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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