The Hubble Space Telescope Primer for Cycle 28

The HST Primer provides an introduction to the Hubble Space Telescope for Phase I proposers.

About This Document

The Hubble Space Telescope (HST) Primer for Cycle 28 is a companion document to the Hubble Space Telescope Call for Proposals for Cycle 28. It provides an overview of HST, with basic information about telescope operations, instrument capabilities, and technical aspects of the proposal preparation process. A thorough understanding of the material in this document is essential for the preparation of a competitive proposal. The Call for Proposals discusses the policies and procedures for submitting a Phase I proposal for HST observing or archival research. Technical documentation on the instruments are available in their respective handbooks, available on the HST Document overview site.

Late Breaking News


  • Long-term GO proposals can be of any size category, Small, Medium, or Large. A typo did not include the Small category in this set. Further details are provided on HST Cycle 28 Proposal Categories.


  • STScI and NASA recognize that measures taken to address the coronavirus COVID-19 are affecting the resources available to some researchers. We will accept late proposals from scientists impacted directly by those issues. Late submissions must be accompanied by an appropriate justification (pending approval) by email to These late submissions will be accepted until March 12, 2020. Contact the HST Help Desk for further details and assistance.


  • We have discovered and largely addressed a problem with MAST archival searches by coordinate in the period of 4 Dec 2019 to 26 Feb 2020, such that a significant fraction of HST observations were not found, instrument footprints were not properly displayed for the affected observations, and retrieved data had incorrect header astrometry. This problem is particularly important for potential HST observers currently performing duplication checks as part of the preparation of HST Phase I proposals (due on 6 Mar 2020). If you or your collaborators performed such searches in the affected timeframe, please resubmit your archival requests and duplication checks to the MAST archive. A small number of datasets (of order 1%) remain uncorrected and undiscoverable in searches using the newer MAST interface, and will remain so until after the Phase 1 deadline, but those data will appear in searches with the classic MAST interface.  We apologize for any adverse impact on your archival research and proposal preparations.


  • Some questions have arisen regarding the Hubble Tess Exoplanet Initiative (HTEI). The following statements are offered to clarify the nature of the initiative:
    • There is no pre-specified orbit allocation for HTEI programs. HTEI proposals will be reviewed by the Cycle 28 TAC and will be assessed along with all other Large and Treasury programs. The total orbit allocation to the TAC is 600 orbits.
    • HTEI programs are long-term programs (up to 3 cycles) that will allow the exoplanet community to maximize the science return from TESS by developing coherent, large-scale programs that can respond to new discoveries. Unlike most other programs, targets can be added after the program is approved. Since the aim is to benefit the broad community, the program must describe how community input will be taken into account in selecting new targets.
    • All data taken for HTEI programs will have no exclusive access period and will be available immediately to the community.
    • HTEI proposal must adhere to dual anonymous guidelines.
    • HTEI proposals are not the only avenue for TESS follow-up with HST. As usual, smaller-scale proposals will be reviewed by the panels in Cycle 28 and in subsequent cycles. Based on past precedents, priority will generally be assigned to accepted smaller programs in the event of duplications with HTEI programs.


  • Support Letters for Proposed Mission Support: Proposers for observations in support of NASA/ESA missions who intend to include mission support letters should email them separately to no later than March 13, 2020. The letters should not be added or included in the PDF attachment as they would not likely be compliant with the guidelines for the dual anonymous review. When necessary, the letters will be provided to the TAC panels in the Team Expertise and Background review.


  • The APT Proposal Tool was released on January 22, 2020.


  • Proposers who are eligible for NASA funding and intend to request funds for supporting resources, including support for ground-based observations, should make those intents clear in the scientific justifications of their Phase I proposals. The Financial Review Committee will not approve requests to cover costs for resources in budget proposals (from successful programs) that are not justified in the approved Phase I proposals.

See also HST Cycle 28 New and Important Features.

Proposing Calendar and Deadlines

  • Phase I proposal deadline: March 6, 2020 at 8:00pm EST

  • Cycle 28 Peer Review: May 11-15, 2020

  • Phase II proposal deadline: June 30, 2020

  • Budget deadline: July 16, 2020

  • Cycle 28 Mid-Cycle Deadline: September 30, 2020

Where to Get Help

Submit questions to the HST Help Desk at See for additional tools and resources.

Who's Responsible

The HST Call for Proposals and related materials for Cycle 28 were edited by Christine Chen. The Associate Directory for Science, Neill Reid, and the Science Mission Office at STScI are responsible for the oversight of the HST science program selection process, whose members include Alessandra Aloisi (Head of Science Mission Office), Claus Leitherer (Head of Science Policies Group), Katey Alatalo, Christine Chen, Andrew Fruchter, Amaya Moro-Martin, Molly Peeples, Louis-Gregory Strolger, Laura Watkins, and Technical Manager Brett Blacker.

Next: HST Cycle 28 Primer: Resources, Tools, and Other Documentation