A projection optical system suitable for projection photolithography is
disclosed. The projection optical system is a modified Wynne-Dyson system
capable of imaging a large field over both a narrow and a broad spectral
range. The projection optical system includes a positive lens group
arranged adjacent to but spaced apart from a concave mirror along the
mirror axis on the concave side of the mirror. The system also includes a
variable aperture stop so that the system has a variable NA. The
projection optical system has two or more common foci within an
ultraviolet exposure band and a third common focus in a visible alignment
band. A projection photolithography system that employs the projection
optical system is also disclosed.