7.2.2 (cont) Target Orientation

ORIENTation <angle1> TO <angle2> FROM <visit>

Specifies that a roll angle or orientation of the spacecraft, relative to another visit’s spacecraft orientation, is required for the exposures within the current visit. <angle1> and <angle2> denote a region of permitted orientation of the current visit relative to <visit>. <angle1> and <angle2> must be between -180 degrees (-180D) and +180 degrees (180D). If necessary, it is possible for <angle1> and <angle2> to be equal, but the size of the region between the two limits should be made as large as possible to make scheduling easier.

Both angles are measured in a counterclockwise direction, so if the orientation region crosses a point 180 degrees from <visit>, <angle1> should be positive and <angle2> negative. Otherwise, <angle1> should be less than <angle2>. It is possible for the orientation region to be larger than 180 degrees.

This Special Requirement is a limited resource and should be used only when necessary. To specify that the current visit be scheduled at the same orientation as another visit, use the SAME ORIENTation AS <visit> Special Requirement

Note that only one "ORIENT ... FROM " may be specified for a visit.

Nominal and Off-Nominal Roll

Orientation constraints translate into timing constraints. Generally, if the spacecraft is unable to roll far enough "off-nominal" to satisfy the required difference in orientation between the two visits, they will be forced apart in time in order to schedule both at near nominal roll. “Nominal roll” is the orientation determined by the necessity of keeping the solar panels perpendicular to the Sun. Targets near the ecliptic have two values of nominal roll through the year 180 degrees apart. Near the ecliptic pole, nominal roll varies by about a degree per day.

In general, the off-nominal roll is limited to less than +/- 30 degrees except when the target is within two degrees of opposition (i.e., exactly opposite the Sun in the sky as viewed from the Earth). Observations scheduled with solar elongations between 90 degrees and 178 degrees can be done at up to 15 to 30 degrees off nominal, depending on the exact elongation. For observations scheduled when the target is within 90 degrees of the Sun, the off-nominal roll is limited to 5 degrees. The legal syntax for <angle1> and <angle2> allows angles between -180 degrees (-180D) and +180 degrees (180D). Please contact your Program Coordinator for details if necessary.

Instrument Orientation Tables

Table 7.4:  Instrument Orientations with respect to U3-Direction

Instrument	Item	Offset Angle1	Comments
FGS (see Figure 10.1: The FGS POS TARG and interferometer coordinate systems.)	+X axis for FGS1	180°
	+X axis for FGS2	90°
	+X axis for FGS3	0°
STIS (see Figure 9.5 STIS coordinate system)	All configurations:		Nominal, +/- 4D
	Long Slit Spatial/+AXIS2/+Y	45D
	Dispersion/+AXIS1/+X	135D
	Toward increasing occultation:
	WEDGEA	45D
	WEDGEB	135D
WFC3 (see Figure 13.1 and Figure 13.2)	UVIS CCD column (same as the Y_POSTARG direction)	137.18D
	IR +Y	135.37D
	UVIS CCD row	221.38D
	IR +X	225.18D
COS (see Figure 12.1 COS Aperture Coordinate System)	FUV +Y (spatial axis)	135D
	NUV +Y (spatial axis)	135D
	FUV +X (dispersion axis)	45D
	NUV +X (dispersion axis)	45D
ACS (see Figure 11.1 ACS Coordinate system)	WFC1 + Y axis	2D
	WFC2 + Y axis	2D
	SBC + Y axis	180D
	WFCENTER + Y axis	2D
	WFC + Y axis	2D

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1

The Offset Angle is the angle from the axis defined in the Item column to the +U3 axis in the counterclockwise (or +U3 through +U2) direction. To compute the angle needed in the ORIENT Special Requirement, add this Offset Angle in column 3 to the Sky Position Angle (measured North through East). These angles are calculated from current alignment calibrations and reflect uncertainties of +/- 1 degrees, except where greater uncertainties are indicated in the table comments. For the best estimate of an error for a specific SI, which is usually much less than 1 degree, consult with the SI team or go to the SI web pages available at http://www.stsci.edu/hst/HST_overview/instruments

WARNING: For any detailed orientation requirements, please describe your requirements clearly in the proposal text and give angles and offsets. Please feel free to contact your Program Coordinator for assistance in preparing your orientations.

 

Table 7.5:  Approximate Separation and Orientations between WFC3 and the other Science Instruments

Instrument	Apertures	Separation1 (arcsec)	Offset Angle2 (degrees)
ACS	WFC3 to ACS/WFC	356	133D
	STIS to WFC	663	315.7D
	STIS to SBC	817	330.0D
STIS	WFC3 to STIS	308	315D
COS	WFC3 to COS	318	45D

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1

The vector separations are calculated from current estimates of the locations of the WFC3, COS, STIS apertures. These values can change with time.

2

The Offset Angle is defined as the angle from the line connecting the two Science Instruments to the +U3 axis in the counterclockwise direction (the +U3 through +U2 direction). These numbers are calculated from current estimates and reflect uncertainties of +/- 1 degree, except where greater uncertainties are indicated in the table.

