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Astronomical Data Analysis Software and Systems IV
ASP Conference Series, Vol. 77, 1995
R. A. Shaw, H. E. Payne, and J. J. E. Hayes, eds.
The SAX­LEGSPC Data Reduction and Analysis System:
An Example of a Minimalist Approach
F. Favata, A. N. Parmar, U. Lammers, G. Vacanti, M. Busetta
Astrophysics Division, ESA/ESTEC, P.O. Box 299, 2200 AG
Noordwijk, The Netherlands
J. J. Mathieu, P. Isherwood
Mathematics and Software Division, ESA/ESTEC, P.O. Box 299, 2200
AG Noordwijk, The Netherlands
Abstract. We present the data reduction and analysis system for the
Low Energy Gas Scintillation Proportional Counter to be flown on the
Italian­Dutch X­ray satellite SAX. The design philosophy of the system
is presented, describing the design constraints and discussing the various
choices made. Also, the problems encountered in following the chosen
approach are described. In particular, the advantages and disadvantages
of using FITS for all the steps in the data analysis chain are discussed.
1. Introduction
The Low Energy Gas Scintillation Proportional Counter (hereafter, LEGSPC)
is one of the instruments flying on the Italian--Dutch X­ray mission SAX (Scarsi
1993), which will be launched in early 1996. The instrument is being provided
from the Astrophysics Division of the European Space Agency at ESTEC, and
is described in detail by Favata & Smith (1989), Parmar et al. (1990), and Erd
& Bavdaz (1992). It is an imaging spectrometer, sitting behind a set of nested
double cone mirrors that approximate Wolter type I.
We are currently implementing the data reduction and analysis system
which will allow the observer to go from the raw telemetry tapes (or ``final ob­
servation tapes,'' FOTs) being provided by the spacecraft operator (Telespazio),
to the extraction of scientific results.
2. Design Choices
Given the characteristics of the detector and the limited resource level, we have
made some radical design choices early on.
2.1. Detector Characteristics, Strong Points, and Limitations
The LEGSPC has a large energy coverage (almost two decades in energy) with
good spectral resolution (actually better than current CCD detectors at the
lowest energies, reaching about 28% full width at half maximum (FWHM) at
1

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the C 280eV line), with relatively modest imaging capabilities (about 1 arcmin
FWHM on­axis, with rather extended wings). Additionally, the front window
has a support structure in the form of a grid with an approximate 4 arcmin pitch,
and the mirror point response function rapidly assumes an irregular shape with
increasing off­axis angle. Therefore the best usage of the instrument will be for
spectroscopy of on­axis point sources, and the complete data­analysis chain is
tuned to this purpose, rather than, for example, the analysis of extended sources.
On the other hand, one of the nice features of the LEGSPC is that the
raw data coming out of the detector are already rather clean, and only need a
relatively small amount of processing before being scientifically useful. Two 55 Fe
calibration sources are constantly shining in the field of view provide continuous
monitoring of the gain, and therefore an easy to use reference point against
which to monitor the performance of the detector.
2.2. Resource Limitations
The level of resources available internally to support the data analysis system
set­up activities was limited to about 2.5 person­years per year, starting about
2 years before launch. This limit meant that nice­to­haves were excluded from
the beginning and, given that we had to handle the data from the raw telemetry
state down to final science, we had to concentrate on the truly essential tasks.
2.3. Baseline Design Requirements
Our baseline design approach has therefore been to set up an analysis system
which would satisfy several criteria: The system must allow for cleaning of the
LEGSPC photon lists, and remove all known instrumental effects (e.g., lineariza­
tion of energy and x­y coordinates). It must allow for extraction of cleaned spec­
tra of on­axis point sources for further scientific analysis. The system must also
be optimized for in­house usage; its eventual integration with software for other
SAX instruments, and eventual distribution and support to guest observers,
must be provided by a team set up by the Italian Space Agency (ASI). Finally,
although the system was written keeping portability in mind, it is being devel­
oped and run only on SunOS platforms, with no effort being made to provide
multi­platform support.
3. Design Choices
To be able to complete the task within the limited amount of resources available
we have decided to adopt a minimalist approach, i.e., to reuse as much available
software as possible, and to concentrate on the instrument­specific stages (i.e.,
event cleaning and linearization, building of the detector response matrix). We
left the generic tasks (image display, spectral extraction and fitting, etc.) to
existing software.
3.1. File Formats: FITS and Others
First and foremost among the design choices was the selection of a file format.
Driven by the aim of recycling existing software as much as possible we have
chosen to adopt FITS, and in particular the use of binary tables for storing event

