HST New and Important Features

What's New for Cycle 31

The following are new policies, opportunities and features for proposers to consider this Cycle:

  • New Target of Opportunity Category (Flexible Thursdays):
    • In addition to the long-standing Target of Opportunity (ToO) categories, a new ToO category is available starting in Cycle 31, anticipating a larger sample of transient events for Hubble follow-up observations.  Once a month, the Hubble schedule will include a Flexible Thursday (beginning at approximately 12:00 UT) with targets that can be rescheduled relatively easily, to accommodate the trigger of a ToO on the preceding Tuesday (fully detailed activation and Phase II submission by 10:00 UT). For more information, see HST Observation Types and Special Requirements.

  • GO Program Completion Limit of N+1 Cycles:
    • STScI aims to complete approved observing programs in a timely manner to ensure that scientific impact is maximised. To formalize this aim, in Cycle 31, we are introducing a policy that all GO programs must be completed in N+1 cycles under nominal operations, where N is the number of cycles requested in the Phase I submission. Most programs will be unaffected by this policy. The majority of programs complete within N+1 cycles, with many actually completing within N cycles. Programs impacted by this policy are most likely to be those with extremely time-constrained observations. To ensure completion within N+1 cycles, observers are advised to be as flexible as possible within their timing constraints. Proposers will have the opportunity to request a 1-year extension (taking the limit to N+2 cycles); requests must be scientifically justified. Further extensions will only be considered in exceptional circumstances. For more information, see HST Proposal Implementation and Execution.

  • Joint HST-JWST Observing Programs:
    • Joint HST-JWST observing proposals will be accepted in HST Cycle 31. Proposers should respond to the HST Call for Proposals if the time request for HST is larger than for JWST (where 1 HST orbit is equivalent to 1 JWST hour). Proposals requesting more JWST time than HST time should be submitted in response to the JWST Call for Proposals. Proposals should NOT be submitted to both TACs. Proposers must justify both the scientific and technical reasons for requesting time on both missions. Joint Target of Opportunity programs will be permitted. Exclusive Access Periods for HST data and JWST data will be set independently following the policies for each observatory according to proposal size and type. See HST Proposal Categories for more information.

  • 1 Mid-Cycle Call in Cycle 31:
    • Owing to the shorter cycle length, we will offer only one Mid-Cycle call in Cycle 31. Mid-Cycle proposals are limited to 15 orbits or less, and must address specific science questions; pilot or preparatory studies are not appropriate. The anticipated deadline is Wednesday November 15, 2023 at 11:59pm EST.

  • ACS Spectropolarimetry:
    • Imaging spectropolarimetry is now offered for ACS in Cycle 31. The ACS polarizers (Instrument Handbook Section 6.1) can be used in conjunction with the G800L grism (Instrument Handbook Section 6.3.1) to provide low spectral resolving power (R~100 @ 8000Å) imaging spectropolarimetry from ~5500Å – 8000Å. This mode is still being calibrated during Cycle 30. Therefore, prior to proposing, potential observers should contact the Help Desk to discuss their specific goals and the current status of the mode. Full descriptions of the polarimetry capabilities of the ACS will be found in the ACS Instrument Handbook, and details about reduction of the data will be described in the ACS Data Handbook.

  • Change in Grant Eligibility:
    • NASA has revised the eligibility requirements for GO/AR grants. Investigators who previously were not eligible to apply for grant support, may now be eligible. This applies to new budget proposals submitted on or after May 25, 2022. For more information, see HST Grant Funding and Budget Submissions.

  • STScI Budget Proposer Guide:

Important Features

The following are important features that proposers should keep in mind when crafting their proposals.


  • >6 Consecutive Orbit Blocks:
    • Programs requiring blocks of more than 6 consecutive orbits must be explicitly described and justified in the Special Requirements section of the Phase I proposal. See HST Proposal Implementation and Execution for more information.

  • Exclusive Access Periods:
    • 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.

