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Astro-D: Plans for the Data Processing System

Charles Day1, Keith Arnaud1,

and N. E. White2


1: Astro-D GOF, 2: HEASARC



Introduction

Astro-D is Japan's fourth cosmic X-ray astronomy mission and the second for which the US is providing a significant part of the scientific payload. Scheduled to fly in February 1993, its four large-area telescopes will focus X-rays from a wide energy range onto a pair of CCDs and a pair of imaging Gas Scintillation Proportional Counters (GSPC). Astro-D will be the first X-ray imaging mission operating over the 0.5-12 keV band with high energy resolution (8 and 2 percent at 5.9 keV for the GSPCs and CCDs, respectively). The spatial resolution of the mirrors, i.e., the half-power diameter of the point spread function, will be 2.9 arcmin. This combination of capabilities will enable a varied and exciting program of research to be carried out by US and Japanese astronomers.

Although the designed lifetime is one year, the mission is expected to last about five years, to generate 200 Mbytes of raw data per day, and may observe up to seven sources per day. The eventual mission archive will contain up to 300 Gbytes of raw data. Since the Astro-D Guest Observer Facility (GOF) and the HEASARC both come under the Office of Guest Investigator Programs (OGIP), the data processing system can be designed to populate the Astro-D archive automatically and in accordance with HEASARC standards. Another advantage of the close association of the HEASARC, Astro-D GOF, as well as the ROSAT GOF, is that software is being written in a multi-mission way as far as possible.

This article describes, in general terms, the plans of the Astro-D GOF to provide an efficient, user-friendly data-processing system.

Data and Data Processing

Overview

The key elements of the design philosophy of the data processing system are:

  • the conversion of the raw data from the mission- specific formats to a HEASARC standard FITS format as soon as possible;
  • a pipeline of simple, modular design which can be run interactively by GOs and automatically by the GOF;
  • the smooth and automatic archiving of data and data products;
  • the separation of mission-dependent and generic modules.

    The data processing system is being designed and built in close collaboration with Japanese astronomers at ISAS, with the aim of ensuring that the same processing software will run in the US and Japan. Furthermore, existing software, such as the IRAF/PROS and XANADU analysis packages, will be used as much as possible. Software is being written with a multi-mission flavor so that it can be used for other missions. This is a feasible design goal, since data from BBXRT, HEAO-1 A2, HEAO-2 SSS etc., will be reformatted to formats closely resembling those used for Astro-D.

    The processing pipeline is shown symbolically in Figures 1-3. Three stages are identified:

  • ingestion and reformatting,
  • concatenation, selection and filtering,
  • analysis.

    Stage-1 processing: ingestion and reformatting

    The first stage of processing is shown in Figure 1. The data from Japan, comprising the telemetry data (First Reduction File, FRF), the Orbit File (OF), the Attitude File (AF), as well as the planned timeline and an observation summary, are ingested by the database management system. After ingestion, the format of the data is changed from the Astro-D specific format to the self-defining FITS format with no loss of information. By reformatting early in the pipeline, we ensure that all the raw data are available to users in a standard format. At this stage, during the reformatting, the scientific and housekeeping data are separated. For each instrument (two SIS, two GIS) and for each change of mode, a Science File (SF) and Housekeeping File (HKF) are produced. The SF will be similar in format to the revised ROSAT FITS files and will contain the attitude and orbit information in the AF and OF.

    Figure 1

    Stage-2: Concatenation, Selection and Filtering

    The second stage of processing is shown in Fig.2. After reformatting, a simple program, the Concatenator, groups together SF and HKF associated with same observation. An observation is defined as the data from a single pointing for a single PI.

    The next important step is the selection and filtering of the data to be analyzed. This is to be done by a program called the Selector which is intended to be the central hub around which data selection and analysis takes place. It will allow the user to select observations, apply filters, check the status and details of these filters, examine the results of such selections, and create a filtered SF as the output. The multi-task nature of the Selector is best realized as a series of free- standing, clearly defined tools, chained in macros if necessary, and mediated by a single user interface program. In order to concentrate first on the selection algorithms, the prototype Selector will be built within the IRAF environment, taking advantage of the flexible parameter-passing mechanism of IRAF. A XANADU-like user interface will be developed in parallel. For both the IRAF and XANADU versions, a command-driven interface will be developed first, the structure of which will be optimized for the subsequent overlaying of a Graphical User Interface (GUI).

    Clearly, the functions performed by the Selector are required by almost any orbiting X-ray or gamma-ray observatory, so, from the outset, the Selector is being designed to be a multi-mission program. From the user point of view, the Selector inputs and outputs are as follows.

    Inputs:

  • concatenated SF and HKF
  • calibration data files
  • user-applied filters, quality flags, either as keyboard
        input or as files
  • macros, script files (for batch or pipeline applications)

    Outputs:

  • filtered SF and HKF in formats that IRAF/PROS and XANADU can read
  • summary products, such as images, spectra, light
        curves, HK and attitude histories
  • access to quick-look display for plotting the summary products
  • log files containing the functions applied in the same
        format as the script files

    Stage-3: Analysis

    the analysis stage of the processing is shown in Figure 3. The principal output from the Selector is the Filtered Science File (FSF) containing the subset of data which the Guest Observer (GO) wishes to analyze. Modifications to XANADU and to IRAF/PROS will enable these programs to read in the FSF and HKF (the SF too, since the FSF and SF differ quantitatively not qualitatively). After displaying the image, with SAOIMAGE (IRAF/PROS) or with XIMAGE (XANADU), the GO will then select part or parts of the image for scientific analysis. The production of light curves for further temporal analysis is the most straightforward: once a region of the field of view, e.g., an X-ray binary, and its immediate vicinity, has been selected, a light curve file can be generated for display and for further analysis by a program such as XRONOS (XANADU). Spectral analysis, on the other hand, is more complex because a response matrix is required. Due to variations across the field of view of the detectors (the GIS and SIS), the response matrix has to be generated each time a spectrum is integrated from the region of interest. This will be done by a free-standing program which will read the header of the spectral file and consult the appropriate calibration files and HKF. The same idea lies behind the generation of background and exposure maps for image analysis: a free-standing program will read the header of the FSF, and consult the appropriate calibration files and HKF to generate the maps.

    Figure 2

    Automatic, "pipeline" processing

    As mentioned above, the processing software will be designed to be run interactively by GOs and in batch by the Astro-D GOF. The products of automatic analysis will be directed to the Astro-D archive which will also include catalogs and the data themselves. Access to data and data products in the archive will be subject to a year-long moratorium in the case of US data. Japanese and US/Japan data will be made available after a longer, as yet undetermined, period.

    Figure 3


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