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lists. Given the large number of tools available for the reduction and analysis of
ASCA data in FITS format, and given that FITS will most likely be adopted by
forthcoming X­ray missions, we decided to adopt, as much as possible, a format
adhering to the specifications for ASCA event lists and house­keeping (HK) files
as produced by the HEASARC group at Goddard Space Flight Center. This
immediately gave us the possibility of using the set of software tools produced by
HEASARC known as FTOOLS. These include tools for performing elementary
operations on FITS binary tables, tools for selecting events from a table based
on arbitrary complex selection criteria, and building 1­D and 2­D histograms
(images) from event lists, etc.
We have additionally decided to use FITS as the format for the calibration
files, adopting an approach similar to the one used by HEASARC for the ASCA
calibration database. We have used, for setting up the calibration database, the
same software tools distributed by HEASARC (caltools), which allow to keep
the indexing of calibration products and to retrieve automatically the correct
calibration product for the observation being analyzed. As for the format of
the calibration files, we have found that many of the HEASARC suggested
formats were, while aiming to be very general, too complex or unnatural for
our instrument, and have therefore decided to define our own formats which
follow the data types at hand in a more natural way.
3.2. Disadvantages of FITS
While the advantages of using FITS for our data from the start of the chain
should be evident from the previous section (e.g., the large body of software
usable as­is, the ease of archiving and of sharing the data), we have found that
this comes at the price of a significant amount of work in defining the files formats
and keywords and in checking that the chosen format is actually compatible with
the large body of software that we are going to use. As for the other supposed
disadvantages of FITS, namely the inefficiencies both in terms of sheer file size
and of I/O requirements, we have found that, with our typical data sizes (an
observation file set being of order 10 MB in size) and with the speed of modern
machines and the size of modern disks, these disadvantages are almost negligible,
and that the choice of a more efficient but non­standard file format is therefore
not justified.
4. The Reduction System
Starting from the raw telemetry tapes the data files go through the following
steps:
1. conversion to FITS
2. non­interactive linearization
3. non­interactive selection of Good Time Intervals (GTI)
4. (optional) interactive selection of Good Time Intervals (GTI)
5. filtering of the data based on the GTI files produced above

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6. interactive extraction of the spectrum of a source (PHA file), using XSE­
LECT and SAOimage as image analysis tools
7. building of the response matrix for the extracted spectrum file
8. spectral fitting of the resulting spectrum using XSPEC
5. Calibration Data
To analyze the calibration data, thanks to the fact that the data produced during
ground calibration by the electronic ground support equipment have essentially
the same format as the flight data, we are using the same tools and the same
approach as we will take for the flight data. This gives us the possibility of
thoroughly testing the data analysis chain before flight.
6. Conclusions
The adoption of FITS and the extensive recycling of existing software has al­
lowed us to put together a complete data analysis chain for the SAX LEGSPC
detector using a relatively small amount of resources. This task would not have
been possible, within the same resource constraint, by using a more traditional
approach of re­designing everything from scratch in­house, even using fancy en­
vironments such as IDL.
References
Erd, C., & Bavdaz, M. 1992, in EUV, X­Ray, and Gamma­Ray Instrumentation
in Astronomy III, SPIE Conference Proceedings, Vol. 1743 (Bellingham,
SPIE), p. 133
Favata, F., & Smith, A. 1989, in EUV, X­ray and Gamma­Ray Instrumentation
for Astronomy and Atomic Physics, SPIE Conference Proceedings, Vol.
1159 (Bellingham, SPIE), p. 488
Parmar, A. N., Smith, A., Bavdaz, M. 1990, in Observatories in Earth Orbit
and Beyond, ed. Y. Kondo, (Dordrecht, Kluwer)
Scarsi, L. 1993, A&AS, 97, 371