day, and [EPOCH = <value>] the reference time for the temporal variation, in UTC (4 digits).
1EPOCH must also be specified with this quantity.
The PGRAPHIC 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; 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. For planets with retrograde rotation, the planetographic longitude increases in a right-handed direction.
If ALT 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. If any coordinate is given as a function of time, then EPOCH is required and the time-varying coordinate is interpreted in the following way.
For this example the longitude at any time, T, is given by:
longitude = LONG + R_LONG * (T Ð EPOCH)
longitude = 20 + 45 * (t Ð 5ÐJANÐ1990:00:15:00)
The same interpretation for time-varying coordinates also applies to the other TRSs described below.
Position Angle Coordinate System
Table 4.6: Parameters for TYPE = POS_ANGLE
RAD = <value> Radius, in arcseconds ANG = <value> Position angle relative to the reference axis, in degrees REF = NORTH REF = SUN
REF = <value>1Reference axis is celestial north, or Reference axis is the apparent direction to the Sun as projected on the sky, or User specified position angle of the reference measured in degrees eastward from north. [R_RAD = <value>]1 Rate of change of RAD, in arcseconds/sec [R_ANG = <value>]1 Rate of change of ANG, in degrees/day [EPOCH = <value>] the reference time for the temporal variation, in UTC (4 digits).
1EPOCH must also be specified with this quantity.
The POS_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 REF = NORTH, angles are measured from celestial north (positive angles are measured in the same sense as rotating from celestial north through east). For REF = 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). For REF = <value>, the proposer must specify the position angle of the reference axis, measured in degrees East of celestial north (once again, positive angles are measured in the same sense as rotating from celestial north through east).
Magnetic Coordinate System
Table 4.7: Parameters for TYPE = MAGNETO
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 ÒcartographicÓ 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 POLE_LAT (latitude) and POLE_LONG (longitude).
- The final step is to translate the origin of the new system to the specified cartographic latitude, longitude, and radius (O_LAT, O_LONG, and O_RAD, 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
Table 4.8: Parameters for TYPE = TORUS
If the optional fields above are left blank, the following default values are used:
The TORUS TRS is defined primarily to support observations of JupiterÕs plasma torus and is closely related to the MAGNETO TRS. 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 (see special instructions below). 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. Therefore, the TORUS TRS does not rotate with the Level 1 body. 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, LONG = 270 (90 for the west ansa), LAT = 0, RAD = 3.57E05). As the planet rotates, the target moves up and down in celestial coordinates as Jupiter rotates. This coordinate system can also be used to support observations of a planetary ring ansa.
Satellite Elements Coordinate System
Table 4.9: Parameters for TYPE = SAT
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 ÒreferenceÓ axis for the angles defined above is the intersection of the EarthÕs equator at the standard epoch implied by the EQUINOX with the parent planetÕs 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, O_RATE, and W_RATE are not supplied, then their values are assumed to be 0. If RAP and DECP are not supplied, then the standard IAU values are used. If RAP and DECP are supplied, then they should be referred to the standard equator and equinox given by EQUINOX. If EQUINOX is not provided, we will assume J2000.
STScI maintains its ephemeris data base with the best available elements, and you should use the STD = form for objects in Table4.1: Solar System Standard 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=SAT.
4.4.3 Target Position Level 3 [Level_3]
The instructions for this field are identical to those for the Level 2 field except that ÒLevel 3Ó should be substituted wherever ÒLevel 2Ó occurs, and ÒLevel 2Ó should be substituted wherever ÒLevel 1Ó occurs.
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Space Telescope Science Institute http://www.stsci.edu Voice: (410) 338-1082 help@stsci.edu |
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