the soft metal circuits. These coils are excited by the same current, hence the symmetry of the lens collimates the beam at the sample. CO lenses are often coupled with an additional small condenser, manipulating the incoming convergence angle before the sample, facilitating convergent or parallel illumination without changing the CO strength.

To demonstrate the applicability of the CO lens to fs-TEM, we include a symmetric twin-field system with the sample equidistant between the condenser and the objective single turn solenoids as shown in figure 2. Again, we do not include pole pieces in this design, rather take care so as maintain field strengths and gradients that can be managed with water-cooled lenses. We make use of the equivalent lens concept, whereby two solenoids of radius r separated by 2r is a good approximation of the field geometry in a CO lens.

The combined influence of C3 and the CO is tested under two operating conditions, parallel and antiparallel excitation polarities. Referring to figure 2, C3 and the upper and lower parts of the CO have the same polarity in the former, and C3 and the CO lower are excited with opposite polarities to the CO upper in the latter. 