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PS reprint
M. Denby
Department of Physics and Astronomy, University of Leicester,
Leicester, LE1 7RH, U. K.
D. J. Allan
Department of Space Research, University of Birmingham, Birmingham,
B15 2TT, U. K.
M. J. Ricketts
DRAL, Chilton, Didcot, OX11 0QX, U. K.
JET-X, the Joint European X-ray Telescope, is one of the core instruments which will comprise the scientific payload of the Russian Spectrum Roentgen Gamma, high energy astrophysics mission. The instrument consists of two identical, co-aligned, X-ray imaging telescopes, each with a spatial resolution of 20 arc seconds. A 20 arc minute image of the sky is formed on a pair of CCDs located in the focal plane of each telescope. The CCDs provide spectral information in the 0.3 to 10.0 keV band with an energy resolution of approximately 2% at the center of the band. The instrument is described in detail by Wells et al. (1990).
Software is currently being developed to generate the Observation and Calibration Datasets which will be distributed to the observer and also to reduce the event information into scientific formats suitable for manipulation by a number of analysis packages.
Limited project resources have led us to a minimalist approach to the software development task. Wherever possible advantage is drawn from existing systems and priority is given to the development of those items of software which are JET-X specific. In practice this means the reduction of telemetry and calibration data into recognized formats and the production of programs which allow instrument specific science analysis tasks to be performed.
The FITS binary extension has been chosen as the storage mechanism for all data which are supplied to the observer. This decision has been driven primarily by the requirements for simplicity and portability. In addition, through the adoption of formats similar to those used for ASCA, effort is saved at the science analysis stage by relying on the use of existing software tools and analysis packages.
Separate event files are generated for each X-ray telescope and for each mode and configuration change of a telescope. Each event file is accompanied by corresponding housekeeping, attitude, orbit and time filter files. These files will be supplied to the observer, together with a contemporary Calibration Dataset, and will form the start point for scientific analysis. The resulting Observation Dataset will conform as closely as possible to the standards developed by the OGIP team at the Goddard Space Flight Center.
This stage of the processing occurs at the UK project data center. It associates the complete set of data files corresponding to an observation request, and applies calibration information to the detected events in order to convert the recorded parameters into absolute time, energy and spatial scales.
Positional parameters stored for each event include the CCD x and y coordinates, the angular coordinates of the photon relative to the optical axis of the telescope and the sky coordinates relative to the nominal pointing. Energy information includes the raw pulse height digitization together with a corrected pulse invariant value which is referenced to a nominal energy scale. Timing information includes the spacecraft and absolute event arrival times.
A `spatial index' will be included in the event files and will provide the potential for efficient spatial discrimination of events. Two tables give the start address and count of the events associated with each of 256 sub-lists corresponding to one of a grid of 16 x 16 celestial boxes covering the nominal field of view.
In keeping with the OGIP philosophy three levels of calibration files have been identified.
For the CPFs a set of keywords has been identified which are required by all files in order to obtain commonality with the OGIP calibration database programs. In addition, each CPF has a set of keywords which are specific to the particular dataset stored.
Calibration filenames have the form product_[selectors]_date.fits where product denotes the form of the calibration product (e.g., rmf for a redistribution matrix file), selectors is a list of configurations required to index the file (e.g., x1 for X-ray telescope number 1, c1 for CCD number 1), and date is the date at which the file became valid in the form yymmdd.
JET-X specific code has been developed to allow interface between the science analysis software and the CPFs. The interface is implemented through a file indexing subroutine followed by calls to subroutines which read the individual CPFs.
The indexing subroutine takes a set of requested instrument configuration parameters such as the telescope, filter, CCD, Modified Julian Day, etc. It then opens the matching CPF and sets up a descriptor block which contains the configuration parameters. The routine returns a unique identifier which may then be used in subsequent calls to recover the data arrays.
The calibration files will be stored in a directory structure which is identical to that used by the OGIP distribution system.
Science analysis begins with the selective transfer of events into science datasets (the images, spectra and time-series). Events may be screened at this stage by rejecting periods when the attitude solution, background rates, instrument configuration, etc., were not optimal. Our choice of file formats will allow the use here of existing FTOOLS such as MAKETIME, XSELECT and others.
After the first stage the requirement for additional, JET-X specific, programs has been identified as follows
The programs will be distributed in a /jetx subdirectory of the main /ftools directory. A Makefile will be supplied by JET-X which will build the JET-X library and FTOOLS and then copy these to the main /ftools/bin.host directory. The OGIP will include a reference to the JET-X Makefile in their own top level Makefile so that the JET-X software is automatically relinked whenever a new release of the FTOOLS occurs.
By adopting existing file format standards the software development effort for JET-X has been minimized. The Observation Datasets which will be delivered to the observers will be compatible with a number of existing software tools and analysis packages and will allow an open approach to scientific analysis.