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ADDENDUM: Hubble Cycle 28 Proposal Selection

L. Watkins (lwatkins[at]stsci.edu) and B. Blacker (blacker[at]stsci.edu)

STScI recently completed the proposal review and time allocation process for Hubble Cycle 28. This year saw a number of changes; some implemented well in advance of the Call for Proposals in anticipation of running Hubble and Webb proposal reviews concurrently, and some implemented in response to the coronavirus pandemic—more details on all of these changes are below. However, many things remained the same: the exceptional quality of the proposals, the exciting science, and the hard work and dedication of all those involved in the process, both as reviewers and behind the scenes. Here we summarize the review process and the results from the Hubble Cycle 28 proposal review.


The Phase Ⅰ deadline this year was earlier than usual, in early March, intended to accommodate scheduling of the now-postponed Webb Cycle 1 proposal review. However, the deadline was extended by one week for those directly impacted by the coronavirus pandemic. We received 16 requests for a deferred deadline; 13 were approved and 12 proposals submitted. All told, we received a total of 1,080 proposals—865 in the General Observer category, 41 in the Snapshot category, 123 in the Archival Research and Archival Legacy categories, and 51 in the Theory category—requesting a total of 22,541 orbits. These proposals included investigators from 44 US States (plus the US Virgin Islands and the District of Columbia), and investigators from 45 countries.

In a departure from our usual process for the main TAC, approximately half of these were sent for external review by 199 reviewers from all over the world; the other half were reviewed in our first fully remote panel meeting. These changes are discussed in more detail below. As in Cycle 26 and 27, this was a Dual-Anonymous review; one proposal was disqualified from consideration for egregious violations of the guidelines. The external review process was conducted through April, and the international members of the proposal review panels and the TAC met in May. Together, the reviewers and panelists provided recommendations to the Director, who approved 190 proposals totalling 2,884 awarded orbits, which will start executing at the beginning of Cycle 28 in October. Up to 200 orbits will be available for Mid-Cycle GO programs targeting recently discovered, non-transient objects. Those proposals may be submitted any time before September 30, 2020 for implementation in late 2020 or early 2021, or submitted by January 30 for implementation in mid-2021.

The Call for Proposals

The Cycle 28 Call for Proposals (CP) was released on December 2, 2019, announcing observing opportunities with HST's current instrumentation: the Advanced Camera for Surveys (ACS), the Cosmic Origins Spectrograph (COS), the Fine Guidance Sensors (FGS), the Space Telescope Imaging Spectrograph (STIS), and the Wide Field Camera 3 (WFC3).

The category of JWST Preparatory proposals (with a default proprietary period of zero months) to complement and enhance the scientific impact of future JWST observations was continued. Also continued was the opportunity to request high-cadence photometric monitoring by the Transiting Exoplanet Survey Satellite (TESS) for individual targets in HST programs. Medium Proposals continued as a separate category for programs requesting between 35 and 74 orbits, to improve the success rate of programs in this historically challenging orbit range. As in previous cycles, as part of the HST proposal it was possible to request time on Chandra, XMM-Newton, and on NOAO and NRAO facilities.

The CP also announced opportunities to request funding for theoretical and archival research. To support the latter, the CP continued the Archival Cloud Computation Studies category introduced in Cycle 27, through which proposers could request to use HST data via the Amazon Web Services (AWS) platform. The CP also encouraged archival proposals that mine the HST Source Catalog, combining tens of thousands of single visit-based WFC3, ACS, and WFPC2 source lists from the HST Legacy Archive into a single master catalog with roughly 100 million individual sources, and the HST Spectroscopic Legacy Archive for high-level data products, containing "science grade" co-added spectra of all usable public data, combining exposures for each target from across visits. The CP also encouraged the community to submit proposals that address questions in fundamental physics, whether they are GO, AR, or theory proposals.

Recognizing the unique and limited availability of HST's ultraviolet (UV) capabilities, the UV Initiative was continued to encourage the community (and the TAC and panelists) to increase the fraction of time and awards dedicated to wavelengths below 3200 Å. The UV initiative applied to all Small, Medium, Large, and Treasury GO Proposal (with the exception of SNAP), as well as Archival Proposals. The available UV instrument modes include ACS/SBC imaging, COS spectroscopy, STIS/MAMA imaging and spectroscopy, STIS/CCD spectroscopy (UV gratings only) and WFC3/UVIS imaging (UV filters only).

In addition, the CP highlighted the Hubble UV Legacy Library of Young Stars as Essential Standards (ULLYSES) program, 600–1000 orbits of Director's Discretionary time spread across Cycles 27–29 designed to leverage Hubble's unique UV capabilities to probe star-formation processes and related stellar astrophysics. In particular, the community was encouraged to submit proposals that would supplement and complement the conceptual program, including Archival proposals to analyze all, or a subset of, the full ULLYSES dataset.

