Introduction

Most general special requirements directly or indirectly restrict the times when observations (and/or their component visits) can be scheduled. These should be used to provide the schedulers at STScI with enough constraints to ensure that the observations are properly scheduled. Special requirements should not be used unless necessary to accomplish the scientific objectives of the proposal.

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. (This Special Requirement must have been indicated in the Phase 1 proposal.)

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. (This Special Requirement must have been indicated in the Phase 1 proposal.)

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.  (This Special Requirement must have been indicated in the Phase 1 proposal.)

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. (This Special Requirement must have been indicated in the Phase 1 proposal.)

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>.

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. (This Special Requirement must have been indicated in the Phase 1 proposal.)

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.  (This Special Requirement must have been indicated in the Phase 1 proposal.)

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.  (This Special Requirement must have been indicated in the Phase 1 proposal.)

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  ( Not possible in Reduced Gyro Mode)

This special requirement overrides the orbital visibility period normally computed for the visit. The computed CORON orbital visibility period 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. It is ordinarily implementation-only; see VISIBILITY INTERVAL <time>.

VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET 

This special requirement overrides the orbital visibility period normally computed for the visit.  This should not be used in conjunction with the CVZ or SCHEDulability <percentage> Special Requirements.

As HST is aging, one of its gyros needs a calibration, called a "gyro bias update", to be conducted no less frequently than every third HST orbit in order to maintain satisfactory performance. Initially, a gyro bias update could be conducted only when the telescope was at fixed pointing. When this was the case, it was necessary to limit the duration of moving target visits to no more than 2 orbits.

More recently, an update to the HST onboard software has enabled a gyro bias update to be conducted during a moving target visit. However, that ability comes at the price of consuming 6 minutes at the end of the target visibility interval. In order to reserve sufficient time for gyro bias updates during moving target visits, APT defaults to showing a target visibility interval that is 6 minutes less than would otherwise be the case.

If VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET is specified, then APT will provide the full target visibility interval without the 6 minute truncation. However, the visit will then be limited to a duration of no more than 2 orbits.

Related Links

6.2.1 General Visit-level Special Requirements
6.2.2 Target Orientation Visit-level Special Requirements

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