Phase 2 Proposal Instructions for Cycle 10

4.7 Observing Windows [Window]

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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, ÒwindowsÓ 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 ÒfacingÓ HST).

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 ÒvisibilityÓ 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 ÒvisibilityÓ windows more than two or three months in advance.

The various keywords used to define windows are given in the following table and described in detail below.

Table 4.10: Keywords for Observing Windows

SEP	angular separation of two bodies as viewed from a third body
RANGE	distance between two bodies in AU
A_VEL	angular velocity in arcsec/sec
R_VEL	radial velocity of one body relative to another in km/sec
SIZE	angular diameter of one body as seen from another in arc-sec
PHASE	phase of one body as seen from another
OCC	when two bodies overlap as viewed from a third body
TRANSIT	when one body crosses another as viewed from a third body
ECL	when one body is in the shadow of another body
CML	central meridian longitude
OLG	orbital longitude

 

Table 4.11: Operators for Observing Windows

LT	short for less than
GT	short for greater than
EQ	short for equals
MAX	short for maximum
MIN	short for minimum.
NOT	logical complement. May be used to specify when a condition does not exist. Each of the above keywords may be preceded by the NOT operator.

 

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 ÒSeparationÓ 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 either be standard bodies, or objects that have been previously defined in the target position fields. The unit for ÒangleÓ 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, GT, or EQ then times are found when the separation of Òobjects 1 and 2Ó, as viewed from <observer>, is less than <angle>, is greater than <angle>, or equals <angle>, respectively. When the <condition> is MAX (MIN), then times are found when Òobjects 1 and 2Ó 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>

A_VEL

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

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 ÒOccultationÓ 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 bodies from Table4.1: 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 bodies from Table4.1: 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

ECL is short for ÒEclipseÓ 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 bodies 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

CML is short for ÒCentral Meridian LongitudeÓ and is used to find times when the sub-observer meridian of an object lies within a particular range. The syntax is:

[NOT] CML OF <object> FROM <observer> BETWEEN <angle 1> <angle 2>

The <object> and <observer> must be standard bodies from Table4.1: 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

OLG is short for ÒOrbital LongitudeÓ 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):

1. Construct a vector from <object 2> (Earth) to the Level 1 parent (planet) of the <object 1> (satellite).
2. Extend the vector ÒbehindÓ 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 0Ð360 degrees.

   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 Òobject 1Ó 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 planetÕs orbital plane, increasing in the direction of the planetÕs 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 ÒelongationÓ or ÒseparationÓ from the sun.

4.7.1 Default Target Windows

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

All targets:

SEP OF <target> SUN FROM EARTH GT 50D

All targets in the Martian system except Mars:

SEP OF <target> MARS FROM EARTH GT 10"

All targets in the Jovian system except Jupiter:

SEP OF <target> JUPITER FROM EARTH GT 30"

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"

All targets in the Saturnian system except Saturn:

SEP OF <target> SATURN FROM EARTH GT 45"

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 EARTH

All targets in the Neptunian system except Neptune:

NOT OCC OF <target> BY NEPTUNE FROM EARTH

All TYPE=PGRAPHIC, 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 observerÕs Io-Callisto separation window will apply and the default will not.