New for Cycle 28: External Reviews

In a recent Newsletter article, the STScI Science Policies Group described a series of changes that would be made to the review process for Hubble Cycle 28, designed to mitigate the impact of running Webb proposal reviews alongside Hubble proposal reviews going forwards. These processes rely on peer review, so adding a second observatory significantly increases the workload and travel demands on the astronomical community. These changes have also been discussed extensively with the Space Telescope Users Committee.

In the past, all proposals have been reviewed in a panel meeting, with a number of mirror panels per field, each with 8–9 reviewers. In advance of the meeting, reviewers read and grade proposals in their panel, the grades are averaged and the proposals ranked. Lower-ranked proposals are triaged; higher-ranked proposals move forward to be discussed at the in-person review meeting. The discussed proposals are re-graded and re-ranked, and the top-ranked are recommended to the Director for approval. In this process, Small (1–34 orbits) and Medium (35–74 orbits) proposals, Snapshot, Archival and Theory proposals are reviewed by the mirror panels; Large (75+ orbits) and Archival Legacy proposals are reviewed by the main TAC.

For Cycle 28, the smaller-sized Small proposals with <16 orbits, Snapshot, Archival and Theory proposals were sent for external review; although this was new for the main cycle, it has been used successfully for HST Mid-Cycle reviews for the past few years. Each proposal was sent to five reviewers; the reviewers graded the proposals and provided brief comments. The grades were averaged and used to rank the proposals within each subfield. The top-ranked proposals in each subfield were recommended to the Director for approval. There was no additional meeting to discuss the proposals. The comments were anonymized and sent to the proposers. External reviewers typically received 10–20 proposals; this workload is much lower than is typical for a panelist, while still large enough to develop a baseline for grading the proposals. This process requires no travel—even in a normal year!

The remaining proposals (Small 16–34 orbits, Medium, Large, Treasury, and Archival Legacy) were intended to be reviewed as normal, that is with preliminary review, grading and triage, culminating in an in-person panel meeting in Baltimore. Due to the decreased number of proposals at the panel meeting, there were no mirror panels.

The Coronavirus Pandemic: A Fully Virtual Panel Meeting

In response to the coronavirus pandemic, we made the decision to move the panel meeting to a fully virtual meeting, whereby all panelists joined the meeting remotely. At the time, the Institute was working under a mandatory work-from-home order, with no staff members on site, so all STScI staff and all NASA observers also joined the meeting remotely. The preliminary phases of the review were unaffected as they required no travel or in-person meetings.

The biggest challenge we faced with a virtual review were time zones, as we had panelists spanning from Eastern Europe to Hawaii. As such, we changed the length of the meetings from the typical 8‑hour days to 6‑hour days, to accommodate those in the most extreme time zones.

The virtual review worked extremely well overall, although, of course, there were some lessons learned that we will improve upon in future virtual reviews. We circulated a survey to the TAC members and are currently analyzing the results. We expect to learn lessons that will help us to improve future in-person panel meetings, as and when we are able to have such again.

The external reviews were not directly affected by the pandemic, as there was no travel or face-to-face component. However we recognize the very real impacts felt by many reviewers due to pandemic, including dependent care, home-schooling, and the added stress and uncertainty at a very unsettled time.

Ensuring an impartial review

We continue to strive for impartiality and fairness in the HST review process. Conflicts of interest for each reviewer are identified based on institution and publication record. Once the proposals are initially distributed to the panel, each panelist must identify any remaining strong conflicts of interest, including competing proposals, mentorship relationships, and close collaborations. Panelists are not permitted to grade proposals for which they are conflicted, and for strong conflicts, e.g., any from which they themselves would directly benefit, panelists are not permitted to participate in the discussion.

Additionally, the Institute has taken steps to address the unconscious biases of the HST TAC process, which has resulted in small but statistically significant over-representation of male PIs relative to female PIs in Cycles 1–26. As with the ΔCycle 26 and Cycle 27 TACs, a dual-anonymous system was used to review the proposals, in which the panelists did not know the identities of the proposers while reviewing the proposals.

Submitted and Approved Proposal Statistics

A total of 1,080 proposals were submitted, requesting 22,541 prime orbits, consistent with the last few cycles (except the unusual Cycles 25 and 26). Of these, 190 were approved for a total of 2,884 orbits. This includes 134 General Observer programs (118 Small for 1665 orbits, 11 Medium for 520 orbits, 5 Large & Treasury for 683 orbits), 11 Snapshot programs for 1098 targets, 3 Archival Legacy programs, 30 Archival Research programs, and 12 Theory programs. These included 81 UV Initiative programs, 8 JWST Initiative programs, 6 Fundamental Physics programs, and 6 ULLYSES programs. In addition, 3 joint HST/NAO programs, 2 joint HST/NRAO programs, 4 joint HST/Chandra programs, 2 joint HST/XMM-Newton programs, and 1 TESS coordinated program were approved. 

