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CHAPTER 1:
Overview of the
HST Archive


Archival research is an important component of the Hubble Space
Telescope (HST) program. Calls for proposals for funded archival research
(by U.S. investigators) occur regularly with the HST Call for Proposals.
Requests for archival data and visits to STScI for archival research projects
that do not involve funding are considered year-round.

Completed HST observations-including both General Observer (GO)
and Guaranteed Time Observer (GTO) data-become available to the
astronomical community upon expiration of their proprietary periods. The
science data, along with the accompanying engineering and telemetry data,
are archived at the Space Telescope Science Institute (STScI) and are
available for analysis by interested scientists through the HST Archival
Research program. A copy of the HST archives is maintained at the Space
Telescope European Coordinating Facility (ST-ECF) in Garching
Germany, to which European requests should normally be addressed.
Another copy of all public HST data is maintained at the Canadian
Astronomical Data Center (CADC). As of December 1, 1992, the HST
archive contained approximately 20,000 public (non-proprietary) datasets,
providing ample opportunity for archival research.

The Data Management Facility (DMF) is the hardware and software
system currently used at STScI for archiving and distributing HST data.
The DMF was jointly developed by the STScI, the ST-ECF, and the CADC.
The DMF consists of an optical disk archive and a catalog that describes
the data. The catalog contains tables of keyword parameters describing
each observation. The DMF catalog can be accessed through the user
interface known as STARCAT, written by the ECF with the support of the
CADC and the STScI.

STARCAT is the menu-driven program used to search the HST
archive's catalog. It provides a window into those tables of the DMF
catalog that contain exposure and proposal level information about each
HST observation. A STARCAT user defines catalog queries using one of
the provided screens or by creating personal screens adapted to a particular
search strategy. By querying the catalog through STARCAT, the user can
locate interesting datasets, ascertain if a particular object or an interesting
region of the sky has been observed with the HST, determine the
instrumental parameters employed in a given observation and select
datasets for retrieval from the archive. Using STARCAT, small amounts of
non-proprietary HST data can be retrieved electronically and large amounts
can be obtained on Exabyte or 9-track tape.

STScI staff recently completed a program to reprocess all of the early
HST data, using the latest STSDAS (Space Telescope Science Data
Analysis System) software. Many of the keyword problems associated with
the original data reduction have been corrected and the data have been
reprocessed with calibration files based on data obtained from the science
verification phase of HST. This reprocessing effort produced a more
homogeneous set of data within the HST optical disk archive and a more
complete and correct description of each HST observation in the archive's
catalog.

There is information about all HST observations in the archive catalog,
which is fully accessible by the public. However, GOs and GTOs have
exclusive access to their scientific data during a proprietary period.
Normally this period is 12 months. Proprietary periods shorter or longer
than a year can be assigned at the discretion of the STScI director. Certain
types of observations, e.g., calibration data acquired by the STScI for the
purposes of instrumental calibration, or observations obtained as part of
orbital or science verification, have shortened (or no) proprietary periods.
The date on which each dataset becomes public is found in a field in the
HST archive catalog (called release_date) and can be determined
using the STARCAT software.
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