6.3.2 Timing Exposure-level Special Requirements

Timing Special Requirements (e.g., LOW SKYPHASE) are used to restrict the scheduling of HST observations by timing constraints. The Astronomer's Proposal Tool (APT) is used to enter the requirements into the proposal.

Expand/Collapse all...

On This Page

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.


Requests that the current exposure be taken when the total background light is no more than 30% greater than the yearly minimum value of the zodiacal background for that target. To minimize Earth shine, the exposure will also be taken when the target is at least 40° from the bright Earth. This limits orbital visibility time to about 48 minutes per orbit. Efficiency and schedulability are reduced with this Special Requirement, but to a much lesser degree than with SHADOW. This Special Requirement may not be combined with SHADOW or used in a CVZ visit.  (This Special Requirement must have been indicated in the Phase 1 proposal.)

PHASE <number1> TO <number2>

Requests that the exposure start in the specified phase range (<number1> to <number2>) of a periodic variation. <number1> and <number2> should be between 0.0 and 1.0. The PERIOD <time> ZERO-PHASE (HJD) <date> used in the calculation should have been already entered at the visit level. With short periods, the phase range (difference between <number1> and <number2>) should be made as wide as possible to make scheduling easier.  (This Special Requirement must have been indicated in the Phase 1 proposal.)

Please note that there is no need to repeat the PHASE Special Requirement for multiple exposures within the same Visit (unless you are attempting to specify different phases for those exposures). Instead, specify PHASE for only the first exposure in the Visit. If the phase window you specify is short, you may need to adjust PHASE for the first exposure so that the subsequent ones start before the phase window ends.

(red star) Note that Number_of_Iterations must be 1 if PHASE is specified.

SEQuence <exposure-list> NON-INTerruptible

(red star) This has been replaced in APT with the use of Exposure Group Containers. See Exposure Containers: Exposure Sequences, Coordinated Parallels and Patterns.

Specifies that exposures defined in the <exposure-list> be observed without gaps due to Earth occultation or SAA passages. Gaps between exposures may still be necessary to allow time for activities which are necessary to set up for the next exposure, such as pointing changes, readouts of the SI buffers, and SI reconfigurations. This requirement must be included for a series of FGS exposures which are to be executed within the same orbit.  (This Special Requirement must have been indicated in the Phase 1 proposal.)

For text file

In the Text Proposal File, the exposure list must be a range of numbers separated by a hyphen, such as SEQ NON-INT 55-60.

Note: If this requirement is placed on one exposure (e.g., SEQ 20 NON-INT) and Number_of_Iterations > 1, the sequence of subexposures will not be split.


Requests that the current exposure be taken while HST is in the umbral shadow of the Earth. It is primarily useful when contamination by geocoronal emission (Lyman alpha, OI 1304Å, etc.) must be minimized. However, it does not minimize zodiacal light, which is the principal source of background at wavelengths longer than 3500Å (see the LOW-SKY Special Requirement).

Exposures using this Special Requirement are limited to about 32 minutes per orbit (including overheads). Scheduling may only be feasible for a small percentage of the year. This Special Requirement may not be combined with LOW-SKY or used in a CVZ visit.  (This Special Requirement must have been indicated in the Phase 1 proposal.)

For available but unsupported parameters (AFTER BY)

AFTER BY <time> [TO <time2>]

This Special Requirement, when specified on an exposure within a visit (as opposed to on a visit level), specifies the minimum amount of <time> between the start of the previous exposure and the current exposure’s start. This <time> should never be longer than one orbit. When a TO <time2> is added, it specifies the maximum time between the start of the previous exposure and the current exposure’s start. If the interval given is shorter than the least interval possible, the exposures will be scheduled as close together as possible. Also note that <time2> may not exceed 36 hours. If you plan to use this Special Requirement, please contact your Program Coordinator for further details and discussions.

Expand/Collapse all...

Table of Contents

Change Log

Version Cycle 31 June 2023

  1. Fixed typo
  2. Fixed Links to CVZ

Version Cycle 30 April 2022

          PROPINST-91383 Emphasize which special requirements should have been preapproved in Phase I (Low-Sky, Phase, Seq Non-Int, Shadow)