HST Cycle 28 New and Important Features

What's New for Cycle 28

The following are the important features for proposers to consider this Cycle:


  • As with Cycle 27, proposals must now be submitted and will be reviewed in an anonymous format. See HST Cycle 28 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 28 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 section 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, using the Phase I section designated for this purpose. 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 28 Preparation of the PDF Attachment).
  • The orbit limit for mid-cycle proposals in Cycle 28 is 15 orbits. This applies to both the September 30, 2020 and the January 30, 2021 deadlines.


  • HST UV Legacy DD program: The STScI Director will devote 600-1000 orbits of Director's Discretionary time in Cycles 28-30 to a 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, uniformly 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 28 proposals to supplement and complement the conceptual program. This includes Archival proposals to analyze all or a subset of the full ULLYSES datasets. GO programs that are recommended by the Cycle 28 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 28 proposals for further information.
  • STScI announces the HST-TESS Exoplanet Initiative, designed to provide the community with an opportunity to submit long-term (multi-cycle) Treasury programs that capitalize on the exciting small exoplanet discoveries generated by the Transiting Exoplanet Survey Satellite. HST-TESS Exoplanet Initiative proposals should be identified as such in the proposal abstract. See HST Cycle 28 Special Initiatives for further information.
  • TESS data are publicly available through MAST.
  • 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 28 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 more frequently, 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 54 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.
  • Users preparing COS proposals are reminded that the COS2025 policies are still in effect. These policies consist of restrictions on the choice of detector segment and FP-POS positions for the G130M observing modes. The policies are designed to maximize the FUV detector lifetime by minimizing the exposure of the FUVB detector to geocoronal Lyman-alpha emission. Under COS2025, there are now four G130M central wavelengths (cenwaves) that can be used with both detector segments on: 1055, 1096, 1222, and 1291. For the other G130M cenwaves (1300, 1309, 1318, 1327) only segment FUVA can be switched on. Observations requiring the Ly-alpha wavelength range can be performed at Lifetime Position 3 and need to be justified in the Phase I. Detailed information about the changes is available at the COS2025 policies page.
  • The zeropoints for the Solar Blind Channel have been updated to correct a longstanding 30% discrepancy in the absolute flux calibration of the imaging modes. The error was found to be caused by inaccuracies in the filter and detector throughput tables used to derive the zeropoints.  The discrepancy is in the sense that the SBC is actually 30% more sensitive than previously estimated: a source of a given astronomical flux should have produced a 30% larger SBC count-rate; conversely, prior conversions of observed SBC count-rate to flux have overestimated the astronomical flux by 30%.  The throughput tables have now been corrected, and new zeropoints have been derived for the relevant imaging modes.
    In the past year, the ACS Team also characterized the time-dependent sensitivity of the SBC.  The sensitivity has been found to decline 9% since launch.  Corrections for this effect are now included in the pipeline, and adjustments to the zeropoints made accordingly.  Additionally, new flatfield reference files have been delivered for all SBC imaging filters. The total effect of these changes brings the SBC absolute flux calibration within 5% accuracy. More information can be found in ISR ACS 2019-04 and ISR ACS 2019-05.

Next: HST Cycle 28 Proposal Checklist