The X-ray Timing Explorer (XTE) is a High Energy Astrophysics mission intended for launch in the second half of 1995. It carries two pointed instruments that together cover the range 2-250 keV at s time resolution and moderate spectral resolution, and one instrument that will monitor the X-ray sky continuously over the 2-10 keV range. XTE's on-board science data systems provide considerable processing power and unprecedented flexibility in telemetry data modes. Events are processed on-board in several simultaneous data modes, chosen from a large repertoire of modes. Consequently, keeping track of the collected data in the database and providing a mechanism to select data that satisfy selection criteria couched in physical terms is a challenging problem. The XTE Guest Observer Facility, in conformity with the practices at the Office of Guest Investigator Programs, will provide the data in FITS format. The design of these FITS files includes two new features that address the cataloging and data selection issues. First, a hierarchy of FITS tables will be used to navigate the database. A master index will allow software to browse through the catalog with the granularity of individual observations, and find references to instrument indices (one index per instrument or subsystem per observation), as well as, for instance, source information. An instrument index table will contain references to data files generated by data-system components for various time intervals during the observation. The emphasis for the data tables is on those containing raw data, but there will be additional ones holding, for instance, data products and calibration information. Thus, given access to the master index and a set of selection criteria, extractor software will be able to determine the location of the requested data. Second, a Data Description Language (DDL) has been developed to label each data item unambiguously and to facilitate data selection browsing. Through the use of tokens along a number of predefined axes the DDL describes the structure of the data (dimensionality of arrays), the physical placement of the data along the axes (e.g., time increments), and the selection criteria (e.g., single layer events, which detector, calibration events, veto layer events) applied when the data were collected. The eventual goal is to develop a data extractor or browser that is capable of parsing these data descriptors and matching data requests with available data. Both schemes can easily be generalized for multi-mission use.
Astronomical Data Analysis Software and Systems III
- Pub Date: