BeppoSAX is an Italian/Dutch spacecraft that was placed into low-Earth
orbit by an Atlas I launch vehicle on 1996 April 30.
The nominal orbit is circular at a height of 600 km with an inclination
of 
to the equator in order to take maximum advantage of the
shielding of the Earth's magnetic field.
During the expected lifetime of up to 4 years,
BeppoSAX will make 2000-3000 observations of cosmic
X-ray sources over the unprecendently wide energy range of 0.1-300 keV.
Descriptions of the mission are to be found in
Spada (1983), Butler & Scarsi (1990), Scarsi (1993) and
Boella et al. (1996b).
The scientific objectives of the mission include broad band spectroscopy
in the energy range 0.1-10 keV with imaging
resolution of 1', non-imaging spectroscopy
in the energy range 3-300 keV and variability studies of X-ray source
intensities and spectra on timescales from milliseconds to months
(Perola 1983). Mission operations will be conducted from Rome, Italy and are
designed to
allow a flexible response to any targets of opportunity.
The payload consists of the Phoswich Detector System (PDS), the High Pressure Gas Scintillation Proportional counter (HPGSPC), two Wide Field Cameras (WFC), three imaging Medium Energy Concentrator Systems (MECS) and the imaging Low Energy Concentrator System (LECS). All the instruments are coaligned except for the WFCs which point in opposite directions along an axis perpendicular to the other instruments. Each imaging GSPC is located at the focal plane of an X-ray concentrator system. In order to extend the sensitive range of the imaging spectrometers to <1.3 keV, the LECS dispenses with the separate drift and scintillation regions of conventional GSPCs and utilizes an extremely thin entrance window. This instrument was provided by SSD and is described here. Its primary objective is to provide better energy resolution at low energies than currently available non-dispersive detectors and so to open up new areas of astrophysical interest.
The last few years have been an exciting time for X-ray astronomy with
the launches of the ROSAT, ASCA and XTE satellites.
ROSAT provides high sensitivity and spatial resolution in the
0.1-2.5 keV energy range, but with only moderate spectral resolution
(Trümper 1983).
XTE concentrates on timing studies in the 2-200 keV energy range with
moderate spectral resolution (Swank et al. 1995).
The ASCA payload consists of both imaging CCD and GSPC detectors
(Tanaka et al. 1994).
The CCD detectors provide an energy resolution, 
,
of 100-150 eV over the 
keV energy range, while the
GSPCs provide 
eV at 6 keV over the energy range
1.0-10 keV.
The good spectral and spatial resolutions, robustness and
low power requirements of CCD detectors have ensured
that they have become the preferred detectors for future X-ray spectroscopic
investigations (Weisskopf 1987; Bignami et al. 1990;
Wells et al. 1992).
While their development is still proceeding,
it is worth comparing their properties with those of GSPCs:
worse than a CCD. However, the resolution,
scales approximately
as 
and so becomes comparable to that of CCDs at low
(
keV) energies.
s. This should be compared with a time of 
s needed
to read out a typical CCD. CCD time resolution can be improved to tens of ms
by continuously reading the device. The spatial distribution
of the observed X-rays then represents timing information.
which can place severe constraints on the maximum source strength allowed.
The maximum count rate of the LECS is limited by the electronics to
be 
.