A reflection type projection optical system suitable for projection of a micropattern object is capable of performing reduction projection. The projection optical system comprises first and second optical subsystems which are combined to set a Petzval sum to zero. A first optical subsystem S1 forms a...http://www.google.com.au/patents/US4812028?utm_source=gb-gplus-sharePatent US4812028 - Reflection type reduction projection optical system
