1. An apparatus for performing correlative optical microscopy and charged particle microscopy, the apparatus comprising: a charged particle column comprising a charged particle source (for producing a beam of charged particles along a particle-optical axis; and a magnetic objective lens comprising two pole shoes for focusing said charged particle beam; a sample position located between the pole shoes; an optical microscope for imaging a thin flat sample located at the sample position, the optical microscope showing an optical axis perpendicular to the particle-optical axis; and a sample holder for holding a thin flat sample in a first orientation in which the thin flat sample can be imaged by the particle-optical axis, and in a second orientation in which the thin flat sample is perpendicular to the optical axis; wherein: the optical microscope has an objective lens that is non-rotationally symmetric and is truncated to fit between the pole shoes of the magnetic objective lens, with the size parallel to the particle-optical axis smaller than the size perpendicular to the particle-optical axis, thereby showing different numerical apertures in two directions, and thus different resolutions in two directions; and the acceptance solid angle for gathering light is larger than the acceptance solid angle of the largest rotationally symmetric lens fitting between the pole shoes.