WARNING: For any detailed orientation requirements, please describe your requirements clearly in the proposal text and give angles and offsets. Please feel free to contact your Program Coordinator for assistance in preparing your orientations. The above tabulated values are meant to be representative, and because of instrument-specific details the value you need may differ. Please refer to the Observatory Pointing and Apertures pages for current values: http://www.stsci.edu/hst/observatory/apertures/siaf.html

SAME ORIENTation AS <visit>

Sometimes any orientation (or any orientation within the range of the ORIENT Special Requirement) is acceptable the first time an object is observed, but must then be the same for subsequent observations of that target. This Special Requirement requests that the exposures in the current visit be made at the same telescope roll angle as the observations specified in <visit>. If timing Special Requirements are also used, then an incompatibility may result or the observations may be difficult to schedule.

7.2.3 Special Observation Requirements

CVZ

Requests that a visit be scheduled within the Continuous Viewing Zone. When this requirement is specified, observers are allowed the entire 96-minute orbit in which to schedule their observations, instead of restricting them to a visibility interval. Only observers with proposals approved by the TAC for CVZ-usage should use this Special Requirement.

Caution: CVZ visits are limited to a few narrow scheduling opportunities during a cycle. A detailed definition of the CVZ is given in the Call for Proposals.

Note: The CVZ Special Requirement does not necessarily enforce scheduling in an uninterrupted manner. Observations could be scheduled in SAA impacted or earth occulted orbits if that would benefit the overall efficiency of the telescope. If it is required that all (or a subset) of the exposures be done without interruption, the exposure level Special Requirement SEQuence <exposure-list> NON-INTerruptible should be used.

SCHEDulability <percentage>

This Special Requirement allows the observer to adjust the amount of target visibility allowed in each orbit. The visibility at a given pointing varies throughout the year with the 56-day precessional cycle of the HST orbit. This variation is small at zero declination (about 4 minutes between best case and worst case) but increases sharply as the Continuous Viewing Zone is approached. There is a trade-off between visibility and schedulability: visits with longer visibilities allow more science to be packed into each orbit, but are harder to schedule because the long visibility may only be attainable for a few short intervals during the year. Visits with shorter visibilities are less efficient in terms of how much can be done in each orbit, but are easier to schedule.

<Percentage> specifies the percent of HST orbits in which the visit should be schedulable. The higher the percentage, the shorter the visibility. For example, SCHEDulability 80 would allow only enough visibility in each orbit for the visit to be schedulable in the best 80% of HST orbits. SCHEDulability 100 would allow the least amount of time per orbit, but would ensure that the visit would “fit” in every available HST orbit. Schedulability values are defined at 10% intervals, so percentages will be rounded to the nearest multiple of 10%. If this Special Requirement is not supplied, visits will default to 30, except in certain cases noted in the following table. <Percentage> values below 30 are not allowed.

Program Type	Default Schedulability
Large GO Programs (Section 3.2.3 of the HST Call for Proposals)	100

The SCHEDulability Special Requirement may be necessary when an ORIENTation <angle1> TO <angle2> or BETWEEN <date1> AND <date2>) visit-level Special Requirement or a PHASE <number1> TO <number2> exposure-level Special Requirement is specified with a very small tolerance, restricting the visit to only a few days during the cycle. In this case <percentage> should be set to a high enough number to ensure that the visit can schedule in orbits within its time window.

NOTRACK

In certain cases a program will observe a moving target, but without tracking it. For example, a fast-moving comet might be observed by first executing a guide star acquisition, to remove pointing uncertainty, followed by an observation on gyros to acquire the comet data. This should be done without tracking both to save time and to avoid unnecessary use of spacecraft hardware.

The default is to track a moving target, but tracking can be turned off by specifying NOTRACK for each exposure as appropriate. NOTRACK has no effect on an exposure unless it is for a moving target.

TARGET OF OPPORTUNITY RESPONSE TIME <time>

This requirement specifies that the current visit is to be executed as a Target of Opportunity. <time> is the requested time after the activation request is submitted to STScI that the observation should be started. The minimum time allowed is 3 weeks for programs approved for non-disruptive targets of opportunity. Shorter times may be specified for programs approved for disruptive targets of opportunity. The minimum response time is 24 hours (See the ToO User Information Report). Note that all Targets of Opportunity will get an automatic ON HOLD requirement added by APT.

7.2.4 Timing Requirements

For examples of formats of times, see Section 7.1 on page 100, and Table 7.1: Supported Formats for Visit Level Special Requirements.

AFTER <date>

Specifies that the visit must start after the date given by <date>. The capability to designate a specific exposure within a visit to start after a certain time is not supported by this Special Requirement; that case is intended to be handled by adjusting timing within the visit.

