The effective area of the SXT to X-rays is described more fully in SXT Calibration Note 31 (J. R. Lemen, 11 Feb 93, Rev A) and also in SXT Calibration Notes 30 and 25.
The on-axis effective area of the SXT depends on four different factors: the double entrance filter transmission, the mirror reflectivity, the analysis filter transmission, and the CCD quantum efficiency (QE). All these factors are functions of wavelength and off-axis angle (see section 3.3.4). Fig. 2 (a) shows the double entrance filter transmission, Fig. 2 (b) shows the mirror reflectivity (including the effects of obscuration by the support structures), and Fig. 2 (c) shows the CCD QE as a function of wavelength. Fig. 2 (d) shows the product of Figs 2 (a)-(c) and corresponds to the SXT effective area when both filter wheel positions are ``open'' and sometimes referred to in SXT documentation as ``noback'' (for no back filter). Fig. 3 contains six panels which show the SXT effective area through the open position and through each of the X-ray analysis filters. Calibration Note 31 gives details about how these curves were derived, but in short, measurements were made a selected wavelengths, to which model calculations were fit and used to predict the responses at any desired wavelength. NOTE: Be and Al12 are mistakenly reversed in Tsuneta et al. (1991) for Fig 4.