Background

The OXS will encounter cosmic background from the diffuse x-ray background and scattered solar x-rays, as well as non-X-ray background in the LEPC. The total non-X-ray background in LEPC is estimated to be $\sim 5\times10^{-4}$ counts cm^-2 s^-1 keV^-1. In a 2$^{\prime}$ diameter circle ($\sim 4.7$ mm for an 8 m focal length), this corresponds to a rate of $\sim 9\times10^{-5}$ counts s^-1 keV^-1.

The diffuse x-ray background may be estimated using the effective area shown in Figure 55 and a model xrb spectrum. We use the ROSAT spectrum of Wang and McCray (1993) below 3 keV, and the HEAO-1 spectrum of Boldt (1987) above 3 keV, and a pixel corresponding to a 2$^{\prime}$ diameter circle, to determine the values shown in Table 7. Readers should be aware, however, that the soft component of the xrb is dominated by galactic contributions and can vary with viewing direction by as much as a factor of 5. The total xrb in the OXS bands will therefore vary by a large factor.