6.2.1 General Visit-level Special Requirements

General Special Requirements (e.g., GUIDING, CVZ) are generally used to restrict the scheduling of HST observations by various constraints. The Astronomer's Proposal Tool (APT) is used to enter the requirements into the proposal.

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Format definitions

Boldface type indicates the name of an APT parameter or a value for a parameter.

(red star) Black text indicates an important note.

Magenta text indicates available but unsupported parameters (requires prior approval from STScI).

Red text indicates restricted parameters (for STScI use only).

Brown text indicates text file parameters.

Items in brackets - <value> - are required values.

Items in square brackets - [<value>] - are optional.


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.


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.

Reduced Gyro Mode

counterclockwise arrows button Guiding on gyros is not possible in Reduced Gyro Mode.


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 (counterclockwise arrows button Not possible in Reduced Gyro Mode)

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> (counterclockwise arrows button Not possible in Reduced Gyro Mode)

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] (counterclockwise arrows button Not possible in Reduced Gyro Mode)

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)

GYRO Mode <value> (counterclockwise arrows button 1G in Reduced Gyro Mode)

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.

Guiding Accuracies

Table 6.1: Accuracies of Guiding Modes

Guiding ModeGuiding Accuracy

Gyro hold

Drift rates can vary from 0.01 to 0.1  arcsec/sec  and can vary during the exposures

 counterclockwise arrows button These numbers are now greatly increased due to Reduced Gyro Mode and therefore gyro hold is no longer available

Fine lock

RMS jitter ~0.005 arcsec (RSS both axes)

Coarse track

RMS jitter ~0.005 arcsec (RSS both axes)

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


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.)

(red star) 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> (counterclockwise arrows button Defaults to 70% in Reduced Gyro Mode)

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 70, except in certain cases noted in the following table. <percentage> values below 30 are not allowed.

Program TypeDefault Schedulability

Large GO Programs (see the HST Proposal Categories)




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%.


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.


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. Ultra Disruptive targets are those approved for 24-48 hour response time. (See the ToO User Information Report).  (This Special Requirement must have been indicated in the Phase 1 proposal.) When a ToO special requirement is specified you will be prompted for the total Number Of Activations on the Proposal Information form.


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

For available but unsupported parameters (BEA, PAR)

BRIGHT EARTH AVOIDance <angle in degrees>

Requests that the current visit schedule with the specified minimum Bright Earth Avoidance (BEA) angle. The specified <angle in degrees> is currently limited to 25. All other allowed angles (0 to 90 degrees) are Restricted.

The minimum, default BEA angles for visits with external observations (visits consisting of all internal exposures will default to 0.0 degrees) are as follows:

Instrumentdefault BEA angle
FGS (astrometry)15.5°

Efficiency and schedulability may be reduced with this Special Requirement. Also, this Special Requirement may not be combined with LOW-SKY or SHADOW. The BEA angle will be the largest of the minimum angles required by any prime or parallel instrument operating in the visit.


Specifies that the current visit is to execute as a parallel. This Special Requirement is only needed for those situations where the ground system does not automatically set up the visit as a parallel.

Visits that include spacecraft (S/C) mode exposures that also need to execute as a parallel typically will need this Special Requirement.

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.)

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Table of Contents

Change Log

Version Cycle 32 April-June 2024

  1. PROPINST-91405 - Updated the section on Target of Opportunity Response Time for new  ToO Number of Activations.
  2. PROPINST-91417 - Update changes to default gyro mode and EFF level
  3. PROPINST-91419 - Document changes in availability of features due to change to RGM

Version Cycle 31 June-August 2023

  1.  PROPINST-91390 Document new "Flexible Thursday ToOs" ToOs
  2.  Removed old PR link from Change Log, fixed two links to CP.
  3.  PROPINST-91403 Gyro Guiding Accuracy

Version Cycle 30 April-May 2022

          PROPINST-91361 ON HOLD Not Needed with TARGET OF OPPORTUNITY

          PROPINST-91383 Emphasize which special requirements should have been preapproved in Phase I (CVZ already had words, but added to ToO and On Hold For)

Version Cycle 26 October 2018

  1. PR.89899 Remove NOTRACK Reference to Exposures
    6.2.1 General Visit-level Special requirements

Initial Conversion from Chapter 4 of HST Cycle 25 Phase II Proposal Instructions (Engineering) Version Cycle 25 (June 2018)