<title>In-orbit performance of the SAX Low-Energy Concentrator System (LECS)</title>

The low energy concentrator system (LECS) is an imaging x- ray spectrometer. I is one of the narrow field instruments onboard the SAX satellite and covers the energy range from 0.1 to 10 keV. The good low energy response of the detector is achieved by using a driftless gas cell and a thin multilayer polyimide foil as an entrance window. SAX was launched on April 30, 1996. Following a two month commissioning phase, the satellite has entered the science verification phase. We report here on the first in-flight data acquired with the LECS. Using back-ground measurements and well known x-ray sources, we present the first results of the in-orbit performance and calibrations an compare them to ground measurements acquired at synchrotron and long beam x-ray sources. After a brief description of the instrument we discuss some aspects related to the ground calibrations. I particular the entrance window characteristics and the particularities of a driftless detector design are reviewed. By correcting for the x-ray absorption depth, we also show how the energy resolution could be enhanced.