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Hubble Space Telescope

Director's Discretionary Time Submission

Up to 10% of the available HST observing time may be reserved for Director's Discretionary (DD) allocation. A DD proposal is appropriate when a truly unexpected transient phenomenon occurs or when developments since the last proposal cycle make a time-critical observation necessary. The pool of orbits for in-cycle DD proposals is limited and typical proposals are less than 5 orbits, although larger requests can be considered under exceptional circumstances.

Scientists wishing to request DD time should first check the Real-time list of all Approved DD Programs. This list is created nightly and posted. Scientists who wish to submit, should use the Astronomer's Proposal Tool (APT) to submit their DD programs. To download and install APT, go to the APT page. For help on using APT to prepare and submit a Phase I program, go to the Phase I Roadmap. You will be providing the "Coverpage" information via the APT tool and you will attach your scientific justification and observation description as a PDF attachment to your submission. More detailed information is included below as well as in the Phase I Roadmap.

If you would like a template to use for your Scientific Justification go to the Current Cycle Announcement Page.

Upon completion of your DD submission, your program will be transferred to the STScI for processing by our DD team. If you run into problems submitting a DD Request, send mail to for investigation/resolution.

Please refer to the section on DD proposals in the latest Call for Proposals for policies regarding this type of proposal.

Proposals for DD time must be sufficiently detailed for adequate evaluation. The required level of detail is the same as for proposals submitted for the regular observing cycles, as described in the current Call for Proposals.
Among other things,

  • both the proposed observations and the use of DD time must be explicitly justified,
  • there must be an adequate description of how the proposed observations relate to the current state of knowledge,
  • and the proposed observations must be described in sufficient detail to allow technical evaluation.

Summary of APT DD Required Items

Coverpage Information
  • Title
  • Abstract
  • Category
  • Cycle
  • Primary orbits
  • Parallel orbits
  • Proprietary Period (Generally zero for DD programs. )
  • Scientific Category
  • Science Keywords
  • Justification PDF Attachment
Investigator Information
  • Full address for PI and limited for CoIs
Observations: Repeat for as many observations as required.
  • Target Name, Coordinates, Magnitude
  • Configuration
  • Science Mode
  • Spectral Elements
  • Total Orbits
  • Observation Flags
Proposal Justification Information PDF Attachment: (Science Justification Templates)
  • Rationale for DD time: Explain why DD time is required; i.e., why the proposal was not submitted to the most recent TAC, or why the proposal cannot wait until the next TAC for evaluation.
  • Scientific Justification: Provide a scientific justification to allow for scientific evaluation.
  • Description of Observations: Provide a description of the proposed observations. Explain the amount of exposure time and number of orbits requested (e.g., number of objects, examples of exposure-time calculations and orbit estimations for some typical observations, etc.). Explicitly describe any non-standard calibration requirements and observations.
  • Scheduling Requirements: Provide any special scheduling requirements (such as required and desired execution windows, special orientation or background requirements, and time links to HST or other observations) to allow for scheduling impact assessment. For minimum schedule disruption, STScI requires that all observing information be submitted at least 2 months prior to execution, in cases where this is possible.

Release Documents

    The Call for Proposals describes the policies and procedures for submitting a HST Phase I proposal. Get the CP as: PDF [US Letter Size], PDF [A4 Size], or as HTML.

    The Primer provides an introductory overview of the Hubble Space Telescope and explains how to calculate the appropriate number of orbits for an observing proposal. Get the Primer as: PDF [US Letter Size], PDF [A4 Size], or as HTML.

    Science Justification Templates for Cycle 24 HST Phase I

  • Full Set of HST Documentation

HST Phase I Roadmap

    The Roadmap is a step-by-step guide to proposing and submitting an HST proposal. Download APT here.

DD Time

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

Mid-Cycle Proposals

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..

Past Proposal Opportunities