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Hubble Space Telescope

Mid-Cycle Time 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-transient objects. As such, they complement Director’s Discretionary programs, which target unexpected transient phenomena and time-critical observations.

Mid-Cycle GO Proposals must meet the following prime criteria:

  1. Proposers must provide an explanation of why the proposal could not have been submitted in response to the standard annual Call for Proposals: for example, the target source may have been identified subsequent to the most recent proposal deadline.
  2. Proposers must provide a clear description of the scientific urgency of these observations and why they should be executed in the present cycle.

Proposals should be submitted via the Astronomer’s Proposal Tool (APT) as type GO, using the Mid-Cycle template for the pdf attachment. Upon completion of your Mid-Cycle submission, your program will be transferred to the STScI for processing. If you run into problems submitting an Mid-Cycle Request, Contact the STScI HST Help Desk web: for a quicker response, or send email to for investigation/resolution.

Proposals for Mid-Cycle time must be sufficiently detailed for adequate evaluation, comparable with 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 Mid-Cycle time must be justified explicitly,
  • 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.

Mid-Cycle GO proposals will have the following characteristics:

  • Proposals are limited to requesting no more than 10 orbits;
  • Observations should have minimal constraints to maximize scheduling flexibility;
  • Observations taken for accepted programs will have a proprietary period of no more than 3 months;
  • Proposals may request only HST time - joint proposals are not permitted;
  • Proposers may apply for all available instruments. Proposals must be compliant with the technical restrictions described in the most recent Call for Proposals.
  • Page limits are the same as Small GO as described in the most recent Call for Proposals.

Review Process and Schedule

Members of the STScI Science Policies Group will undertake an initial review of GO Mid-Cycle proposals to determine whether the proposals meet prime criterion #1. Proposals that do not meet that criterion will not be distributed for further review; the Principal Investigator will be informed of that decision, and is free to submit the proposal at the next standard cycle deadline. Re-submissions of rejected past cycle proposals will be rejected automatically.

Mid-Cycle proposals will receive scientific review by members of the community who have contributed to recent HST TAC reviews. Each proposal will be distributed to four reviewers.

Mid-Cycle proposals may be submitted at any time. Proposals received between September 1, 2018 and January 30, 2019 will be considered for implementation in the April 1, 2019 to September 30, 2019 time-frame. January 30th is the only Mid-Cycle Deadline for Cycle 26.

Reviewers are required to address the following issues:

  1. The scientific merit of the proposed investigation and its potential contribution to the advancement of scientific knowledge – grade 1-5;
  2. The program’s importance to astronomy in general – grade 1-5;
  3. The scientific urgency of the observations and their suitability for a Mid-Cycle proposal – grade 1-5;
  4. The program science goals can only be achieved by observations with Hubble Space Telescope – yes/no.

Grades are as follows:

  1. - Excellent
  2. - Very Good
  3. - Good
  4. - Fair
  5. - Poor

Reviewers are also required to provide brief text feedback on the strengths and weaknesses of the proposal, and a rationale for their grades.

The STScI Science Policy group will collect the reviews and rank order the proposals. The Director will make the final selection of proposals to be awarded time.

Summary of APT Mid-Cycle Required Items

Coverpage Information
  • Title
  • Abstract
  • Category
  • Cycle
  • Primary orbits
  • Parallel orbits
  • Proprietary Period (A proprietary period of no more than 3 months)
  • 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)
  • Proposals must now be submitted and reviewed in an anonymous format. See HST Cycle 26 Anonymous Proposal Reviews for more information on the review process. Guidelines are provided on how to anonymize a proposal.
  • Rationale for Mid-Cycle time: Explain why Mid-Cycle 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. Go to the CP.

    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. Go to the Primer.

    Science Justification Templates for Cycle 26 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..

Special Programs from Past Cycles

Status of Current and Past Programs:

Past Proposal Opportunities