HST Cycle 27 Primer Resources, Tools, and Other Documentation

This page contains general information on the resources, tools, and other documentation that may be useful in the preparation of an HST proposal.


The Phase I Proposal Roadmap is a high-level step-by-step guide to writing a Phase I Proposal. At each step, links are provided to relevant information. 

The Cycle 27 Announcement webpage contains links to information needed for preparing an HST proposal. It also contains late-breaking updates regarding the Phase I process and a FAQ (frequently asked questions).

The Call for Proposals discusses policies and procedures for submitting a Phase I proposal for HST observing or Archival Research. It also provides a summary of the proposal process from proposal submission to the execution of observations. 

Instrument handbooks, the primary source of information for HST instruments, provide additional information beyond what’s presented in this Primer. Please use current versions of the handbooks when preparing the Phase I proposal. The latest handbook versions for active and decommissioned instruments are available at the HST Documents webpage. Other potentially useful documents, such as instrument science reports, data handbooks, and calibration conference proceedings are also accessible from that website.

Observation Planning Tools

  • The Astronomers Proposal Tool (APT) is the software interface for all Phase I and Phase II proposal submissions for HST. Please refer to the APT webpage for information regarding the installation and use of APT.
  • The Aladin Sky Atlas, available through APT, can be used to display HST apertures on images of the sky. This software interface provides access to a wide variety of images and catalogs; note that the GALEX catalog is available to assist in checking for potentially dangerous objects for the UV detectors. Training documentation and videos can be found on the APT Training Materials webpage.  

Information in this Primer, together with the instrument handbooks, provides the means for estimating acquisition times, exposure times, and other observational parameters. Values provided in document tables, or as illustrations, are only approximations; reliable calculations that take into account the complex telescope and instrument operation are best obtained using software tools provided by STScI, such as the Exposure Time Calculators (ETCs) and APT. The ETCs, for example, provide warnings for target count rates that exceed saturation and safety limits. Note, however, that Signal-to-Noise (S/N) predictions from the ETCs do not include the effects of degrading CTE (Charge Transfer Efficiency) for CCD detectors.

Descriptions of the ETCs for active instruments, including determinations of exposure time as a function of instrument sensitivity and S/N ratio, are available in these documents: Chapter 9 of the ACS Instrument Handbook, Chapter 7 of the COS Instrument Handbook, Chapter 6 of the STIS Instrument Handbook, and Chapter 9 of the WFC3 Instrument Handbook.

HST Data Archive

The HST Data Archive is part of the Mikulski Archive for Space Telescopes (MAST). The HST Data Archive contains all the data taken by HST. Completed HST observations from both General Observer (GO) and Guaranteed Time Observer (GTO) programs are available to the community upon the expiration of their exclusive access periods. Observations taken in Large, Calibration, Treasury (see HST Cycle 27 Proposal Categories), and Large GO Pure Parallel programs (see HST Cycle 27 Observation Types and Special Requirements) generally carry no exclusive access period.

The HST Archive webpage provides links to information about getting started, search and retrieval, documentation, etc. (see also the introductory description in HST Cycle 27 Primer Data Processing and the HST Data Archive). You can search for HST data using either of two main search pages: the dedicated HST search page or the Data Discovery Portal. The Canadian Astronomy Data Centre (CADC) and the European Space Agency Centre (ESAC) maintain copies of the HST science data, and are the preferred sources for Canadian and European astronomers.

The Hubble Legacy Archive (HLA) is a project designed to enhance science from HST data by augmenting the HST Data Archive and by providing advanced browsing capabilities. Features of the HLA include a preview viewer, an interactive image display, a footprint service, individual, combined and mosaicked images, improved astrometric positions, object catalogs, and selected grism extractions. The HLA is a joint project of the Space Telescope Science Institute, the Canadian Astronomy Data Centre (CADC), and the European Space Astronomy Centre (ESAC).

The HLA provides source lists for tens of thousands of HST images. The Hubble Source Catalog (HSC) combines visit-based WFC3, ACS, and WFPC2 source lists from the HLA into a master catalog with roughly 300 million sources. Searches that would have required months or years to perform in the past can be completed in a matter of seconds using the HSC. Version 1 of the HSC was released in February 2015, and Version 2 was released in Fall 2016. The HSC is an invaluable resource for exploring a wide range of new archival proposals, a few potential examples of which are included in HST Cycle 27 Primer Data Processing and the HST Data Archive.

Cycle 27 proposers will be able to mine the HST Spectroscopic Legacy Archive for high-level data products intended to accelerate the scientific use of existing spectroscopic data. This archive will contain “science grade” co-added spectra of all usable public data, combining exposures for each target from across visits, programs, and cycles. This data will be organized into “smart archives” by target type (such as “hot stars” and “white dwarfs”) and by scientific purpose (“IGM absorption sources”) so that samples can be readily constructed and downloaded without manual interaction with MAST. The second generation of these products for the FUV modes of COS is available online via MAST. We encourage the development and submission of Archival Programs based on these new products.

New for Cycle 27, all non-exclusive access data for current Hubble instruments (ACS, COS, STIS, WFC3, FGS), have been made available as part of the Amazon Web Services (AWS) public dataset program. More information is available in the Archival Research (AR) Proposals section.

Questions about the Archive and archival data should be sent to the Archive Help Desk at http://masthelp.stsci.edu.

Duplication Checking

The HST Data Archive provides access to several tools that allow you to check whether planned observations duplicate any previously executed or accepted HST observations. For details see HST Cycle 27 Data Rights and Duplications.

Data Reduction and Calibration 

The Introduction to the HST Data Handbooks is a general overview of HST data formats and software tools. It complements the instrument data handbooks that contain more details about calibration and data analysis. The latest versions of the instrument data handbooks are available at the Documents webpage.

The Space Telescope Science Data Analysis Software (STSDAS) webpage has links to the software used to calibrate and analyze HST data, along with documentation on its use. More details are available in HST Cycle 27 Primer Data Processing and the HST Data Archive. The DrizzlePac Handbook provides information about the DrizzlePac software package that has replaced MultiDrizzle in pipeline calibration and post-pipeline processing. Some information about dither patterns, drizzling, and various observing considerations are included in it, but for more detailed information, please refer to the Phase II Proposal Instructions and instrument handbooks.

Next: HST Cycle 27 Primer System Overview

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