Vignetting correction

Vignetting is a decrease in the effective area of the telescope mirror with increasing off-axis angle. It is due to a combination of a real decrease in the mirror area and a degradation of the mirror reflection efficiency at larger angles of reflection. It becomes significant for $>12^{\prime}$ (Einstein IPC) or $>6^{\prime}$ (ROSAT PSPC). Both the ROSAT and Einstein exposure maps exposure maps are corrected for vignetting.