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.

Guiding

There are currently two tracking modes available during HST observations. One of the modes employs guide stars and the Fine Guidance Sensors: Fine Lock. Alternatively, observations can be made while the HST is stabilized with gyros. (In this scenario no guide star acquisition occurs.) The typical guiding accuracies for the two modes are listed in the Table 6.1: Accuracies of Guiding Modes.

PCS MODE Fine

Specifies the use of Fine Lock as the guiding mode for the exposures defined in that particular visit. This Special Requirement is the default and any use of the GUIDing TOLerance <angle> overrides this Special Requirement.

PCS MODE Gyro

Specifies the use of Gyro hold (see Table 6.1: Accuracies of Guiding Modes) as the guiding mode for the exposures defined in that particular visit. This mode is available ONLY with the ACS/WFC, S/C, STIS/CCD and WFC3. It is prohibited with other SIs because of concern that gyro drift will result in inadvertent exposure to bright objects. Note that the RMS absolute pointing error for gyro hold is 14 arcsec with a drift rate that can vary from 0.01 to 0.1 arcsec/sec and can vary during exposures. This Special Requirement overrides the default and any use of the GUIDing TOLerance <angle> Special Requirement.

GUIDing TOLerance <angle>

This special requirement specifies a non-default guiding tolerance for the exposures contained within that particular visit. The <angle> is the allowed maximum pointing error for the observations; the units must be given in arcsec. It should be used in situations when it is permissible for a portion of the observation to be taken while guiding on gyros. It is also used as a trigger for the guide star handoff capability and is useful for fast moving targets. The handoff process involves using a single pair of guide stars for as long as possible. When a given exposure cannot be completed with a given pair of guide stars, guidance is transferred to gyro control. The fine guidance sensors are slewed to and acquire a new pair of guide stars before the exposure in question begins. The error in this procedure is due to the accumulated drift during gyro control, typically a few tenths of an arcsec at most. Note that this is much less than the nominal error between guide star pairs. The planning system will schedule the observations so that the expected pointing-error buildup remains below the <angle> specified. Guide star handoff will only be used if a single pair of guide stars cannot be found for the observation. If you need to use the guide star handoff capability, then set GUIDing TOLerance to at least 0.11".

Note that the difficulty in finding guide stars for these types of observations may prevent them from being scheduled. If you plan to use this Special Requirement, please contact your Program Coordinator for further details and discussions.

DROP TO GYRO [NO REACQuisition]

This Special Requirement allows the system to drop guide-star control for exposures with pointings (target/aperture positions) that are too far (more than 2 arcmin) from the first pointing in the visit to use the same guide stars. If this Special Requirement is not used, such exposures will have to be taken using a separate pair of guide stars. The NO REACQ qualifier will disable guide-star reacquisition after dropping to gyro control, even if subsequent pointings are close enough to the first pointing in the visit to allow guide-star control to be resumed. This Special Requirement has no meaning for internal observations and visits with PCS MODE Gyro. This Special Requirement is allowed ONLY with the ACS/WFC, WFC3 and STIS/CCD SI/detector combinations (see the above discussion of PCS MODE Gyro). DROP TO GYRO is not allowed with the ACS/SBC, STIS/MAMA and COS configurations because of concern about bright objects.

See the discussion of the GUIDing TOLerance <angle> Special Requirement concerning the pointing drift that will occur while the spacecraft is under gyro control. Due to this drift, this requirement is useful primarily for short observations (bright targets) where pointing is not critical (e.g., imaging). This capability might be used if, after spending most of an orbit obtaining spectra of a target, you want to get a quick image without having to acquire a new set of guide stars.

For engineering parameters (GYRO Mode)

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GYRO Mode <value>

This specifies the gyro mode in which the visit is to be executed. The allowed values for <value> are None Selected, 1G, 2G, 3GFHST or 3GOBAD. Normally, the value is set to None Selected; do not change it unless your PC tells you to do otherwise.

For Two Gyro Mode guiding accuracy information, see the Two Gyro Mode Handbook at: http://www.stsci.edu/files/live/sites/www/files/home/hst/documentation/_documents/two-gyro/HST-Two-Gyro-Handbook_Version-3-2006.pdf

Guiding Accuracies

Table 6.1: Accuracies of Guiding Modes

All observations will be performed using Fine Lock guiding by default whenever possible. If difficulties are encountered finding guide stars, you will be contacted by STScI. It has been determined that using Coarse Track guiding for long periods of time is hazardous to the health of the Fine Guidance Sensors, so use of this tracking mode will be considered for extreme cases only. If you cannot find what you feel to be an appropriate tracking mode, you are urged to contact your Program Coordinator for help resolving the issue.

Special Observations 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 an orbital visibility period.  (This Special Requirement must have been indicated in the Phase 1 proposal.)

Caution: CVZ visits are limited to a few narrow scheduling opportunities during a cycle. A detailed definition of the CVZ is given in the HST Observation Types.

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 orbital visibility period 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.

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.

There are special rules that govern orbital visibility for Moving Target. See explanation in the Special Requirement VISIBILITY INTERVAL NO GYRO BIAS UPDATE ON MOVING TARGET.

Servicing mission proposals need to be schedulable at any time in order to accommodate changes in the schedule. The default SCHED value for SM proposals is therefore 100%.

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 visit as appropriate. NOTRACK has no effect on a visit 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> (Hours or Days) 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).  (This Special Requirement must have been indicated in the Phase 1 proposal.)

TARGET OF OPPORTUNITY FLEX DAY (FLEXIBLE THURSDAY)

Once a month, the Hubble schedule will include a "Flexible Thursday" (beginning at approximately 12:00 UT and generally running for 24 hours) with targets that can be rescheduled relatively easily.  The trigger for Flexible Thursday observations must be received by 10:00 UT on the preceding Tuesday, with a fully executable Phase II proposal and no further changes allowed.  This requirement specifies that the current visit is to be executed in this manner.  (This Special Requirement must have been indicated in the Phase 1 proposal.)

Flex Day ToO visits are subject to a number of constraints to ensure easy scheduling:

1. No observations of solar system targets
2. No links to other visits
3. Must be maximally schedulable (use the Sched 100 Special Requirement)
   
4. No use of bright object sensitive detectors (COS/FUV, COS/NUV, STIS/FUV, STIS/NUV, ACS/SBC)
5. No longer than 5 orbits
6. No constraints relative to the South Atlantic Anomaly
7. No spatial scanning
8. No timing constraints or orient restrictions

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

Related Links

6.2.2 Target Orientation Visit-level Special Requirements
6.2.3 Timing Visit-level Special Requirements

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