Call for Proposals Instructions

The Call for Proposals (CP) invites the astronomical community to propose for observing time on HST in a given cycle (nominally one year in duration). It summarizes the policies and procedures for proposing in that cycle of HST observing, including requests for funding research on archival HST data. It also provides an overview of HST's expected capabilities for that observing cycle, including information about the telescope and the available scientific instruments.


Phase I: Writing and Submitting a Proposal for HST Observing Time

A Phase I proposal provides:

  • A summary of the observing program.
  • The scientific justifications for the program.
  • A list of targets with their celestial coordinates.
  • The instrument(s) to be used with desired modes, filters, and dispersers.
  • An estimate of the number of spacecraft orbits needed to accomplish the observing program (a means to calculate this number is provided in the Phase I instructions).

Proposers should consult all relevant technical documentation about the capabilities and sensitivities of the instrument(s) to be used in their programs and should discuss the requirements of their observing programs with appropriate STScI experts. Contacts are provided via the STScI HelpDesk. To avoid duplication conflicts proposers also should consult up-to-date exposure catalogs, available on-line at the STScI Web site.

Learn More About Phase I

DD Time Submission

Up to 10% of the available HST observing time may be reserved for Director's Discretionary (DD) allocation. Learn More

Mid-Cycle Submission

Up to 200 orbits per cycle will be available for Mid-Cycle GO programs. Mid-Cycle programs were initiated in Cycle 23 to provide the community with an opportunity to propose for in-cycle observations of recently-discovered, non-variable objects. As such, they complement Director’s Discretionary programs, which target unexpected transient phenomena and time-critical observations. Learn More


Completion of Phase I: The TAC Review of Phase I Submissions

A Telescope Allocation Committee (TAC), organized by the Space Telescope Science Institute (STScI), reviews and evaluates the submitted Phase I proposals. The TAC recommends a list of programs to the STScI Director for preliminary approval and implementation. Upon final approval by the Director, a successful proposal enters into the Phase II stage of proposal preparation.

Learn More About Peer Review


Phase II: Providing the Observing Details

Following a favorable TAC review and approval by the director, proposers must provide the details required by the ground system to schedule HST and obtain their observations.

Observers are assigned two points of contact at STScI: a Program Coordinator (PC) and a Contact Scientist (CS). The PC helps the observer to deliver a Phase II proposal which is syntactically correct and will schedule successfully on the telescope. The CS provides advice on observing strategies to meet the scientific objectives of the program, answers questions about instrument performance, and can provide technical help in the data analysis phase of the observing program. The CS is an Instrument Scientist involved in the calibration and characterization of the primary instrument used in the observer's program.

Programs are not considered fully accepted until the following items have been completed:

  • An error-free Phase II proposal has been submitted.
  • Duplication checks against similar exposures in previous and current HST programs have been performed and any conflicts have been resolved.
  • All technical feasibility reviews (such as bright object checking, special requirements, etc.) have been completed and approved.


Learn More About Phase II

Proposing Resources

Please Contact the HST Help Desk with any Questions