The HST Cycle 28 TAC Process

N. Reid, C. Leitherer, L. Strolger, and A. Aloisi

Selecting the science program is the most important activity undertaken by every observatory—the best instruments are only as good as the science results they generate. The selection process generally involves peer review, usually through annual (or semi-annual) meetings of a Telescope Allocation Committee (TAC). These TACs involve a growing number of astronomers dealing with an increasing number of proposals for each facility and a correspondingly increased workload for the community. JWST will join the fray in 2020 (Figure 1). With that in mind, we are revising the HST Cycle 28 review process to mitigate the impact.

Chart showing TAC dates through JWST launch
Figure 1:  Proposal deadlines and TAC dates for a number of major astronomical facilities.

We are making two types of adjustment:

  • Schedule—the dates for releasing the Call for Proposals, the Phase Ⅰ deadline and the TAC meeting are approximately six weeks earlier than this year's schedule. Cycle 28 will still start on October 1, 2020, and the Cycle 28 TAC process will include all types of proposals, covering the full cycle.
  • Structure—most smaller proposals will be assessed and graded by external panelists, allowing us to distribute the workload more broadly and reduce the number of astronomers involved in reviewing the larger proposals at the on-site TAC meeting.

In changing the structure, we are building on four cycles of experience with mid-cycle proposals. Those proposals are distributed to experts in the appropriate field who grade against specific criteria on an absolute scale; the resulting grades show a high degree of consistency, giving us confidence that we can extend this approach to include smaller proposals submitted in response to the general call. Mid-cycle proposals are limited to no more than 15 orbits; approximately half of the proposals submitted to the annual call meet these limits.

The Cycle 28 on-site review will comprise eight topical panels, one each for solar system, exoplanets and disks, stellar physics, stellar populations, galaxies, circumgalactic medium, black holes and their hosts, and cosmology. With one exception, the on-site panels will assess and grade medium GO proposals and regular GO proposals requesting 16–34 orbits; the exception is solar system, where the number of proposals is not sufficient for a split review. Based on past cycles, we anticipate each panel receiving between 35 and 65 proposals for assessment. The remaining GO proposals (≤15 orbits), regular ARs and SNAP proposals will be distributed for external review. Each on-site panel will include seven panelists, a chair and a co-chair. The eight chairs and co-chairs, 2–3 at-large members, and the TAC chair will assess and grade Large, Treasury, and Legacy proposals.

Five experts will assess the externally reviewed proposals, grading on an absolute scale against three criteria: scientific merit within the field, broader importance for astronomy, and the strength of the data analysis plan. HST's unique capabilities must also be required to achieve the scientific goals. Each external panelist will receive a limited number of proposals (≤8), with the grades due four weeks prior to the TAC meeting. The proposals will be grouped by subject area; the rank-ordered list will be provided to the chair and co-chair of the appropriate panel to allow them to identify potential conflicts with the proposals to be reviewed by the panel. In addition, the chairs will have an opportunity to flag for discussion proposals with divergent grades. The initial orbit allocations will be set following the usual procedures based on orbit and proposal pressure, but once the panels have completed their discussions and ranking there will be an opportunity to revise those allocations.

A request to the community: For this process to be successful, the Institute needs to recruit ~500 experts who will be willing to serve as external panelists for a period of two years, covering both the annual and mid-cycle calls. Each external panelist will be assigned no more than 12 proposals over the course of a single cycle, and will not attend the on-site HST TAC meetings. We will be starting the recruitment process later this year, so please keep an eye on your mailboxes.