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).

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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.


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  results in a 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 31 Call for Proposals, and with the Instrument Handbooks.

A PDF version is available here: HST Phase II Proposal Instructions for Cycle 31

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.

Significant Changes Since the Previous Cycle


Affected articles

Ticket (Internal link)

Ten new moons for Saturn

3.2.1 Solar System Standard Targets


Enhanced documentation for Central Meridian Longitude (Solar System Observing Window)

3.2.4 Solar System Targets Observing Windows


Generic targets now require either ON HOLD or ToO Special Requirement

3.3 Generic Targets


Add "Flexible Thursday" Target of Opportunity

6.2 Visit-level Special Requirements

6.2.1 General Visit-level Special Requirements


Update Gyro Guiding Accuracy

6.2.1 General Visit-level Special Requirements


Add 4 Guide Star Acquisition Scenarios to support Solar System targets and Spatial Scans

6.4 Implementation Only Special Requirements


FLASHCUR and FLASHEXP now allowed then CTE is not NONE (restricted mode)

10.2 ACS Wide Field Channel, WFCPROPINST-91391

Change ACS Spectro-Polarimetry from Available to Supported mode

10.5.2 ACS WFC Permitted Filter CombinationsPROPINST-91396

Technical Content

Available Science Instruments for Cycle 31

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 31 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 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.