The overall success rate for Cycle 28 proposals was 17.6%, or 5.7:1, consistent with the acceptance rate of 17.8% from Cycle 27. The oversubscription ratio for GO programs was approximately 6.5:1 by proposals (higher than the 5.6:1 for Cycle 27) and 7.8:1 by orbits (lower than the 9.1:1 for Cycle 27). The oversubscription ratio for Archival Research and Archival Legacy programs was 3.7:1 (slightly lower than for Cycle 27), and for Theory proposals was 4.5:1 (almost double that for Cycle 27). The estimated oversubscription of Archival and Theory proposals by nominal funding was 4.7:1, consistent with Cycle 27. PIs from ESA member countries led 24% of the accepted Cycle 28 programs, higher than the 21% in Cycle 27. The success rate of Medium proposals was 8.8% by proposal (11 out of 130), consistent with Cycle 27.

Once again, WFC3 was the most-awarded instrument, with 46.1% of the allocated time; this was split between IR imaging (12.9%), IR spectroscopy (12.6%), and UVIS imaging (20.7%). A further 22.4% of the allocated time was for STIS with 19.6% split fairly evenly across CCD, FUV and NUV spectroscopic modes, and 2.8% split between FUV and CCD imaging. 20.6% of the allocated time was for COS FUV (20.0%) and NUV (0.6%) spectroscopy. The remaining 10.9% was for ACS, with the majority split between WFC (6.6%) and SBC (4.2%) imaging. The success rate for the proposals under the UV Initiative was 20% by proposal (12 out of 44 for archival research and 69 out of 359 for GO), and 53% by orbit (1512 orbits out of 2880 requested).

The Cycle 28 orbit distribution per science category is as follows:

  • 13.7% for Exoplanets and Planet Formation (relative to 18.4% submitted),
  • 19.9% for Galaxies (24.6% submitted),
  • 6.8% for Intergalactic Medium and Circumgalactic Medium (13.9% submitted),
  • 10.3% for Large-Scale Structure (7.6% submitted),
  • 4.7% for Solar System (2.8% submitted),
  • 17.8% for Stellar Physics (15.3% submitted),
  • 12.2% for Stellar Populations (9.6% submitted), and
  • 14.6% for Supermassive Black Holes (7.8% submitted).


We are extremely grateful to everyone who was involved in this process at any stage. It takes an enormous amount of time and energy from a huge number of people to see the review through from start to finish, and the complications and stresses of the coronavirus pandemic made this process more intensive than usual.

We thank all of the Hubble TAC members, review panelists, and external reviewers for their service on the Hubble Cycle 28 TAC.

In addition, numerous STScI personnel supported this review process:

From the Science Policies Group, Andy Fruchter, Claus Leitherer, Molly Peeples and Laura Watkins were responsible for selecting panelists and external reviewers, assigning proposals for review based on expertise while avoiding conflicts of interest, and coordinating and processing external reviews. Katey Alatalo, Alessandra Aloisi, Christine Chen, John Debes, Amaya Moro-Martín, Neill Reid, and Lou Strolger assisted with policy coordination, and oversight during the panel meeting.

From the HST Mission Office, Helmut Jenkner, John MacKenty, and Rachel Osten were observers.

Technical Manager Brett Blacker received, organized, and distributed the proposals, oversaw the proposal database, announced the results, and prepared the statistical summaries and figures provided here.

The TAC logistics were devised and coordinated by Brett Blacker and Crystal Mannfolk. Additional administrative support came from Robin Auer, Nick Austin, Martha Devaud, Sherita Hanna, Flory Hill, Jean-Baptiste Regnard, Karen Sealover, and Darlene Spencer.

Panel support was provided by Ramona Augustin, Nimish Hathi, Tuomas Kangas, Brian Kilpatrick, Michael Leveille, Shelly Meyett, Tony Roman, and Jenna Ryon.

Christopher Clark, Alec Hirschauer, Matthew Maclay, Ivelina Momcheva, Elizabeth Nance, Karla Peterson, Blair Porterfield, Mitchell Revalski, and Lou Strolger served as levelers to ensure discussions remained unbiased and focused on the science of the proposal, not the proposers.

Instrument expertise was provided by Norman Grogin and Marco Chiaberge for ACS; Marc Rafelski, Bethan James and Will Fischer for COS; Ed Nelan and Matt Lallo for FGS/OTA; John Debes, Joleen Carlberg, and TalaWanda Monroe for STIS; and Sylvia Baggett, Annalisa Calamida, and Linda Dressel for WFC3. William Januszewski provided expertise on observation planning and scheduling issues. David Adler and Bill Workman provided additional telescope support.

