less than 1 mm fig9 demonstrates a further characteristic of the projection optical system 6 namely the free working distance d w this is defined as the distance between the image plane 8 and the closest portion 34 thereto of one of the mirrors of the projection optical system 6 i e mirror 19 in the embodiment shown in fig2 in the projection optical system 6 the free working distance d w is 40 mm the fifth mirror 19 which is closest to the image plane 8 can therefore be constructed having a thickness that provides sufficient stability of the fifth mirror 19 materials for mirrors of this type include for example quartz zerodur or silicon carbide compounds other materials with ultra low expansion properties may also be used examples of materials of this type are known from products sold by corning usa under the name u201c ule u201d the optical data of the projection optical system 6 are summarised in the following the image side numerical aperture na is 0 5 the size of the image field is 1 u00d7 13 mm 2 the reducing magnification level is 8 u00d7 the image field 7 is rectangular the wavelength of the illumination light is 13 5 nm the sequence of the optical effects of the mirrors m 1 to m 6 negative n positive p is npnpnp principal rays enter the projection optical system in a convergent manner from the object plane an aperture stop is arranged on the mirror m 3 for limiting the illumination light at the edge the z distance between the object plane.