Introduction

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:

• Times when the target is not occulted by the Earth.
• Times when the target is not too close to the Sun, Moon, or the bright Earth limb.
• If the target is a planetary satellite, the times when it is not occulted by any other object in the planetary system.
• If the target is a surface feature on a body, the times when the feature is within the field of view of the HST (i.e. the feature is on that part of the body “facing” HST).

Some of these windows are enforced by Default Target Windows (see list near the bottom of this article). Those can actually be removed if required.

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 target “availability” 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 target “availability” windows more than two or three months in advance.

The various keywords used to define windows are given in Table 3.24: Keywords for Observing Windows and described in detail below. Table 3.25: Operators for Observing Windows contains the list of operators that can be used to define a window.

Table 3.24: Keywords for Observing Windows

Table 3.25: Operators for Observing Windows

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

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

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>

ANGULAR_RATE

[NOT] ANGULAR_RATE <object1> [RELATIVE <object2>] FROM <observer> <condition> <rate>

This requirement is used to select windows based on the angular rate of objects in terms of arcsec/sec. When <condition> is greater than (GT) or less than (LT), then <rate> is interpreted as the apparent angular rate of <object1> as observed from <observer> - unless <object2> is also specified. If <object2> is specified, then <rate> is interpreted as the apparent angular rate of <object1> relative to <object2> as observed from <observer>. When <condition> is local minimum (MIN) or local maximum (MAX), then <rate> is interpreted as a tolerance relative to local minimum or maximum inflection points rather than as an absolute angular rate.

R_VEL

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

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

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>

where <angle> is the observer-object-sun angle, in degrees.

OCC

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 the list in 3.2.1 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

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  the list in 3.2.1 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

ECLIPSE is used to find times when one body is in the shadow (cast in sunlight) of another body. The syntax is: [NOT] ECLIPSE <shadow type> <completeness> OF <eclipsed object> BY <eclipsing object> FROM <observer>. The <eclipsed object>, <eclipsing object>, and <observer> must be standard targets from the list in 3.2.1 Solar System Standard Targets. <shadow type> must be specified as either PENUMBRAL or UMBRAL. <completeness> must be specified as either FULL or PARTIAL. <observer> defaults to EARTH.

A partial eclipse is defined to begin when the observer would see the limb of the eclipsed sphere enter the umbra or penumbra of the eclipsing sphere. The partial eclipse ends when the last part of the limb of the eclipsed body would be observed to leave the umbra or penumbra of the eclipsing sphere. A full eclipse is defined to begin when the observer would see the entire eclipsed sphere enter the umbra or penumbra of the eclipsing sphere. The full eclipse ends when the entire eclipsed body would be observed to leave the umbra or penumbra of the eclipsing sphere.

CML

CML is short for "Central Meridian Longitude" and is used to find times when the sub-observer meridian of an object lies within a particular range. For planets and their satellites, planetographic coordinates are used (in the case of Jupiter, lambda(III)). For dwarf planets, a right-handed coordinate system 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 the list in 3.2.1 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>.

By definition, east is the direction to one's right hand side if you were on the body and facing north. For the Sun, the planets, and their satellites, the north pole is defined as parallel to the rotation axis and pointing to the north side of the plane whose normal vector is the angular momentum vector of the solar system (i.e. the north side of the ecliptic plane). For those dwarf planets and their satellites that have been visited by spacecraft (currently Ceres, Pluto, and Charon), north is defined as the spin axis around which they rotate in the right hand sense.

Planetographic longitude only equals planetocentric longitude (e.g. as produced by JPL Horizons) if the target body's rotation is retrograde.

OLG

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 3.1: Orbital Longitude for Satellites):

1. Construct a vector from <object 2> (Earth) to the Level 1 parent (planet) of the <object 1> (satellite).
2. Extend the vector “behind” the planet and project it onto the orbital plane of the satellite. This is the reference axis.
3. The orbital longitude is the angle from the reference axis to the position of the satellite measured in the direction of motion of the satellite. Valid values for the orbital longitude lie in the range 0–360°.
   
   Orbital Longitude of 0° corresponds to superior conjunction, Orbital Longitude of 180° corresponds to inferior conjunction, and 90° and 270° 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 3.2: Orbital Longitude for Planets).
   
   Orbital Longitude of 0° corresponds to opposition, Orbital Longitude of 180° corresponds to conjunction with the Sun. However, Orbital Longitude of 90° or 270° does not correspond with quadrature. Orbital Longitude is not synonymous with “elongation” or “separation” from the sun.

Figure 3.1: Orbital Longitude for Satellites

GALACTIC LATITUDE

GALACTIC LATITUDE is used to find times when the Galactic latitude of one object (as seen from a selected observer) is within a specified range. The syntax is:

[NOT] GALACTIC LATITUDE OF <object> FROM <observer> <condition> <angle 1>  [angle 2]

The<observer> defaults to Earth. <condition> can be GREATER THAN, LESS THAN, or BETWEEN. <angle 1> and [angle 2] are the Galactic latitude in degrees. [angle 2] is only available when <condition> is BETWEEN.

Default Target Windows

Please note that the following defaults apply for solar system targets. 

All targets in the Martian system except Mars:

• SEP OF <target> MARS FROM EARTH GT 10"
• NOT OCCULTATION OF <target> BY MARS FROM <observer>
  

Observations of Jupiter, Jupiter surface features, and offsets from Jupiter:

• NOT ECLIPSE PENUMBRAL PARTIAL OF <target> BY IO FROM EARTH
• NOT ECLIPSE PENUMBRAL PARTIAL OF <target> BY EUROPA FROM EARTH
• NOT  ECLIPSE  PENUMBRAL PARTIAL OF  <target>  BY GANYMEDE FROM EARTH
• NOT  ECLIPSE  PENUMBRAL  PARTIAL  OF  <target>  BY  CALLISTO FROM EARTH

All targets in the Jovian system except Io:

• SEP OF <target> IO FROM EARTH GT 10"

All targets in the Jovian system except Europa:

• SEP OF <target> EUROPA FROM EARTH GT 10"

All targets in the Jovian system except Ganymede:

• SEP OF <target> GANYMEDE FROM EARTH GT 10"

All targets in the Jovian system except Callisto:

• SEP OF <target> CALLISTO FROM EARTH GT 10"

Observations of Saturn, Saturn surface features, and offsets from Saturn:

• NOT ECLIPSE PENUMBRAL PARTIAL OF <target> BY TITAN FROM EARTH

All targets in the Saturnian system except Rhea:

• SEP OF <target> RHEA FROM EARTH GT 10"

All targets in the Saturnian system except Titan:

• SEP OF <target> TITAN FROM EARTH GT 10"

All targets in the Uranian system except Uranus:

• NOT OCC OF <target> BY URANUS FROM <observer>
  

Observations of Neptune, Neptune surface features, and offsets from Neptune:

• NOT ECLIPSE PENUMBRAL PARTIAL OF <target> BY TRITON FROM EARTH

All targets in the Neptunian system except Neptune:

• NOT OCC OF <target> BY NEPTUNE FROM <observer>
  

All TYPE=PGRAPHIC, TYPE=PCENTRIC, and TYPE=MAGNETIC targets:

• NOT OCC OF <target> BY <parent body> FROM <observer>

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.

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