How STScI Conducts the Proposal Review

HST programs are selected through competitive peer review. A broad range of scientists from the international astronomical community evaluates and ranks all submitted proposals, using a well-defined set of criteria and paying special attention to any potential conflicts of interest. The Telescope Allocation Committee (TAC) comprises sub-discipline review panels and the Executive Committee (EC), which offer their recommendations to the STScI Director. Based on these recommendations, the STScI Director makes the final allocation of observing time. Full details on the peer review process are given in the HST Peer Review Guide.

The Review Panels

Dependent on their size, proposals in Cycle 34 will be reviewed either by external panelists or by discussion-based review panels.

Discussion Panels

The Cycle 34 discussion-based (i.e., synchronous, but virtual) review will comprise eight topical panels, one each for:

• solar system astronomy,
• exoplanets and exoplanet formation,
• stellar physics and stellar types,
• stellar populations (and the ISM),
• galaxies,
• circumgalactic medium and intergalactic medium,
• supermassive black holes and active galaxies,
• high-energy transients.

The science topics covered by each panel are listed in the Appendix B: Science Keywords, and each panel will cover the full range of science topics within that science area.

Proposers should note the breadth of topics covered within each science area. Each proposal is read by multiple reviewers. For the topical panels, all reviewers will have expertise within the broad science area. Few, or even none, may have expertise in the specific subfield of the proposal. For the Executive Committee, all reviewers will have expertise with the field of astronomy, but few will have expertise within the broad science area of the proposal, and it is possible none will have expertise in the specific subfield of the proposal. Proposers should take care to write for an appropriately broad audience.

Each panel will be managed by a Panel Chair and most also have a Vice Chair, and there will also be one overall TAC Chair overseeing the review process. Panelists are chosen based on their expertise in one or more of the areas under review by the panels.

Whether proposals go through Discussion review or External review depends on the class and size of the proposal, and the overall number of submissions in the science area. In general, Discussion panels will assess and grade Small GO proposals, Medium GO proposals, and Target of Opportunity (ToO) proposals of any size. The CGM-IGM, Solar System, and SMBH Discussion panels also assess the Very Small, Archival, and Snapshot proposals in their respective science areas as there are not enough proposals in these areas to support a separate External panel. Additionally, the High-Energy Transients panel will review Target of Opportunity proposals for high-energy phenomena, regardless of size.

The orbits allocated to each panel is proportional to the orbits requested by the proposals assigned to that panel; there are separate allocations for Small and Medium proposals. Discussion Panels do not adjudicate Large GO proposals, Treasury GO proposals, or Pure Parallel proposals, but they will advise their chair on the scientific merit of the subset of those proposals assigned to their panel.

External Panels

The Very Small GO proposals, regular AR proposals, and SNAP proposals will be distributed for external review, except for the science areas mentioned above. Those proposals will be assessed by five experts who will grade on an absolute scale against the primary criteria, as discussed below. External Panelists are chosen based on their expertise in one or more of the scientific topics covered by the panel. Each external panelist will receive a limited number of proposals. The proposals will be grouped by subject area, following the same Science Categories used for the Discussion Panels (except those with too few proposals to be split into external/discussion). The proposals likely to be recommended to the Director for acceptance will be provided to the Panel Chairs of the appropriate Discussion panels prior to the meeting to allow them to identify potential duplications with the proposals reviewed by their panel.

The Executive Committee (EC)

The EC will include the TAC Chair, the Discussion Panel Chairs and Vice Chairs, and a few At-large members to ensure broad expertise across the full range of scientific categories. The EC will review Large and Treasury GOs, GO Calibrations, large SNAPs, and Pure Parallel programs. The EC is provided additional input on proposals through feedback from the discussion panels via the Panel Chair and Vice Chair.  

Selection Criteria

Primary Criteria for All Proposals

Evaluations of HST proposals are based on the following three primary criteria. Each criterion is given a separate grade, then the grades are combined with equal weight. Reviewers will be instructed to evaluate these criteria based on what is written in the proposal.

(A) In-field Impact:

• The scientific merit of the program within its immediate sub-field, and its contribution to advancement of knowledge.
• The immediate sub-field of the proposal is the niche area of the program, not the whole broad science area (e.g. Trans-Neptunian Objects, not Solar System Astronomy).
• Proposals should address how the proposed program will improve understanding of the objects, classes of object, or specialist topics under study, and why the work is relevant and timely.

