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.

Introduction


Default Single Star Guiding

Starting with Cycle 34 some GO visits will default to single star guiding, similar to imaging SNAPs in Cycle 33. These are visits with only WFC3 or ACS/WFC imaging, with all exposures shorter than 1001 seconds. (Moving target and spatial scans visits will continue to use pairs.) As demonstrated in WFC3 ISR 2025-07 and ACS ISR 2026-04, the PSF quality under single guide star guiding is indistinguishable from that under two guide star guiding for exposures shorter than 1000 seconds. This is being done to extend the life of the FGSs, reduce failures and enhance schedulability. An article overviewing this change, the motivation, and image quality considerations is available in the April 2026 STScI Newsletter.

Most observers do not need to take any action. If you believe your GO science requires two guide stars (such as programs sensitive to sub-pixel pointing drifts in ways that cannot be corrected in post-processing), you may request an exception by sending a scientific justification to your Program Coordinator, and, if applicable, Contact Scientist by August 3, 2026. If approved, you can select the visit level guiding special requirement No Single Star Guiding. Starting in Cycle 35, this will be a special requirement which needs to be specified in the Phase I proposal. SNAPs are not eligible to request an exception. 


Reduced Gyro Mode

counterclockwise arrows button In June 2024 Hubble operations moved from Three Gyro Mode to  Reduced Gyro Mode (RGM). 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. 

One important limitation that APT is not enforcing is that under RGM you cannot have multiple Guide Star Acquisitions within an orbit. So if there are multiple targets within the orbit, none can be far enough away as to cause a second Guide Star Acq. 


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 34 Call for Proposals, and with the Instrument Handbooks.

A PDF version is available here: HPIOM-vCycle34-1.pdf

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



Cycle 34 ChangesAffected articlesTicket (Internal link)

TARGET OF OPPORTUNITY RESPONSE TIME now requires a minimum and maximum response time (and the difference must be at least one day)

6.2 Visit-level Special Requirements

6.2.1 General Visit-level Special Requirements

PROPINST-91442

Correct the documentation of the Bright Earth Avoidance special requirement  (to match the implementation)

6.2 Visit-level Special Requirements

6.2.1 General Visit-level Special Requirements

PROPINST-91440

Added capability to opt out of the new single-star guiding defaults

New restrictions on use of CR=SPLIT special requirement for ACS and WFC3


6.2 Visit-level Special Requirements

6.2.1 General Visit-level Special Requirements

10.1 Introduction to ACS

10.2 ACS Wide Field Channel, WFC

12.1 Introduction to WFC3

12.2 WFC3 UVIS Observing Mode

PROPINST-91452

Added new visit-level special requirement GYRO BIAS UPDATE

Removed the NO GYRO BIAS UPDATE ON MOVING TARGET special requirement

6.2 Visit-level Special Requirements

6.2.3 Timing Visit-level Special Requirements

PROPINST-91421

New COS Lifetime Positions and how to specify all Lifetime Positions

11.1 Introduction to COS

11.2.1 COS FUV Target Acquisition Modes

11.3.1 COS FUV Observing Modes

11.5.2 COS Spectral Elements and Central Wavelengths

PROPINST-91445


Technical Content

Available Science Instruments for Cycle 34

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 34 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 34 July 2026

  1. Added information about the new default single star guiding.

Version Cycle 34 April 2026

  1. Updated the Cycle number.

Version Cycle 33 May 2025

  1. Updated the Cycle number.
  2. Updated the change log.
  3. PROPINST-91428 Additional reduced Gyro Mode limitation

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.