This routine computes the mirror vignetting factor for a particular energy band at a specific position in the focal plane as the ratio between the mirror effective area values for that position and on-axis. Thus, calling the routine with theta = 0 will return 1.0 for all azimuthal angles and energies. Vignetting by the X-ray baffles and by the reflection grating spectrometer is taken into account. The state variable AccuracyLevel determines the scheme to be used to calculate the effective area:
| Accuracy Level | scheme | description |
| NORMAL | library | the vignetting factor at any position in the field of view is calculated from a library of vignetting tables provided in the CCF. Interpolations in energy and field angle are performed to provide the value of the vignetting function at the requested energy and field position. |