(B) Out-of-field Impact:

• The program’s impact outside of its immediate sub-field.
• A proposal does not have to impact all of astronomy. The out-of-field impacts could be in other sub-fields within the broader science area of the proposal, or in other broad science areas (e.g. in the case of a TNO proposal, this could be solar system formation or planet formation in general, among others).
• Proposals should discuss implications for other fields or sub-fields, and their breadth, significance, and timeliness.

(C) Suitability & Feasibility:

• The suitability of HST observations or datasets, or relevance to HST science. The necessity of special requirements. The feasibility of the science program.
• Proposals should demonstrate that the capabilities of HST are required to achieve the scientific goals, or demonstrate the relevance of the work to HST science. Technical issues will be adjudicated by STScI instrument scientists.
• Proposals should include a clear observing or analysis plan that demonstrates a clear path to science. Proposals should justify time requests and any special requirements, including duplications or joint observatory time.

Further information on the grading rubric, including example cases, are included in the Selection Criteria and Scoring System guide.

Additional Criteria by Proposal Type

Additional criteria exist as well, depending on the Proposal Category, as listed below. Letters in parentheses indicate the affected Primary Criteria.

All GO and SNAP Proposals

• What is the rationale for selecting the type and number of targets? Reviewers will be instructed to recommend or reject proposals as they are and to refrain from orbit- or object trimming. Therefore, it is very important to justify strongly both the selection and the number of targets in your proposal, as well as the number of orbits requested. (C)
• Is there evidence that the project has already been pursued to the limits of ground-based and/or other space-based techniques? (C)
• What are the demands made on HST and STScI resources, including the requested number of orbits or targets, and the efficiency with which telescope time will be used? (C)
• Is the project technically feasible and what is the likelihood of success? Quantitative estimates of the expected results and the needed accuracy of the data must be provided. (C)

Archival Research Proposals

• What will be the improvement or addition of scientific knowledge with respect to the previous original use of the data? In particular, a strong justification must be given to reanalyze data if the new project has the same science goals as the original proposal.

• What are the demands on STScI resources (including funding, technical assistance, feasibility of data requests, archiving and dissemination of products)?

• Is there a well-developed analysis plan describing how the scientific objectives will be realized?

• Will the project result in the addition of new information that can be linked to the Hubble Source Catalog (HSC)?

Large GO and Treasury GO Proposals

• Is there a plan to assemble a coherent database that will be adequate for addressing all of the purposes of the program? (C)

• Is there evidence that the observational database will be obtained in such a way that it will be useful also for purposes other than the immediate goals of the project? (A,B)

Treasury GO Proposals

• What scientific investigations will be enabled by the data products, and what is their importance? (A,B)
• What plans are there for timely dissemination of the data products to the community? High-level science products should be made available through the HST data archive or related channels. (C)

SNAP Proposals

• Willingness to waive part or all of the exclusive access period. While this is not the primary criterion for acceptance or rejection, it can provide additional benefit to any proposal and will be weighed by the reviewers as such. (A,B)

Calibration Proposals

• What is the long-term potential for enabling new types of scientific investigation with HST and what is the importance of these investigations? (A,B)

Recommendations for Acceptance

The task of each panel is to provide a rank-ordered list of proposals. Each panel is allocated a pool of orbits based on the submitted time pressure. Proposals are recommended for acceptance in rank order until the pool of orbits allocated to the panel is exhausted. The allocation is typically N and the cut-off in ranking is called the 1-N line.

Some proposals do not request prime observing hours, including AR, SNAPs, and Pure Parallel. These programs, together with Calibration programs, cost zero orbits to the panel, however, they are expensive in other resources. As such, all proposals in a panel are ranked together based on their scientific merit, and Zero-Orbit proposals are required to rank competitively with the other accepted programs. That is, only programs ranked above the 1-N line are recommended for acceptance, regardless of their orbit cost.

There are further restrictions on some proposals that are considered across all panels together, such as Disruptive ToO activations or SNAP targets. For these, first only proposals above the 1-N line in their panel are recommended for acceptance. Then, if the set of all recommended proposals exceed the allowed activations or targets, the panel rankings are used to further down-select proposals until the activation or target limits are not exceeded.

Next: HST Guidelines and Checklist for Phase I Proposal Preparation