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Hubble Space Telescope
Cycle 21 Announcement

Release Notice: January 07, 2015

NASA and The Space Telescope Science Institute (STScI) are pleased to announce the Cycle 23 Call for Proposals for Hubble Space Telescope (HST) Observations and funding for Archival Research and Theoretical Research programs. Participation in this program is open to all categories of organizations, both domestic and foreign, including educational institutions, profit and nonprofit organizations, NASA Centers, and other Government agencies.

Cycle 23 will extend from October 1, 2015 to September 30, 2016. We will accept proposals for the following instruments: ACS, COS, FGS, STIS, and WFC3.

This solicitation for proposals will be open through April 10, 2015 8:00pm EDT. The Astronomer's Proposal Tools (APT), which is required for Phase I Proposal Submission will be made available/released for Cycle 23 Phase I use during the 2nd week of February 2015. Results of the selection will be announced by the end of June 2015.

Questions can be addressed to the STScI Help Desk (email:; phone: 410-338-1082).


  • Phase I Proposals: Friday March 01, 2013 8:00 pm EST
  • Phase II Proposals & Budgets: Thursday June 27, 2013
  • E/PO Proposals: Wednesday August 21, 2013 5:00 pm EDT

Late Breaking News

What's New for Cycle 23

  • Joint XMM-HST Programs from XMM OTAC for Duplication Checking. For XMM-Newton's Cycle 14 and HST's Cycle 23, the XMM-Newton Observing Time Allocation Committee (OTAC) met in the fall of 2014 and reviewed their proposals. The four joint XMM-Newton proposals listed on the results page have been approved for execution in HST Cycle 23. These observations will be processed during the normal HST cycle ingest after the HST Telescope Allocation Committee (TAC) meets in June of 2015 with those results released to the community by the end of June. Since these programs haven't been loaded into our duplication checking system you will need to review the results page to check for duplications against any proposed HST Cycle 23 Phase I Observations that you might be considering.
  • JWST-related proposals: STScI encourages the community to actively consider Cycle 23 AR, Theory and GO proposals that will develop the scientific landscape for JWST and help maximize its scientific return.
  • COS Lifetime Position. During Cycle 22 COS FUV spectra will be moved from the second COS Lifetime Position (LP2) to the third (LP3) in order to ameliorate the continuing effects of gain sag. While the exact change in performance is not yet known, it is anticipated that the typical spectral resolution (λ/Δλ) may decline by about 15%. Depending on the details of the LP3 implementation, it is possible that the resolution decline for the COS FUV G140L grating may be somewhat larger. Changes in the instrument performance at LP3 will be fully characterized over the Winter 2014/Spring 2015. Changes in the COS extraction and calibration pipeline required by the change to LP3 will result in poorly calibrated FUV observations of extended sources, and FUV observations made with the BOA apertures. Beginning Cycle 23, FUV observations of extended sources, and those with BOA apertures, will be offered as "Available-but-Unsupported". Observers should consult the most recent COS-related newsletters and documentation available on the COS website while preparing their Phase I.
  • New STIS Supported Apertures for STIS MAMA Spectroscopy. STIS has three newly supported neutral-density filtered slits for use with the first-order, and echelle observations. These three apertures, 31X0.05NDA, 31X0.05NDB, and 31X0.05NDC, provide attenuation factors of 6X, 14X, and 33X, respectively. See the STIS Instrument Handbook, Section 13.4 for more information on the basic properties of these apertures.
  • Version 1 of the Hubble Source Catalog (HSC) is scheduled for release in February, 2015. The HSC combines tens of thousands of single visit-based WFC3, ACS, and WFPC2 source lists from the Hubble Legacy Archive (HLA) into a single master catalog with roughly 100 million individual sources. Searches that would require months or years to perform in the past can, in many cases, be done in seconds with the HSC. This resource may be used to support a wide range of new archival proposals. A few potential examples are listed in Section 7.4 of the HST Primer (e.g., variability over more than a 20 year lifetime in many cases, astrometry with better than 10 milli-arcsecond accuracy, cross-matching with a wide variety of catalogs). Currently, a Beta 0.3 version of the HSC is available to support exploratory projects.
  • The Phase I page limit for Medium GO proposals has been increased to 4 pages for the Scientific Justification section, for a total limit of 9 pages for the PDF attachment. Figures and tables may now be embedded in the text.
  • The budget categories for Regular Archival proposals (including Theory and Calibration) are no longer required, removing the Small and Medium categories used for planning purposes in previous cycles. See Section 3.4 for further details. Proposers should still provide a Management Plan as described in Section 9.7.

Important Features Carried Over From Cycle 22:

  • Medium Proposals are being continued as a separate proposal category. General Observer (GO) Proposals are classified as Small (1-34 orbits), Medium (35-74 orbits) or Large (75 or more orbits) based on orbit request. Medium Proposals are reviewed by the panels and ranked together with the Small Proposals./li>
  • The Ultraviolet (UV) Initiative introduced in Cycle 21 will continue in Cycle 23. This initiative is designed to take full advantage of the unique UV capabilities of Hubble while they still exist. It uses orbit allocation targets to increase the share of primary GO observing time dedicated to UV observations (central wavelengths < 3200 Angstroms). Small, Medium, Large, and Treasury GO Proposals are all eligible to benefit, as are all types of Archival proposal, but SNAP Proposals are not eligible.
  • Frontiers Field Program: The STScI Director has devoted a significant fraction of his discretionary time in Cycles 21 and 22 to the Frontiers Field Program. This involves deep imaging observations of four moderate-redshift galaxy clusters and offset blank fields using ACS and WFC3 operating in parallel. Following a favorable mid-term review, the Director has decided to proceed with observations of the final two clusters, Abell S1063 and Abell 370, in Cycle 23. Full details are given at the Frontier Fields website. Data taken for the Frontiers Field Program have no proprietary period, and the community is encouraged to submit AR Proposals, Theory Proposals, and supplementary GO Proposals for the scientific exploitation of these data for all six clusters.
  • Click here for details on these and other changes for Cycle 23

Release Documents

The Call for Proposals describes the policies and procedures for submitting a HST Phase I proposal. Get the CP as: PDF [US Letter Size], PDF [A4 Size], or as HTML.

The Primer provides an introductory overview of the Hubble Space Telescope and explains how to calculate the appropriate number of orbits for an observing proposal. Get the Primer as: PDF [US Letter Size], PDF [A4 Size], or as HTML.

Science Justification Templates for Cycle 21 HST Phase I

HST Phase I Roadmap

The Roadmap is a step-by-step guide to proposing and submitting an HST proposal.

DD Time

Up to 10% of the available HST observing time may be reserved for Director's Discretionary (DD) allocation.

Mid-Cycle Proposals

Up to 200 orbits per cycle will be available for Mid-Cycle GO programs. Mid-Cycle programs were initiated in Cycle 23 to provide the community with an opportunity to propose for in-cycle observations of recently-discovered, non-variable objects. As such, they complement Director’s Discretionary programs, which target unexpected transient phenomena and time-critical observations..


Scheduling Considerations