A method and apparatus for maintaining an optical gap between optical structures in a photolithography system is described. A frame defines first and second opposing surfaces. The first opposing surface defines a first opening and the second opposing surface defines a second opening. A plurality of spacing members are spaced apart on the first opposing surface around the first opening. The spacing members have substantially co-planar surfaces configured to mate with a surface of a first optical structure. A bonding agent seals a space around the first opening between the first opposing surface and the first optical structure. The frame thereby encloses the optical gap between the first optical structure and a second optical structure.