Chapter 1 HST Phase II Proposal Instructions Introduction

The HST Phase II Proposal Instructions describe how to prepare proposals to use the Hubble Space Telescope, with the Astronomer's Proposal Tool (APT).

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.


Reduced Gyro Mode

counterclockwise arrows button In June 2024 Hubble operations moved from Three Gyro Mode to  Reduced Gyro Mode. Updates and notes have been included in these Phase II Proposal Instructions to highlight changes due to this switch. Until all the details are confirmed these notes will be denoted with this change symbol: counterclockwise arrows button for greater visibility. 

Proposals to observe with Hubble Space Telescope (HST) are reviewed in two  phases managed by the Space Telescope Science Institute (STScI). In Phase I,  proposers submitted an Observation Summary for review by the Telescope  Allocation Committee (TAC). The Observation Summary provides only general  descriptions of the targets and of the proposed observations. The TAC review process provides a final list of accepted proposals.

During Phase II, General Observers (GOs) with accepted proposals must provide  complete details of their proposed observations so that STScI can conduct a full technical feasibility review. The Phase II information will then be used to schedule  the actual observations and obtain data.

The Phase II Proposal Instructions has the following purposes:

  • To describe the information that must be submitted to STScI by GOs during  Phase II.
  • To describe what you have to provide in APT (Astronomer’s Proposal Tool).
  • To show how to submit the information to STScI.

Proposers should be familiar with the Cycle 32 Call for Proposals, and with the Instrument Handbooks.

A PDF version is available here: HST Phase II Proposal Instructions for Cycle 32

Format Definitions

Some of the modes/parameters are not generally available to proposers, and these are indicated as follows:

    • Available but Unsupported items (shown in magenta) may be used by GOs if there is strong scientific justification. In such cases calibrations will not be provided by STScI. There is limited user support from STScI for the reduction and analysis of data taken with Available-but-Unsupported modes, and that support may be provided at a low priority. Available-but-Unsupported-mode observations will be taken on a shared-risk basis, which means that observations that fail due to the use of this capability will not be repeated. Available-but-Unsupported modes can only be invoked if the Avail_Ok flag is set in the Phase II program.
    • Restricted items (shown in red) generally have no valid scientific use at all and are only used for instrumental calibration and maintenance. These should never be used without the approval of an Instrument Scientist for the science instrument (SI) in question. Use these items with caution!
    • Engineering text (shown in green).

To assist proposers with large programs, there is an option for proposers to export their APT file and then edit their proposal in a Text Proposal File. Text in brown has been added for proposers who elect to use this option. For guidelines on when you might use this template, see Using the Text Proposal File.

Changes Since the Previous Cycle


Affected articles

Ticket (Internal link)

counterclockwise arrows button Document change of gyro mode to 1G and associated schedulability level to 70%

 6.2.1 General Visit-level Special RequirementsPROPINST-91417
counterclockwise arrows button Document changes in availability of features due to change to reduced gyro mode

 Chapter 1 HST Phase II Proposal Instructions Introduction

6.2 Visit-level Special Requirements

6.2.1  General Visit-level Special Requirements

6.2.2 Target Orientation Visit-level Special Requirements

6.2.3 Timing Visit-level Special Requirements

6.3 Exposure-level Special Requirements

6.3.1 General Exposure-level Special Requirements

6.4  Implementation Only Special Requirements

counterclockwise arrows button Document new GSACQ scenarios (for use by program coordinators supporting moving targets and spatial scans)

6.4 Implementation Only Special Requirements

Added an explanation of the new fields for Target of Opportunity Number of ActivationsChapter 2 The Basics of Phase II ProposalsPROPINST-91405
Updated the section on Target of Opportunity Response Time special requirement for new  ToO Number of Activations6.2.1 General Visit-level Special RequirementsPROPINST-91405
Changes to the Fixed Target form to make Coordinate System clearer

3.1 Fixed Targets


Added a cautionary footnote about filter vignetting with the polarizing filters and filter F892N

10.5.1 ACS WFC Spectral Elements


Added a footnote about the ACS/WFC imaging spectropolarimetry capability with the G800L Grism

10.5.1 ACS WFC Spectral Elements

Added Clear filter rules to table 10.4

10.5.2 ACS WFC Permitted Filter CombinationsPROPINST-91413

Technical Content

Available Science Instruments for Cycle 32

ACS/WFC, ACS/SBC, COS, WFC3, STIS and FGS are available for this cycle.

Instrument Modes, Special Requirements, etc.

Some changes have been made to these Instructions since the previous version to correct errors and omissions, and to document new modes or options. There are too many to list, but as long as you use the most recent version the information provided should reflect our current state of knowledge.

Where to Find Additional Information

As you write your Phase II proposal, you will probably need to consult the Instrument Handbooks for additional information on one or more Science Instruments. These handbooks are available on the Web and a PDF version at HST Instrument Handbooks.

There are some policies that were delineated in the  Cycle 32 Call for Proposals, which asked for Phase I proposals. You may wish to review those, particularly the sections discussing limited resources. 

Table of Contents

Change Log

Version Cycle 32 April - June 2024

  1. Updated the Cycle number
  2. Updated the change log.
  3. Fixed an awkward sentence.
  4. Added to the Introduction and explanation of the temporary counterclockwise arrows button symbol to denote changes for reduced gyro mode.

Version Cycle 31 June - August 2023

  1. Started updating the Cycle number
  2. Added change log.

Version Cycle 30 September 2022

  1. Added PDF to parent page and added link to PDF from this page.
  2.  Added change log.