1 Introduction

The Low-Energy Concentrator Spectrometer (LECS) (Parmar et al. 1997b) is an imaging gas scintillation proportional counter on-board the Italian-Dutch BeppoSAX X-ray astronomy mission. BeppoSAX (Boella et al. 1997a) was launched into a 600 km 3$.\!\!^\circ$9 inclination orbit on 1996 April 30. The LECS is one of four co-aligned narrow field instruments (NFIs). The other NFIs are the imaging Medium-Energy Concentrator Spectrometer (MECS; 1.3-10 keV; Boella et al. 1997b), the High Pressure Gas Scintillation Proportional Counter (HPGSPC; 5-120 keV; Mazo et al. 1997) and the Phoswich Detection System (PDS; 15-300 keV; Frontera et al. 1997). In addition, the payload includes two wide field cameras (WFC; 2-30 keV; Jager et al. 1997) which observe in directions perpendicular to the NFI. The MECS background is discussed in Chiappetti et al. (1998).

Accurate background subtraction is important for virtually all X-ray observations and especially for studies of extended and diffuse objects. In this paper we discuss background subtraction techniques appropriate to the LECS. Background subtraction is complicated in the LECS because (1) the wings of the mirror point spread function distribute a significant fraction of hard X-rays over the entire field of view (FOV), (2) the broad detector response spreads a fraction of low-energy X-rays over the entire FOV, (3) the off-axis response of the mirrors is complicated and single scattered X-rays from outside the direct FOV can reach the focal plane (Conti et al. 1994), and (4) at low-energies the X-ray sky has a great deal of spatial structure (see e.g., Snowden et al. 1995, 1997). This spatial structure has a complex energy dependence. Below 0.28 keV there is a galactic plane-to-pole variation of a factor $\sim$3, while at energies between 0.5 and 2.0 keV the North Polar Spur and the galactic bulge dominate. For energies >0.5 keV and $50^\circ < l_{\rm II} < 290^\circ$ the X-ray sky is relatively smooth with few discrete features visible.