The observability of solar system targets is often constrained by various geometrical conditions (e.g. satellites observed at greatest elongation from their parent planet), or the desirability of coordinated observations (e.g. the observation of a planetary system at the same time as a spacecraft encounter with the system). The
Window field is provided to allow the proposer to define geometric and timing constraints. The proposer should specify any constraints necessary to achieve the scientific objectives of the program. However, care should be taken in specifying constraints, since they can render the observations difficult or impossible to schedule.
In general, “windows” which define when the target is visible to HST need not be explicitly identified, since these windows will be calculated by the STScI. Windows in this category include:
If you require other specific conditions to be satisfied (e.g. to observe when a satellite is near elongation, to observe when the central meridian longitude lies in a particular range, etc.), then these conditions must be specified in the
Window field. However, the proposer must recognize that proposer-supplied windows might not overlap with the “visibility” windows defined above (calculated by STScI), in which case the observation cannot be scheduled. Note that atmospheric drag and other effects make it difficult to predict the exact position of the HST in its orbit far in advance. This leads to uncertainty in the exact timing of the “visibility” windows more than two or three months in advance.
Table 4.11:
Keywords for Observing Windows
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short for local maximum (i.e. inflection point). Accompanied by a non-zero tolerance value.
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short for local minimum (i.e. inflection point). Accompanied by a non-zero tolerance value.
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The operator NOT, if present, should precede the keyword for the solar system target observing window, as in these examples:
NOT SEP OF IO JUPITER FROM EARTH GT 10
NOT RANGE JUPITER EARTH GT 10
NOT A_VEL IO RELATIVE JUPITER FROM EARTH GT 10
SEP is short for “Separation” and is used to find the times when the apparent separation between two objects, as observed from a third object, satisfies certain conditions. The separation between two bodies is defined as the angle between the closest points on the observed limbs of the spheres representing the objects as viewed from the observer (the radius of the sphere is equal to the largest radius of the tri-axial ellipsoid representation of the object). The syntax is:
[NOT] SEP OF <object 1> <object 2>
FROM <observer> <condition> <angle>
where <object 1>, <object 2>, and <observer> must be standard targets that have been previously defined in the target position fields. The units for “angle” must be chosen from one of
D (degrees),
' (arc-minutes), or
" (arc-seconds). The interpretation of the
SEP keyword is as follows: when the <condition> is either
LT or
GT then times are found when the separation of “objects 1 and 2”, as viewed from <observer>, is
less than <angle> or
greater than <angle> respectively. When the <condition> is
MAX (
MIN), then times are found when “objects 1 and 2” are at
maximum elongation (
minimum separation), as viewed from <observer>.
RANGE is used to select windows based on the separation of objects in terms of distance (AU). The syntax is:
[NOT] RANGE <object 1> <object 2> <condition> <distance>
A_VEL is used to select windows based on the angular velocity of objects in terms of arcsec/sec. The syntax is:
[NOT] A_VEL <object 1> [
RELATIVE <object 2>]
FROM <object 3> <condition> <velocity>
<Velocity> is the angular velocity of <object 1> as observed from <object 3>. If
RELATIVE is used, <velocity> is the apparent angular velocity of <object 1> relative to <object 2> as observed from <object 3>.
R_VEL is used to select windows based on the change in distance between two objects (i.e. the Radial Velocity) in km/sec. The syntax is:
[NOT] R_VEL <object 1> <object 2> <condition> <velocity>
Positive values of <velocity> mean that the objects are moving away from each other while negative values mean that the objects are moving closer to each other.
SIZE is used to select windows based on the apparent angular diameter of an object in arc-seconds. The syntax is:
[NOT] SIZE <object> <condition> <angle>
PHASE is used for solar phase angle, and is used to find times when the angular phase of one body as seen from another is within a specified range. The syntax is:
[NOT] PHASE OF <object>
FROM <observer>
BETWEEN <angle 1> <angle 2>
OCC is short for “Occultation” and is used to find times when one body appears to pass behind another body as viewed from a third body. The syntax is:
[NOT] OCC OF <occulted object>
BY <occulting object>
FROM <observer>
The <occulted object>, <occulting object>, and <observer> must be standard targets from
Table 4.1: Solar System Standard Targets. An occultation is defined to begin when the limb of the sphere representing the <occulted object> first touches the limb of the sphere representing the <occulting object>, as seen from the vantage point of the <observer>.