Maria Bertch, Jeff Bucklew, Paul Douglas, Alex Framerini, Craig Hollinshead, Lauretta Nagel, and Joel Richon provided support for the SPIRIT online reviewing tool.

Audio Visual specialist Thomas Marufu ensured that the virtual meetings ran smoothly. Additional IT support was provided by Scott Bennett, Reena DeBerry, Larry Doering, Tanner Ferro, Dante Frias, Gary Gilbert, Romeo Gourgue, Phil Grant, Michael Gutzat, Mike Jackson, David Le, Jamie Lipinski, Justin Mauritz, Leigh McCuen, Ivan Overton, Anupinder Rai, Corey Richardson, Leyla Rutz, Greg Sachs, Patrick Taylor, Jeff Wagner, Thomas Walker, and Otto Wassenius.

Pam Jeffries provided support from the Office of Public Outreach.

Figures and Tables provided by Brett Blacker and formatted by OPO.


Table 1. Summary of Cycle 28 Results
Proposals Requested Approved % Accepted ESA Accepted ESA % Total
General Observer   865   133 15.4% 32 24.1%
Snapshot       41     11 26.8%   2 18.2%
Archival Research       96     28 29.2%    
AR Legacy       27       3   11.1%    
Theory       51     12 23.5%    
Total   1080   187 17.3%   34 23.6%
Primary Orbits 22541 2872 12.7% 557 19.4%
Primary Orbits Includes 6 Orbits from the Calibration Pool



Table 2. Cycle 28 Instrument Statistics
Configuration Mode Prime % Coordinated Parallel %   Total Instrument Prime Usage Instrument Prime + Coordinated Parallel Usage Pure Parallel Usage Snap Usage
ACS/SBC Imaging   4.2%   0.0%   3.8%     0.0%   0.0%
ACS/SBC Spectroscopy   0.0%   0.0%   0.0%     0.0%   0.0%
ACS/WFC Imaging   6.6% 41.6%   9.5%     0.0%  11.9%
ACS/WFC Ramp Filter   0.1%   0.0%   0.1% 10.9% 13.5% 0.0%   0.0%
ACS/WFC Spectroscopy   0.0%   0.0%   0.0%     0.0%   0.0%
COS/FUV Spectroscopy 20.0%   0.0% 18.3%     0.0%   0.0%
COS/NUV Imaging   0.0%   0.0%   0.0% 20.6% 18.9% 0.0%   0.0%
COS/NUV Spectroscopy   0.6%   0.0%   0.5%     0.0%   0.0%
FGS POS   0.0%   0.0%   0.0%   0.0%   0.0% 0.0%   0.0%
FGS TRANS   0.0%   0.0%   0.0%     0.0%   0.0%
STIS/CCD Imaging   1.8%   0.0%   1.6%     0.0% 18.2%
STIS/CCD Spectroscopy   5.2%   0.0%   4.7%     0.0%   0.0%
STIS/FUV Imaging   1.0%   0.0%   0.9% 22.4% 20.5% 0.0%   0.0%
STIS/FUV Spectroscopy   7.5%   0.0%   6.8%     0.0%   0.0%
STIS/NUV Imaging   0.0%   0.0%   0.0%     0.0%   0.0%
STIS/NUV Spectroscopy   7.0%   0.0%   6.4%     0.0%   8.6%
WFC3/IR Imaging 12.9%   6.0% 12.3%     0.0% 13.8%
WFC3/IR Spectroscopy 12.6%   0.0%   11.6% 46.1% 47.2% 0.0% 0.0%
WFC3/UVIS Imaging 20.7% 52.4% 23.3%     0.0% 48.5%
WFC3/UVIS Spectroscopy   0.0%   0.0%   0.0%     0.0% 0.0%
    100% 100.0% 100.0% 100.0% 100.0% 0.0%



Table 3. Proposal Breakdown by PI Country
Country Submitted Approved Country Submitted Approved
Australia   9 1 Japan     3     0
Austria   3 1 Korea     1     0
Belgium   3 2 Mexico     5     1
Brazil   2 0 Norway     2     0
Canada 11 2 Poland     1     0
Chile   7 1 Russia     2     0
China   4 1 Serbia     1     0
Czech Republic   2 0 Spain   11     0
Denmark   3 0 Sweden   16     4
Finland   2 0 Switzerland   19     2
France   8 2 The Netherlands   15     1
Germany 32 6 United Kingdom   56     9
Hungary   1 0 United States 823 148
India   1 0 Ukraine     2     0
Israel   2 0 Vatican     1     0
Italy 32 5      
      ESA Proposals 213   34


Proposal Success Rate as a Function of Orbit Requests

Science Category Distribution for Orbits