As a preliminary stage in manufacturing a lens or lens part for an
objective, in particular a projection objective for a microlithography
projection system, an optical blank is made from a crystal material. As a
first step in manufacturing the optical blank, one determines the
orientation of a first crystallographic direction that is defined in the
crystallographic structure of the material. The material is then machined
into an optical blank so that the first crystallographic direction is
substantially perpendicular to an optical blank surface of the optical
blank. Subsequently, a marking is applied to the optical blank or to a
mounting element of the optical blank. The marking has a defined
relationship to a second crystallographic direction which is oriented at
a non-zero angle relative to the first crystallographic direction.