A minimal does technique in 3d-electron microscopy

Abstract

As discussed elsewhere a minimal dose technique for the exposure of a 3Dtilt series is very important. This can be achieved by choosing one area of the specimen for tilting and focussing and another for exposure. For 2D-microscopy this principle was already used by Williams and Fisher. We avoid mechanical movements by shifting the illumination spot by a magnetic deflection system in the microscope, and by simultaneously bringing back the image by an electrostatic field, generated by additional built-in deflection plates (which allow shifts up to 50μ). Electromagnetic shifts of the illumination spot (but without the synchronous shifts of the image) have already been applied for 2D-microscopy. Our method has the advantage that large shifts (which are independent of the magnification) can be used. In practice the shifts are of the order of 10μ. The deflection is orientated along the tilt axis of the microgoniometer.