AFTER <visit> [BY <time1> TO <time2>]

Specifies that the visit must start after the indicated <visit>. The BY <time1> TO <time2> option allows specification of the time interval (and its allowable range) that must elapse between the start of the referenced visit and the start of the current visit. For example, AFTER 6 BY 7H TO 9H requests that the current visit start no earlier than 7 hours and no later than 9 hours after the start of visit 6. The capability to designate a specific exposure within a visit to start after a certain time is not supported by this Special Requirement; that case is intended to be handled by adjusting timing within the visit.

Note: If the difference between <time1> and <time2> is too small, the visit may be impossible to schedule. A difference of at least 90 minutes (about 1 orbit) is recommended. Also note that <time1> must be as long as the anticipated duration of the referenced visit.

BEFORE <date>

Specifies that the current visit must start before the <date> given. The capability to designate a specific exposure within a visit to start before a certain time is not supported by this Special Requirement; that case is intended to be handled by adjusting timing within the visit.

BETWEEN <date1> AND <date2>

Specifies that the current visit must start between <date1> and <date2>. For example, BETWEEN 14-SEP-1999 AND 21-SEP-1999 indicates that the visit must be started after 14 September 1999 and before 21 September 1999. The capability to designate a specific exposure within a visit to start at a certain time is not supported by this Special Requirement; that case is intended to be handled by adjusting timing within the visit.

Multiple BETWEEN Special Requirements may be specified on a visit. The visit will be allowed to execute during any of the time intervals specified. For example, the combination of BETWEEN 14-SEP-1999 AND 21-SEP-1999 and BETWEEN 10-OCT-1999 AND 1-NOV-1999 means that the visit must be started either between 14 September 1999 and 21 September 1999, or between 10 October 1999 and 1 November 1999. Multiple BETWEEN intervals on the same visit may not overlap: all the other intervals must either end earlier than <date1> or start later than <date2>.

Note: The BEFORE, AFTER <date>, and BETWEEN Special Requirements are mutually exclusive. A visit which specifies one may not specify either of the other two, although multiple BETWEENs are allowed. Note that any BEFORE or AFTER <date> may be replaced by a BETWEEN with a sufficiently early start date or late end date. However, AFTER <visit> may be combined with BETWEEN or BEFORE.

GROUP <visits> WITHIN <time>

Specifies that visits included in the visit list provided must all start within the <time> given. The number of visits in a Group Within set cannot exceed 32. If the interval given is shorter than the least interval possible, the visits will be scheduled as close together as possible. For example, GROUP 7-10 WITHIN 12H requests that visits 7 through 10 all start execution within a 12-hour interval.

Note that GROUP WITHIN is only a timing Special Requirement, and it implies nothing about relative ordering. GROUP 7-10 WITHIN 12H could possibly execute in the order 10, 7, 9, 8, for example.

PERIOD <time> ZERO-PHASE (HJD) <date>

Supplies the period and zero-phase for observations to be made at a specific phase of a periodically variable target. <time> is the period in days, hours, minutes or seconds, and <date> is the date of the zero-phase with respect to the Sun (i.e., HJD, not a calendar date). Note that, while this requirement is at the visit level, the actual PHASE Special Requirement is on the exposure level.

If a target has multiple periods which must be satisfied simultaneously, the PERIOD ZERO-PHASE Special Requirement should refer to the shorter of the two periods and the longer period can be specified using multiple BETWEEN Special Requirements that cover the next year and a half. Be sure to discuss this with your Program Coordinator. (Example: The target is a X-ray pulsar. The observation needs to occur in a particular phase of the 35-hour binary period as well as a particular phase of the 2-month on/off period. Use the PERIOD ZERO-PHASE Special Requirement for the 35-hour period and then specify the 2-month period with multiple BETWEENs.)

SEQuence <visits-checked> WITHIN <time>

Specifies that visits included in the provided visit list must start within the <time> given, and must be ordered according to their visit number. If the interval given is shorter than the least interval possible, the visits will be scheduled as close together as possible. For example, SEQuence Visits 7-10 WITHIN 10H means that visit 10 must begin execution within 10 hours of the start time of visit 7, with visits 8 and 9 executing between. SEQuence does not change the order of visits.

Note that all SEQ WITHIN visits will be executed in numerical order, so SEQuence Visits 2, 1, 4 WITHIN and SEQuence Visits 1, 2, 4 WITHIN do the same thing.

VISIBILITY INTERVAL CORON

This special requirement overrides the visibility interval normally computed for the visit. The calculated CORON visibility interval is based on the target declination and the amount of slew time needed to execute the orientation change specified in the proposal. This should not be used in conjunction with the CVZ or SCHEDulability <percentage> Special Requirements.

Please note that this special requirement reduces the visibility interval used to plan your orbits to a half orbit.

7.2.5 Conditional Requirements

ON HOLD [FOR <visit>]

When the FOR <visit> is specified, an AFTER <visit> [BY <time1> TO <time2>] requirement must also be specified in order to indicate the necessary time separation between the two visits to allow for time to receive/analyze the first visit's data, and rewrite/resubmit/reschedule the second visit