  • Combined GO-Archival Proposals:
    • GO programs that require funding for Archival research alongside new observations should be submitted as a single GO Proposal, regardless of the relative size of the Archival component. Both the GO and Archival aspects of the program must be clearly described and justified. See HST Proposal Categories for more information.

  • Comparison of HST and JWST Capabilities:
  • Observations of Transient Phenomena:

    • Proposers are reminded that Director’s Discretionary (DD) proposals are intended for follow-up transient phenomena whose occurrence is unexpected. Target of Opportunity (ToO) proposals are intended for observations of transient phenomena whose exact timing is unknown but whose occurrence is expected on a timescale of 1-3 years. As much as possible, we encourage ToO proposals through the main cycle over DD proposals for transient phenomena. For further guidance, see Targets of Opportunity, Mid-Cycle, and Director's Discretionary Programs.

  • Page Limits:
  • Special Requirements & Scheduling Constraints:
    • 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 Preparation of the PDF Attachment for more information.
    • We encourage accepted programs to minimize scheduling constraints. STScI recognizes that some of the scheduling restrictions for successful programs may not be apparent to an observer using APT. Constraints may need to be relaxed to enable scheduling of the program. See HST Proposal Implementation and Execution for more information.

  • Targets of Opportunity in Long-Term GO Programs:
    • Long-term GO programs may only include non-disruptive ToOs for execution in future cycles.


  • Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES):
    • As of Cycle 31, the majority of ULLYSES observations are complete and multiple data releases have been announced. The community is encouraged to consider submitting Cycle 31 proposals to supplement and complement the program. This includes Archival proposals to analyze all or a subset of the full ULLYSES datasets.

  • HST-TESS Exoplanet Initiative (HTEI):
    • The HTEI will continue in Cycle 31, 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. HTEI proposals should be identified as such in the proposal abstract. See Special Initiatives for further information.

  • NASA High-End Computing (HEC) Time:

  • Hubble Data on AWS:

  • Software Development AR Programs:
    • Proposers have an opportunity under the HST AR Program to obtain financial support for the development of software products that will be made available to the community for the purposes of analyzing HST data. The ‘Scientific Justification’ section of the proposal should describe the proposed software plan and also its impact on observational investigations with HST. For more information, see Cycle 30 Proposal Categories for further information, and please contact the Data Science Mission Office (dsmo@stsci.edu) for additional guidance.


  • ACS Solar Blind Channel:
  • COS2025:
    • 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. Detailed information about the changes is available at the COS2025 policies page.

  • COS Lifetime Positions:
    • To extend the lifetime of the COS/FUV detector, spectra are recorded at multiple lifetime positions (LPs) along the cross-dispersion direction, depending on which setting is in use. Details are provided in Section 2.1 of the COS Instrument Handbook (IHB). The main consideration for users is that G160M exposures longer than approximately half an orbit use LP6, while shorter G160M exposures may use LP4 to reduce overheads, if requested in the Phase I proposal. For the conditions under which G160M may be used at LP4, see Section 9.5.1 of the IHB.

  • STIS Spatial Scanning:
    • Spatial scanning with the STIS CCD is an available-but-unsupported mode for obtaining high signal-to-noise ratio spectra of bright targets. A recent analysis of this mode (as reported in the September 2020 STAN) demonstrated that after de-trending, the white light flux measurements can achieve an rms scatter of only 30 ppm.

  • WFC3 Spatial Scanning:
    • Each orbit must have 6 minutes of visibility time under FGS control (i.e. 6 minutes without scanning) to allow for pointing control system updates.

  • WFC3 IR Grism Imaging:
    • Specify the direct image filter in the Phase I and use at least one direct image at the beginning and end of each orbit for optimum calibration, ideally close to the wavelength of the grism.

  • WFC3/UVIS Flash Level:
    • The ETC (rather than APT) provides the most accurate estimate of the image background levels.

    • Any ORIENT restrictions should be included in the Phase I proposal, as these will affect schedulability.

Next: HST Proposal Checklist