TRANSIT is used to find times when one body appears to pass across the disk of another body as viewed from a third body. The syntax is:
[NOT] TRANSIT OF <transiting object>
ACROSS <transited object>
FROM <observer>
The <transiting object>, <transited object>, and <observer> must be standard targets from
Table 4.1: Solar System Standard Targets. A transit is defined to begin when the disk representing the <transiting object> is entirely in front of the disk representing the <transited object>, as seen from the vantage point of the <observer>. The transit ends when the limbs of the two disks come into contact again. Thus at any time in the transit the <transiting object> is entirely surrounded by the <transited object>.
ECL is short for “Eclipse” and is used to find times when one body is in the shadow (cast in sunlight) of another body. The syntax is:
[NOT] ECL <type>
OF <eclipsed object>
BY <eclipsing object>
The <eclipsed object> and <eclipsing object> must be standard targets from
Table 4.1. An eclipse is defined to begin when the trailing limb of the <eclipsed object> enters the penumbra (<type> =
P) or the umbra (<type> =
U) of the <eclipsing object>. An eclipse is defined to end when the leading limb of the <eclipsed object> exits the penumbra (<type> =
P) or the umbra (<type> =
U) of the <eclipsing object>. One of the values
P or
U must be specified.
CML is short for “Central Meridian Longitude” and is used to find times when the planetographic sub-observer meridian of an object lies within a particular range (in the case of Jupiter, lambda(III) is used). The syntax is:
[NOT] CML OF <object>
FROM <observer>
BETWEEN <angle 1> <angle 2>
The <object> and <observer> must be standard targets from
Table 4.1: Solar System Standard Targets. The keyword specifies those times when the central meridian longitude lies between <angle 1> and <angle 2> (both in degrees) as seen by the <observer>.
OLG is short for “Orbital Longitude” and is used to select observation times based on a geocentric view (usually) of the object.
OLG can be used on either a Level 1 or a Level 2 object. The syntax is:
[NOT] OLG OF <object 1> [
FROM <object 2>]
BETWEEN <angle 1> <angle 2>
where <angle 1> and <angle 2> are in degrees. OLG specifies those times when the orbital longitude lies between <angle 1> and <angle 2>. The default for <object 2> is the Earth. If <object 1> refers to a Level 2 body, usually a satellite, the orbital longitude is defined as follows (see
Figure 4.1 Orbital Longitude for Satellites):
Orbital Longitude of 0 degrees corresponds to superior conjunction, Orbital Longitude of 180 degrees corresponds to inferior conjunction, and 90 degrees and 270 degrees correspond to greatest eastern and western elongation, respectively.
If “object 1” refers to a Level 1 body, e.g. a planet, asteroid, or comet, the orbital longitude is defined to be the angle between the Sun-Earth vector and the Sun-Planet vector, projected onto the planet’s orbital plane, increasing in the direction of the planet’s orbital motion (see
Figure 4.2: Orbital Longitude for Planets).
Orbital Longitude of 0 degrees corresponds to opposition, Orbital Longitude of 180 degrees corresponds to conjunction with the Sun. However, Orbital Longitude of 90 degrees or 270 degrees does
not correspond with quadrature. Orbital Longitude is
not synonymous with “elongation” or “separation” from the sun.
SEP OF <target>
MARS FROM EARTH GT 10"
SEP OF <target>
JUPITER FROM EARTH GT 30"
SEP OF <target>
IO FROM EARTH GT 10"
SEP OF <target>
EUROPA FROM EARTH GT 10"
SEP OF <target>
GANYMEDE FROM EARTH GT 10"
SEP OF <target>
CALLISTO FROM EARTH GT 10"
SEP OF <target>
SATURN FROM EARTH GT 45"
SEP OF <target>
RHEA FROM EARTH GT 10"
SEP OF <target>
TITAN FROM EARTH GT 10"
NOT OCC OF <target>
BY URANUS FROM EARTH
NOT OCC OF <target>
BY NEPTUNE FROM EARTH
All TYPE=PGRAPHIC,
TYPE=PCENTRIC, and
TYPE=MAGNETIC targets:
NOT OCC OF <target>
BY <parent body>
FROM EARTH
These default windows will be superseded by any similar windows specified in the solar system target list. For example, if the target is Io and an Io-Callisto separation window is specified by the observer, then the observer’s Io-Callisto separation window will apply and the default will not.
