Time series are corrected by assuming a mean photon energy and finding the ratio of the on axis response at this energy to the response at the position of the datafile. The data array is then multiplied by this ratio. Spectra are left unchanged by the vignetting correction, however an array containing the effective area for each energy at this position is inserted into the datafile and used to modify the detector response matrix by XRTRESP.
For a circular source box centred on the optical axis, the vignetting is calculated using an off-axis angle of half the outer radius. The optimum vignetting to use in the case of a large source box is dependent on the photon distribution within the box; for instance a box with most of its counts in the centre should really have an off-axis angle of 0.0. On the other hand a box with evenly distributed photons should have the area weighted mean off-axis angle of that box, i.e. SQRT((IRAD**2 + ORAD**2)/2), where IRAD ,ORAD are the inner and outer box radii. The default off-axis angle may be overridden on the command line by setting OFFAX1=??. For radial files, an off axis angle may be specified for each bin, by OFFAX1=??, OFFAX2=?? etc...