HST Cycle 27 New and Important Features

The Hubble Space Telescope Cycle 27 Call for Proposals differs from Cycle 26 and other previous cycles in the following ways.

What's New for Cycle 27

Proposers to Cycle 27 should be aware that the instrument complement is subject to change. The following are the important features for proposers to consider this Cycle:


  • As with Cycle 26, proposals must now be submitted and will be reviewed in an anonymous format. See HST Cycle 27 Anonymous Proposal Reviews for more information on the review process. Guidelines are provided on how to anonymize a proposal.
  • Proposers must also submit a brief "Team Expertise and Background" section, incorporated in the Astronomer's Proposal Tool. This section will be available to the review panel after the final ranked list is complete, at which point, the review panel may disqualify proposals that are not sufficiently poised to carry out the proposed work. See also HST Cycle 27 Anonymous Proposal Reviews.
  • The term "exclusive access period" is used throughout this Call, as a replacement for "proprietary period." The connotations on data rights, however, remain much the same.
  • Phase I proposals must include in their description of observations bright object protection information sufficient to establish the safety of any proposed measurements which utilize instruments subject to health and safety concerns. Programs that do not contain this information may be subject to cancellation.
  • Phase I proposals that are awarded observing time will be held to a strict deadline for subsequent Phase II and budget submissions.  Programs that submit Phase II proposals that are either late or insufficient for long-range planning will be subject to cancellation.  Programs that submit late budgets may not receive funding.
  • Phase I proposals must itemize and briefly justify the special requirements that will be implemented in Phase II. This includes the potential for orientation constraints; the detailed orientations do not need to be specified until Phase II. All visit-level special requirements and exposure-level special requirements must be justified (see HST Cycle 27 Preparation of the PDF Attachment).
  • The orbit limit for mid-cycle proposals in Cycle 27 is increased to 15 orbits. This applies to both the September 30, 2019 and January 30, 2020 deadlines.


  • HST UV Legacy DD program: The STScI Director will devote 600-1000 orbits of Director's Discretionary time in Cycles 27-29 to a new Legacy program that takes advantage of Hubble's unique UV capabilities to probe star formation process and related stellar astrophyics. The Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) will serve as a UV spectroscopic reference sample of young high and low-mass stars, unifomly sampling fundamental astrophysical parameter-space for each class of star. STScI will constitute an implementation team to work with the community to define target lists and detailed observing modes. No observations will have exclusive access periods, and all will be immediately available to the community. Further details are provided on the working group site. The community is encouraged to consider submitting Cycle 27 proposals to supplement and complement the conceptual program. GO programs that are recommended by the Cycle 27 will have priority over the UV Legacy DD program.
  • Joint HST-TESS proposals: proposers may request high-cadence photometric monitoring by the Transiting Exoplanet Survey Satellite (TESS) for individual targets in their HST program. There is no guarantee that the TESS data will be obtained simultaneously with the HST observations. See Joint HST Cycle 27 proposals for further information.
  • TESS data are now becoming publicly available through MAST. Data for sectors 1 and 2 were released on 12/6/2018 and succeeding sectors will be released periodically. STScI strongly encourages the community to take advantage of this access to develop proposals for follow-up observations with HST in Cycle 27, either through the standard call or through the mid-cycle opportunities.
  • Successful HST proposers will be eligible to apply for NASA High-End Computing Time. Please indicate whether you intend to apply for HEC time in the text of the ‘Special Requirements’ section of the PDF submission. See HST Cycle 27 General Information, Resources, Documentation, and Tools. More information on NASA HEC Program can be found on https://www.hec.nasa.gov.

  • All non-exclusive access data for current Hubble instruments (ACS, COS, STIS, WFC3, FGS) have been made available as part of the Amazon Web Services (AWS) public dataset program. Proposers may request to make use of this dataset under the Archival Cloud Computation Studies category.


  • With the current performance of the pointing control system, the gyro bias drift must be updated periodically, and this is not possible when pointing under gyro control or during slewing (e.g., during moving target tracking or spatial scanning).  For moving target programs, visits cannot be longer than two contiguous orbits.  For spatial scanning programs, each visibility period must have at least 6 minutes of time under FGS control (i.e., 6 minutes without scanning).
  • Due to the current performance of the pointing control system, the Sun avoidance angle has been increased from 50 to 55 degrees to maintain the safety of the observatory and its instruments.
  • COS NUV observations with the G285M grating are discouraged, because of declining throughput. The available COS gratings are described in the COS Instrument Handbook. Users interested in medium-resolution spectroscopic coverage of the wavelength region from 2500 to 3200 Angstroms are encouraged to use STIS instead.
  • The new ACS SBC-LODARK aperture has been defined to meet these conditions: “Any user that needs to observe for longer than ∼ 2 orbits, which is when the temperature of the detector goes above the limit (N.B. at which dark rates become elevated), and has a small target that would fit inside the SBC- LODARK aperture, can place the target at that location. We have therefore defined SBC- LODARK as a new aperture located at (175,185) on the detector. This option will be available to users beginning with APT v26.2.” See ISR 2018-07, by R. Avila et al. for further information.

Next: HST Cycle 27 Anonymous Proposal Reviews

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